| Hoàng thành Thăng Long có giá trị nổi bật toàn cầu | |||
Phải bàn giao càng
sớm càng tốt để nhất thể hóa quản lý, việc này đang được đẩy nhanh tiến
độ. Hà Nội đã thỏa thuận được với Bộ QP những vị trí để di chuyển, còn
thời gian cụ thể thì hai phía đều phải nỗ lực phấn đấu...
29/9/2008, UNESCO đã tiếp
nhận Hồ sơ đăng ký Khu trung tâm Hoàng thành Thăng Long (HTTL) là Di
sản văn hóa thế giới. Bộ VH-TT&DL hồ sơ nộp trước hạn đúng 1 ngày
(30/9/2008 là hạn cuối cùng) là niềm vui lớn không chỉ cho những người
đã ngày đêm cố gắng vì HTTL, mà còn cho những người yêu lịch sử, yêu văn
hóa nước nhà.
 Dấu vết nền cung điện thời Lý ở Hố A20 - Di tích Hoàng Thành Thăng Long (Chụp lại từ cuốn Hoàng Thành Thăng Long, quà tặng cho các đại biểu quốc tế tham dự Hội nghị APEC 2006.) Bởi HTTL mới chỉ được công nhận là di tích quốc gia vào cuối năm ngoái (28/12/2007), và phần di tích khảo cổ học 18 Hoàng Diệu hiện tại vẫn chưa thuộc quyền quản lý của UBND thành phố Hà Nội. Phức tạp hơn, có tới hai đơn vị quản lý phần diện tích này, là Bộ Xây dựng (quản lý đất vì thuộc dự án xây dựng Nhà Quốc hội) và Viện Khảo cổ học Việt Nam (quản lý nghiên cứu, khai quật khảo cổ). Đã có lúc, tưởng như việc đăng ký hồ sơ sẽ phải lùi lại, bởi có những ý kiến muốn chờ công trình Nhà Quốc hội hoàn tất mới tiến hành lập hồ sơ. Khu Trung tâm HTTL luôn được coi là công trình văn hóa tiêu biểu nhất, được trông đợi nhất, và đúng nhất với tính chất chiều dài lịch sử 1000 năm. Hồ sơ được tiếp nhận, Hà Nội tiến thêm một bước đến hy vọng có di sản văn hóa thế giới quý giá vào đúng dịp kỷ niệm đại lễ 1000 năm Thăng Long - Hà Nội… Báo giới đã trao đổi với TS Nguyễn Văn Sơn, GĐ Giám đốc trung tâm Bảo tồn khu di tích Cổ Loa - Thành cổ Hà Nội nhân cột mốc này. Đã có những thời điểm, việc nộp hồ sơ đúng thời hạn là "nhiệm vụ bất khả thi", bởi thời gian quá gấp rút, bởi việc nghiên cứu vẫn đang tiến hành,... Là người trực tiếp chịu trách nhiệm, ông có chịu nhiều áp lực không?
- Quả thật, việc hoàn
thành hồ sơ đúng hạn là cuộc chay đua, nhưng chúng tôi luôn xác định
phải đến đích, vì đăng ký di sản văn hóa thế giới không chỉ là chuyện
nội bộ, mà còn là thể diện quốc gia, vì Nhà nước Việt Nam đã có đề nghị
đưa khu di tích vào dự thảo danh sách DSVHTG từ 21/6/2006, đã có cam kết
của tổng GĐ UNESCO với Nguyên Thủ tướng Phan Văn Khải.
Việc soạn thảo hồ sơ được
sự tham gia trực tiếp, góp ý kiến chi tiết, sát sao không chỉ của các
chuyên gia trong nước, mà cả các chuyên gia của UNESCO. Họ tận tình giúp
ta vì chính họ bị thuyết phục bởi những giá trị lớn của tự thân di
tích. Rất mừng vì sau khi hoàn thành, dù chỉ trong một thời gian ngắn,
nhưng hồ sơ được đánh giá là tổ chức khoa học nhất, hình thức trình bày
đẹp.
HTTL được đăng ký
theo tiêu chí 2, 3 và 6 trong "Hướng dẫn thi hành công ước quốc tế".
Vậy đâu là tiêu chí quan trọng nhất, là giá trị tiêu biểu "độc nhất vô
nhị" của HTTL so với các kinh thành khác trên thế giới?
Tiêu chí quý giá nhất, HTTL là trung tâm chính trị, kinh tế, văn hóa kéo dài hơn 1000 năm, đến giờ vẫn là thủ đô. Những phát lộ khảo cổ học thời gian qua minh chứng rằng, chúng ta có cả kho tàng di sản vĩ đại trong lòng đất, quá trình nghiên cứu còn rất lâu dài, và cần nhiều sự trợ giúp của quốc tế. Liệu việc xây nhà Quốc hội sát cạnh di tích có ảnh hưởng đến việc được công nhận di sản văn hóa thế giới không?  Phần diện tích đăng ký di sản hiện nay so với quy mô của Hoàng thành Thăng Long là rất nhỏ, dưới 1/5 diện tích. Nhưng đây là phần có giá trị lớn nhất, đậm đặc nhất. Quan điểm của Đảng, Chính phủ là vừa xây dựng NQH, vừa đảm bảo hồ sơ di sản văn hóa thế giới được UNESCO công nhận. Phải đạt được cả 2 mục đích, nên không lấy cái này để loại trừ cái kia. Có nhiều hình thức để bảo quản, giữ gìn di sản. Trên cơ sở những khuyến cáo của UNESCO, tôi tin ta sẽ tìm ra được những giải pháp hợp lý để dung hòa. Tạm dừng phần việc xây dựng hồ sơ, chờ phản hồi của UNESCO (vào 15/11/2008) để tiếp tục chỉnh sửa. Dư luận cũng rất quan tâm đến việc bàn giao những phần diện tích thuộc quản lý của Bộ Quốc phòng, để Hà Nội có thể tiếp quản, chỉnh trang? Trong hồ sơ trình UNESCO có lộ trình cụ thể của việc bàn giao không? - Đúng là phải bàn giao càng sớm càng tốt để nhất thể hóa quản lý, việc này đang được đẩy nhanh tiến độ. Hà Nội đã thỏa thuận được với Bộ QP những vị trí để di chuyển, còn thời gian cụ thể thì hai phía đều phải nỗ lực phấn đấu, Trong hồ sơ chúng ta đã trình trình bày rõ hiện trạng, và tiến trình triển khai các bước tiếp theo. Hà Nội và Bộ QP đang quyết tâm giải phóng phần diện tích phía trước (từ cửa Đoan Môn ra phía đường Điện Biên Phủ), riêng Bảo tàng Lịch sử quân sự thì cần thời gian lâu hơn để chuyển địa điểm. Nhận bàn giao đến đâu, chúng tôi sẽ tiến hành tháo dỡ những công trình không có giá trị, chỉnh trang cảnh quan. Riêng việc tu bổ, tôn tạo phải là quá trình dài, phụ thuộc vào quá trình và kết quả nghiên cứu, không thể là ý chí chủ quan. Việc tháo dỡ những công trình không có giá trị, chỉnh trang phần diện tích đang thuộc quyền quản lý của trung tâm đã bắt đầu chưa, thưa ông?
- Đó là cột mốc đặt ra để phấn đấu, nhưng chúng tôi chưa dám khẳng định sẽ làm được, bởi còn nhiều khó khăn trong việc bàn giao, chưa phải một đơn vị quản lý toàn bộ diện tích của di tích nên việc nghiên cứu cũng hạn chế. Riêng thông tin trên không chính xác, vì trong khuôn viên của Khu di tích sẽ có một bảo tàng để trưng bày toàn bộ hiện vật khai quật được, đó mới là cách làm theo chuẩn quốc tế.
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http://www.baoxaydung.com.vn/Main.aspx?MNU=1135&Style=1&ChiTiet=11033
World Heritage Criteria
To qualify for inscription on the World Heritage List, nominated properties must have values that are outstanding and universal. The Operational Guidelines for the Implementation of the World Heritage Convention provide guidance to the World Heritage Committee in deciding which nominations should be included on the List. These guidelines state that nominations should be based on cultural, natural and/or mixed cultural and natural criteria.
In Australia , The Great Barrier Reef, the Tasmanian Wilderness, the Wet Tropics of Queensland and Shark Bay meet all four World Heritage criteria for natural heritage, with Kakadu National Park , Uluru-Kata Tjuta National Park , Willandra Lakes Region and the Tasmanian Wilderness being listed for both natural and cultural criteria.
The Australian Fossil Mammal Sites (Naracoorte/Riversleigh), Lord Howe Island Group, Central Eastern Rainforest Reserves of Australia, Fraser Island, Macquarie Island, Heard and McDonald Islands, and the Greater Blue Mountains Area are listed under the World Heritage criteria for natural heritage.
Cultural criteria
Article 1 of the World Heritage Convention defines cultural heritage as:"MONUMENTS: architectural works, works of monumental sculpture and painting, elements or structures of an archaeological nature, inscriptions, cave dwellings and combinations of features, which are of outstanding universal value from the point of view of history, art or science;
GROUPS OF BUILDINGS: groups of separate or connected buildings which, because of their architecture, their homogeneity or their place in the landscape, are of outstanding universal value from the point of view of history, art or science;
SITES: works of man or the combined works of nature and of man, and areas including archaeological sites which are of outstanding universal value from the historical, aesthetic, ethnological or anthropological points of view."
Cultural Landscapes represent the "combined works of nature and of man" They are illustrative of the evolution of human society and settlement over time, under the influence of the physical constraints and/or opportunities presented by their natural environment and of successive social, economic and cultural forces, both external and internal. Cultural landscapes include diverse examples of the interaction between humans and the natural environment and fall into three main categories:
(i) the clearly defined landscape designed and created intentionally by man;
(ii) the organically evolved landscape; and
(iii)the associative cultural landscape.
For a property to be included on the World Heritage List as cultural heritage, the World Heritage Committee must find that it meets one or more of the following criteria, the test of authenticity and be adequately protected. Sites nominated should therefore:
i. represent a masterpiece of human creative genius; or
ii. exhibit an important interchange of human values, over a span of time or within a cultural area of the world, on developments in architecture or technology, monumental arts, town-planning or landscape design; or
iii. bear a unique or at least exceptional testimony to a cultural tradition or to a civilization which is living or which has disappeared; or
iv. be an outstanding example of a type of building or architectural or technological ensemble or landscape which illustrates (a) significant stage(s) in human history; or
v. be an outstanding example of a traditional human settlement or land-use which is representative of a culture (or cultures), especially when it has become vulnerable under the impact of irreversible change; or
vi. be directly or tangibly associated with events or living traditions, with ideas, or with beliefs, with artistic and literary works of outstanding universal significance (the Committee considers that this criterion should justify inclusion in the List only in exceptional circumstances and in conjunction with other criteria cultural or natural);
Natural Criteria
Article 2 of the World Heritage Convention, defines natural heritage:
(i) "natural features consisting of physical and biological formations
or groups of such formations, which are of outstanding universal value from the
aesthetic or scientific point of view;
(ii) geological and physiographical formations and precisely delineated
areas which constitute the habitat of threatened species of animals and plants
of outstanding universal value from the point of view of science or
conservation;(iii) natural sites or precisely delineated natural areas of outstanding universal value from the point of view of science, conservation or natural beauty."
For a property to be included on the World Heritage list as natural heritage, the World Heritage Committee must find that it meets one or more of the following criteria and fulfils the conditions of integrity. Sites nominated should therefore:
i. be outstanding examples representing major stages of earth's history, including the record of life, significant ongoing geological processes in the development of landforms, or significant geomorphic or physiographic features; or
ii. be outstanding examples representing significant ongoing ecological and biological processes in the evolution and development of terrestrial, fresh water, coastal and marine ecosystems and communities of plants and animals; or
iii. contain superlative natural phenomena or areas of exceptional natural beauty and aesthetic importance; or
iv. contain the most important and significant natural habitats for in-situ conservation of biological diversity, including those containing threatened species of outstanding universal value from the point of view of science or conservation.
(A more detailed description of World Heritage natural and cultural criteria can be found in: UNESCO. Intergovernmental Committee for the Protection of the World Cultural and Natural Heritage. Operational Guidelines for the Implementation of the World Heritage Convention. World Heritage Centre.
This document can be found in full at the World Heritage Centre web site at http://www.unesco.org/whc/nwhc/pages/doc/main.htm
Selection criteria
Site #772: The Banaue Rice Terraces in the mountains of Ifugao (Philippines).
Site #936: The Cueva de las Manos in a remote region of Patagonia (Argentina).
Until the end of 2004, there were six criteria for cultural heritage and
four criteria for natural heritage. In 2005, this was modified so that there is
only one set of ten criteria. Nominated sites must be of "outstanding
universal value" and meet at least one of the ten criteria.[2]
[edit] Cultural criteria
- I. "to represent a masterpiece of human creative genius";
- II. "to exhibit an important interchange of human values, over a span of time or within a cultural area of the world, on developments in architecture or technology, monumental arts, town-planning or landscape design";
- III. "to bear a unique or at least exceptional testimony to a cultural tradition or to a civilization which is living or which has disappeared";
- IV. "to be an outstanding example of a type of building, architectural or technological ensemble or landscape which illustrates (a) significant stage(s) in human history";
- V. "to be an outstanding example of a traditional human settlement, land-use, or sea-use which is representative of a culture (or cultures), or human interaction with the environment especially when it has become vulnerable under the impact of irreversible change";
- VI. "to be directly or tangibly associated with events or living traditions, with ideas, or with beliefs, with artistic and literary works of outstanding universal significance. (The Committee considers that this criterion should preferably be used in conjunction with other criteria)";
[edit] Natural criteria
- VII. "to contain superlative natural phenomena or areas of exceptional natural beauty and aesthetic importance";
- VIII. "to be outstanding examples representing major stages of earth's history, including the record of life, significant on-going geological processes in the development of landforms, or significant geomorphic or physiographic features";
- IX. "to be outstanding examples representing significant on-going ecological and biological processes in the evolution and development of terrestrial, fresh water, coastal and marine ecosystems and communities of plants and animals";
- X. "to contain the most important and significant natural habitats for in-site conservation of biological diversity, including those containing threatened species of outstanding universal value from the point of view of science or conservation."
ICOMOS CHARTER- PRINCIPLES FOR THE
ANALYSIS, CONSERVATION AND STRUCTURAL RESTORATION OF ARCHITECTURAL HERITAGE
(2003)
(2003)
Ratified by the ICOMOS 14th General Assembly, in Vicoria Falls , Zimbabwe , October 2003
PRINCIPLES
PURPOSE OF THE DOCUMENT
Structures of architectural heritage, by their very nature and history (material and assembly), present a number of challenges in diagnosis and restoration that limit the application of modern legal codes and building standards. Recommendations are desirable and necessary to both ensure rational methods of analysis and repair methods appropriate to the cultural context.
These Recommendations are intended to be useful to all those involved in conservation and restoration problems, but cannot in anyway replace specific knowledge acquired from cultural and scientific texts.
The Recommendations presented in the complete document are in two sections: Principles, where the basic concepts of conservation are presented; Guidelines, where the rules and methodology that a designer should follow are discussed. Only the Principles have the status of an approved/ratified ICOMOS document.
The guidelines are available in English in a separate document. [Word - 164 Kb]
PRINCIPLES
1 General criteria
1.1 Conservation, reinforcement and restoration of architectural heritage requires a multi-disciplinary approach.
1.2 Value and authenticity of architectural heritage cannot be based on fixed criteria because the respect due to all cultures also requires that its physical heritage be considered within the cultural context to which it belongs.
1.3 The value of architectural heritage is not only in its appearance, but also in the integrity of all its components as a unique product of the specific building technology of its time. In particular the removal of the inner structures maintaining only the façades does not fit the conservation criteria.
1.4 When any change of use or function is proposed, all the conservation requirements and safety conditions have to be carefully taken into account.
1.5 Restoration of the structure in Architecture Heritage is not an end in itself but a means to an end, which is the building as a whole.
1.6 The peculiarity of heritage structures, with their complex history, requires the organisation of studies and proposals in precise steps that are similar to those used in medicine. Anamnesis, diagnosis, therapy and controls, corresponding respectively to the searches for significant data and information, individuation of the causes of damage and decay, choice of the remedial measures and control of the efficiency of the interventions. In order to achieve cost effectiveness and minimal impact on architectural heritage using funds available in a rational way; it is usually necessary that the study repeats these steps in an iterative process.
1.7 No action should be undertaken without having ascertained the achievable benefit and harm to the architectural heritage, except in cases where urgent safeguard measures are necessary to avoid the imminent collapse of the structures (e.g. after seismic damages); those urgent measures, however, should when possible avoid modifying the fabric in an irreversible way.
2 Researches and diagnosis
2.1 Usually a multidisciplinary team, to be determined in relation to the type and the scale of the problem, should work together from the first steps of a study - as in the initial survey of the site and the preparation of the investigation programme.
2.2 Data and information should first be processed approximately, to establish a more comprehensive plan of activities in proportion to the real problems of the structures.
2.3 A full understanding of the structural and material characteristics is required in conservation practice. Information is essential on the structure in its original and earlier states, on the techniques that were used in the construction, on the alterations and their effects, on the phenomena that have occurred, and, finally, on its present state.
2.4 In archaeological sites specific problems may be posed because structures have to be stabilised during excavation when knowledge is not yet complete. The structural responses to a “rediscovered” building may be completely different from those to an ”exposed” building. Urgent site-structural-solutions, required to stabilise the structure as it is being excavated, should not compromise the complete building’s concept form and use.
2.5 Diagnosis is based on historical, qualitative and quantitative approaches; the qualitative approach being mainly based on direct observation of the structural damage and material decay as well as historical and archaeological research, and the quantitative approach mainly on material and structural tests, monitoring and structural analysis.
2.6 Before making a decision on structural intervention it is indispensable to determine first the causes of damage and decay, and then to evaluate the safety level of the structure.
2.7 The safety evaluation, which is the last step in the diagnosis, where the need for treatment measures is determined, should reconcile qualitative with quantitative analysis: direct observation, historical research, structural analysis and, if it is the case, experiments and tests.
2.8 Often the application of the same safety levels as in the design of new buildings requires excessive, if not impossible, measures. In these cases specific analyses and appropriate considerations may justify different approaches to safety.
2.9 All aspects related to the acquired information, the diagnosis including the safety evaluation, and the decision to intervene should be described in an “explanatory report”.
3 Remedial measures and controls
3.1 Therapy should address root causes rather than symptoms.
3.2 The best therapy is preventive maintenance
3.3 Safety evaluation and an understanding of the significance of the structure should be the basis for conservation and reinforcement measures.
3.4 No actions should be undertaken without demonstrating that they are indispensable.
3.5 Each intervention should be in proportion to the safety objectives set, thus keeping intervention to the minimum to guarantee safety and durability with the least harm to heritage values.
3.6 The design of intervention should be based on a clear understanding of the kinds of actions that were the cause of the damage and decay as well as those that are taken into account for the analysis of the structure after intervention; because the design will be dependent upon them.
3.7 The choice between “traditional” and “innovative” techniques should be weighed up on a case-by-case basis and preference given to those that are least invasive and most compatible with heritage values, bearing in mind safety and durability requirements.
3.8 At times the difficulty of evaluating the real safety levels and the possible benefits of interventions may suggest “an observational method”, i.e. an incremental approach, starting from a minimum level of intervention, with the possible subsequent adoption of a series of supplementary or corrective measures.
3.9 Where possible, any measures adopted should be “reversible” so that they can be removed and replaced with more suitable measures when new knowledge is acquired. Where they are not completely reversible, interventions should not limit further interventions.
3.10 The characteristics of materials used in restoration work (in particular new materials) and their compatibility with existing materials should be fully established. This must include long-term impacts, so that undesirable side-effects are avoided.
3.11 The distinguishing qualities of the structure and its environment, in their original or earlier states, should not be destroyed.
3.12 Each intervention should, as far as possible, respect the concept, techniques and historical value of the original or earlier states of the structure and leaves evidence that can be recognised in the future.
3.13 Intervention should be the result of an overall integrated plan that gives due weight to the different aspects of architecture, structure, installations and functionality.
3.14 The removal or alteration of any historic material or distinctive architectural features should be avoided whenever possible.
3.15 Deteriorated structures whenever possible should be repaired rather than replaced.
3.16 Imperfections and alterations, when they have become part of the history of the structure, should be maintained so far so they do not compromise the safety requirements.
3.17 Dismantling and reassembly should only be undertaken as an optional measure required by the very nature of the materials and structure when conservation by other means impossible, or harmful.
3.18 Provisional safeguard systems used during the intervention should show their purpose and function without creating any harm to heritage values.
3.19 Any proposal for intervention must be accompanied by a programme of control to be carried out, as far as possible, while the work is in progress.
3.20 Measures that are impossible to control during execution should not be allowed.
3.21 Checks and monitoring during and after the intervention should be carried out to ascertain the efficacy of the results.
3.22 All the activities of checking and monitoring should be documented and kept as part of the history of the structure.
© ICOMOS
http://www.international.icomos.org
centre-doc-icomos@unesco.org
----------------------http://www.international.icomos.org
centre-doc-icomos@unesco.org
RECOMMENDATIONS FOR THE ANALYSIS, CONSERVATION AND
STRUCTURAL RESTORATION OF ARCHITECTURAL HERITAGE
GUIDELINES
1 General criteria
2 Acquisition of data: Information and Investigation
2.1 Generally
2.2 Historical, structural and architectural investigations
2.3 Survey of the structure
2.4 Field research and laboratory testing
2.5 Monitoring
3 The structural behaviour
3.1 General aspects
3.2 The structural scheme and damage
3.3 Material characteristics and decay processes
3.4 Actions on the structure and the materials
4 Diagnosis and safety evaluation
4.1 General aspects
4.2 Identification of the causes (Diagnosi)
4.3 Safety evaluation
4.3.1 The problem of safety evaluation
4.3.2 Historical analysis
4.3.3 Qualitative analysis
4.3.4 The analytic approach
4.3.5 The experimental approach
4.4 Decisions and explanatory report
5 Structural damage, materials decay and remedial measures
5.1 General aspects
5.2 Masonry building
5.3 Timber
5.4 Iron and steel
5.5 Reinforced concrete
Appendix 1. – Committee members.
Appendix 2 – Glossary
GUIDELINES
1 General criteria
A combination of both scientific and cultural knowledge and experience is indispensable for the study of all architectural heritage. Only in this context can the guidelines help to the better conservation, strengthening and the restoration of buildings. The purpose of all studies, research and interventions is to safeguard the cultural and historical value of the building as a whole and structural engineering is the scientific support necessary to obtain this result.
Conserving architectural heritage usually requires a multidisciplinary approach involving a variety of professionals and organisations. These guidelines have been prepared to assist this work and facilitate communication between those involved.
Any planning for structural conservation requires both qualitative data, based on the direct observation of material decay and structural damage, historical research etc., and quantitative data based on specific tests and mathematical models of the kind used in modern engineering. This combination of approaches makes it very difficult to establish rules and codes. While the lack of clear guidelines can easily lead to ambiguities and arbitrary decisions, codes prepared for the design of modern structures are often inappropriately applied to historic structures. For example, the enforcement of seismic and geotechnical codes, can lead to drastic and often unnecessary measures that fail to take account of real structural behaviour.
The subjective aspects involved in the study and safety assessment of an historic building, the uncertainties in the data assumed and the difficulties of a precise evaluation of the phenomena, may lead to conclusions of uncertain reliability. It is important, therefore, to show clearly all these aspects, in particular the care taken in the development of the study and the reliability of the results, in an Explanatory Report. This report requires a careful and critical analysis of the safety of the structure in order to justify any intervention measures and will facilitate the final judgement on the safety of the structure and the decisions to be taken.
The evaluation of a building frequently requires a holistic approach considering the building as a whole rather than just the assessment of individual elements.
2 Acquisition of data: Information and Investigation
2.1 Generally
The investigation of the structure requires an interdisciplinary approach that goes beyond simple technical considerations because historical research can discover phenomena involving structural behaviour while historical questions may be answered by considering structural behaviour. Therefore it is important that an investigating team be formed that incorporates a range of skills appropriate to the characteristics of the building and which is directed by someone with adequate experience.
Knowledge of the structure requires information on its conception, on its constructional techniques, on the processes of decay and damage, on changes that have been made and finally on its present state. This knowledge can usually be reached by the following steps:
- definition, description and understanding of the building’s historic and cultural significance;
- a description of the original building materials and construction techniques;
- historical research covering the entire life of the structure including both changes to its form and any previous structural interventions;
- description of the structure in its present state including identification of damage, decay and possible progressive phenomena, using appropriate types of test;
- description of the actions involved, structural behaviour and types of materials;
A ‘pre-survey’ of both the site and the building should guide these studies.
Because these can all be carried out at different levels of detail it is important to establish a cost effective plan of activities proportional to the structure’s complexity and which also takes into account the real benefit to be obtained from the knowledge gained. In some cases it is convenient to undertake these studies in stages beginning with the simplest.
2.2 Historical, structural and architectural investigations
The purpose of the historical survey is to understand the conception and the significance of the building, the techniques and the skills used in its construction, the subsequent changes in both the structure and its environment and any events that may have caused damage. Documents used for this should be noted.
The sources should be assessed for their reliability as a means of reconstructing the history of construction. Their careful interpretation is essential if they are to produce reliable information about the structural history of a building.
Assumptions made in the interpretation of historical material should be made clear. Particular attention should be paid to any damage, failures, reconstructions, additions, changes, restoration work, structural modifications, and changes of use that lead to the present condition.
It should be remembered that documents which may be used were usually prepared for purposes other than structural engineering and may therefore include technical information which is incorrect and/or may omit or misrepresent key facts or events which are structurally significant.
2.3 Survey of the structure
Direct observation of the structure is an essential phase of the study, usually carried out by a qualified team to provide an initial understanding of the structure and to give an appropriate direction to the subsequent investigations.
The main objectives include:
- identifying decay and damage,
- determining whether or not the phenomena have stabilised,
- deciding whether or not there are immediate risks and therefore urgent measures to be undertaken,
- identifying any ongoing environmental effects on the building.
The study of structural faults begins by mapping visible damage. During this process interpretation of the findings should be used to guide the survey, and the expert already developing an idea of the possible structural behaviour so that critical aspects of the structure may be examined in more detail. Survey drawings should map different kinds of materials, noting any decay and any structural irregularities and damage, paying particular attention to crack patterns and crushing phenomena.
Geometric irregularities can be the result of previous deformations, can indicate the junction between different building phases or alterations to the fabric.
It is important to discover how the environment may be damaging a building, since this can be exacerbated by poor original design and/or workmanship (e.g. lack of drainage, condensation, raising damp), the use of unsuitable materials and/or by poor subsequent maintenance.
Observation of areas where damage is concentrated as a result of high compression (zones of crushing) or high tensions (zones of cracking or the separation of elements) and the direction of the cracks, together with an investigation of soil conditions, may indicate the causes of this damage. This may be supplemented by information acquired by specific tests
2.4 Field research and laboratory testing
The schedule of tests should be based on a clear preliminary view of which phenomena are the most important to understand. Tests usually aim to identify the mechanical (strength, deformability, etc.), physical (porosity, etc.) and chemical (composition, etc.) characteristics of the materials, the stresses and deformations of the structure and the presence of any discontinuities within the structure.
As a rule, the schedule of tests should be divided into stages, starting with the acquisition of basic data, followed by a more detailed examination with tests based upon an assessment of the implications of the initial data.
Non-destructive tests should be preferred to those that involve any alterations to a structure; if these are not sufficient, it is necessary to assess the benefit to be obtained by opening up the structure in terms of reduced structural intervention against the loss of culturally significant material (a cost-benefit analysis).
Tests should always be carried out by skilled persons able to gauge their reliability correctly and the implication of test data should be very carefully assessed. If possible different methods should be used and the results should be compared. It may also be necessary to carry out tests on selected samples taken from the structure.
2.5 Monitoring
Structural observation over a period of time may be necessary, not only to acquire useful information when progressive phenomena is suspected, but also during a step-by-step procedure of structural renovation. During the latter, the behaviour is monitored at each stage (observational approach) and the acquired data used to provide the basis for any further action.
A monitoring system usually aims to record changes in deformations, cracks, temperatures, etc. Dynamic monitoring is used to record accelerations, such as those in seismic areas.
Monitoring can also act as an alarm bell.
The simplest and cheapest way to monitor cracks is to place a ‘tell-tale’ across them. Some cases require the use of computerised monitoring systems to record the data in real time.
As a general rule, the use of a monitoring system should be subjected to a cost-benefit analysis so that only data strictly necessary to reveal progressive phenomena are gathered.
3. The structural behaviour
3.1 General aspects
The behaviour of any structure is influenced by three main factors: the shape and the connections of the structure, the construction materials and the imposed forces, accelerations and deformations (the actions); these factors are here examined in detail
3.2 The structural scheme and damage
The structural behaviour depends on the characteristics of the materials, the dimensions of the structure, the connections between different elements, the soil conditions, etc.
The real behaviour of a building is usually too complex to fully model so that we are obliged to represent it with a simplified 'structural scheme', i.e. an idealisation of the building which shows, to the required degree of precision, how it resists the various actions.
The structural scheme shows how the building transforms actions into stresses and ensures stability.
A building may be represented by different schemes with different complexity and different degrees of approximation to reality.
The original structural scheme may have changed as a result of to damage (cracks, etc.), reinforcements, or other modifications of the building. The scheme used in the structural analysis is usually a compromise between one close to reality but too complex for calculation and one easy to calculate but too far from the reality of the building.
The scheme used has to take into account any alterations and weakening, such as cracks, disconnections, crushing, leanings, etc., whose effect may significantly influence the structural behaviour. These alterations may be produced either by natural phenomena or by human interventions. The latter includes the making of openings, niches, etc.; the elimination of arches, slabs, walls, etc., which can create unbalanced forces; increases in height of the structure, which can increase weights; excavations, galleries, nearby buildings, etc., which can reduce the soil bearing capacity.
3.3 Material characteristics and decay processes
Material characteristics (particularly strengths), which are the basic parameters for any calculation, may be reduced by decay caused by chemical, physical or biological action. The rate of decay depends upon the properties of the materials (such as porosity) and the protection provided (roof overhangs, etc.) as well as maintenance. Although decay may manifest itself on the surface, and so be immediately apparent from superficial inspection (efflorescence, increased porosity, etc.), there are also decay processes that can only be detected by more sophisticated tests (termite attack in timber, etc.).
3.4 Actions on the structure and the materials
'Actions' are defined as any agent (forces, deformations, etc.) which produce stresses and strains in the structure and any phenomenon (chemical, biological, etc.) which affects the materials, usually reducing their strength. The original actions, which act from the beginning of construction and the completion of the building (dead loads, for example), may be modified during its life and it is often these changes that produce damage and decay.
Actions have very different natures with very different effects on both the structure and the materials.
Often more than one action (or, change to the original actions), will have affected the structure and these must clearly be identified before selecting the repair measures.
Actions may be divided into mechanical actions that affect the structure and chemical and biological actions that affect the materials. Mechanical actions are either static or dynamic the former being either direct or indirect (see Table 1).
Table 1 – Classification of the different kinds of action on structures and their materials
|
1 - Mechanical actions – acting on the structure |
i) Static actions |
a) Direct actions(i.e. applied loads) |
b) Indirect actions(i.e. applied strains) |
||
ii) Dynamic actions(imposed accelerations) |
|
|
|
2
i) Physical, ii) Chemical and iii) Biological actions – acting on the materials |
|
|
1 Mechanical actions acting on the structure produce stresses and strains in the material possibly resulting in visible cracking, crushing and movement. This can be static or dynamic
i) Static actions can be of two kinds:
a). Direct actions i.e. applied loads such as dead loads (weight of the building, etc.) and live loads (furniture, people, etc.). Changes, and mainly increases in loads, are sources of increased stresses and thus of damage to the structure. In some cases reductions in load can also be a source of damage to the structure.
b) Indirect actions (comprising deformations imposed on the boundaries of the structure, such as soil settlements, or produced within the body of the materials, such as thermal movements, creep in timber, shrinkage in mortar, etc. These actions, which may vary continuously or cyclically, produce forces only if deformations are not free to develop. The most important and often most dangerous of all indirect actions are soil settlements (produced by change in the water table, excavations, etc.) which may create large cracks, leaning, etc.
A number of indirect actions are cyclic in nature, including temperature changes and some ground movements due to seasonal variation in ground water levels. The effects are usually cyclic too but it is possible for there to be progressive deformation or decay because each cycle produces some small but permanent change within the structure. The temperature gradient between external surfaces and the internal body may cause differential strains in the material and therefore stresses and micro-cracks, which further accelerate the decay. Indirect actions can also be produced by the progressive reduction of the stiffness of elements of an indeterminate (hyperstatic) structure (weakening, decay processes, etc.), resulting in a redistribution of stresses.
ii) Dynamic actions are produced when accelerations are transmitted to a structure, due to earthquakes, wind, hurricanes, vibrating machinery, etc.
The most significant dynamic action is usually caused by earthquakes. The intensity of the forces produced is related to both the magnitude of the acceleration and to the natural frequencies of the structure and its capacity to dissipate energy. The effect of an earthquake is also related to the history of previous earthquakes that may have progressively weakened the structure.
2) Physical, chemical, and biological actions are of completely different nature from those described above and act on the materials changing their nature often resulting in a different kind of decay and in particular affecting their strength.
Material properties may change over time due to natural processes characteristic of the material, such as slow hardening of lime mortar or slow internal decay.
These actions may be influenced and accelerated by the presence of water (rain, humidity, ground water, wetting and drying cycles, organic growth, etc.), variations in temperature (expansion and contraction, frost action, etc.) and micro-climatic conditions (pollution, surface deposition, changes in wind speeds due to adjacent structures, etc.). Fire can be considered as an extreme change of temperature.
A very common action is the oxidation of metals. This may be visible on the surface or may be occurring to metal reinforcing placed inside another material and therefore only apparent through secondary effects, such as splitting and spalling of the other material.
Chemical changes may occur spontaneously because of the inherent characteristics of the material or be produced as a result of external agents, such as the deposition of pollutants, or the migration of water or other agents through the material.
Biological agents in timber are often active in areas not easily inspected.
4 Diagnosis and safety evaluation
4.1 General aspects
Diagnosis and safety evaluation of the structure are two consecutive and related stages on the basis of which the effective need for and extent of treatment measures are determined. If these stages are performed incorrectly, the resulting decisions will be arbitrary: poor judgement may result in either conservative and therefore heavy-handed conservation measures or inadequate safety levels.
Evaluation of the safety of the building should be based on both qualitative (as documentation, observation, etc.) and quantitative (as experimental, mathematical, etc.) methods that take into account the effect of the phenomena on structural behaviour.
Any assessment of safety is seriously affected by two types of problem:
- the uncertainty attached to data (actions, resistance, deformations, etc.), laws, models, assumptions, etc. used in the research;
- the difficulty of representing real phenomena in a precise way.
- It therefore seems reasonable to try different approaches, each giving a separate contribution, but which when combined produce the best possible ‘verdict’ based on the data at our disposal.
When assessing safety, it is also necessary to include some indication, even if only qualitative, of the reliability of the assumptions made, of the results and of the degree of caution implicit in the proposed measures.
Modern legal codes and professional codes of practice adopt a conservative approach involving the application of safety factors to take into account the various uncertainties. This is appropriate for new structures where safety can be increased with modest increases in member size and cost. However, such an approach is not appropriate in historic structures where requirements to improve the strength may lead to the loss of historic fabric or to changes in the original conception of the structure. A more flexible and broader approach needs to be adopted for historic structures to relate the remedial measures more clearly to the actual structural behaviour and to retain the principle of minimum intervention.
The verdict on a structure's safety is based on an evaluation of the results obtained from the three diagnostic procedures that will be discussed below. These bear in mind that the qualitative approach plays a role as important as the quantitative approach.
It also has to be noted that the safety factors established for new buildings take into account the uncertainties of construction. In existing buildings these uncertainties may be reduced because the real behaviour of the structure can be observed and monitored. If more reliable data can be obtained, reduced theoretical factors of safety do not necessarily correspond to a real reduced safety. However there are cases where the contrary is true and data are more difficult to obtain for historic structure. (This is dealt with in more detail in paragraphs 4.3.1 & 4.3.4 below)
4.2 Identification of the causes (Diagnosis)
The diagnosis is to identify the causes of damage and decay, on the basis of the acquired data. This comes under three headings:
- Historical analysis (see 4.3.2.)
- Qualitative analysis (see 4.3.3)
- Quantitative analysis, which includes both mathematical modelling (see 4.3.4) and testing (see 4.3.5).
The diagnosis is often a difficult phase, since the data available usually refer to the effects, while it is the cause or, as it is more often the case, the several concomitant causes that have to be determined. This is why intuition and experience are essential components in the diagnostic process. A correct diagnosis is indispensable for a proper evaluation of safety and a rational decision on the treatment measures to be adopted.
4.3 Safety evaluation
4.3.1 The problem of safety evaluation
Safety evaluation is the next step towards completion of the diagnostic phase. Whilst the object of diagnosis is to identify the causes of damage and decay, safety evaluation must, determine whether or not the safety levels are acceptable, by analysing the present condition of both structure and materials. The safety evaluation is therefore an essential step in the project of restoration because this is where decisions are taken of the need for and the extent of any remedial measures.However, safety evaluation is also a difficult task because methods of structural analysis used for new construction may be neither accurate nor reliable for historic structures and may result in inappropriate decisions. This is due to such factors as the difficulty in fully understanding the complexity of an ancient building or monument, uncertainties regarding material characteristics, the unknown influence of previous phenomena (for example soil settlements), and imperfect knowledge of alterations and repairs carried out in the past. Therefore, a quantitative approach based on mathematical models cannot be the only procedure to be followed. As with the diagnosis, qualitative approaches based on historical research and on observation of the structure should also be used. A fourth approach based on specific tests may also be useful in some situations.
Each of these approaches, which are discussed below, can inform the safety evaluation, but it is the combined analysis of the information obtained from each of them, which may lead to the 'best judgement'. In forming this judgement both quantitative and qualitative aspects should be taken into account having been weighed on the basis of the reliability of the data and the assumptions made. All this needs to be set out in the explanatory report already discussed.
It must be clear, therefore, that the architect or engineer charged with the safety evaluation of an historic building should not be legally obliged to base his decisions solely on the results of calculations because, as already noted, they can be unreliable and inappropriate.
Similar procedures have to be followed to evaluate the safety levels after the design of some kinds of intervention (see paragraph 5) in order to assess their benefits and to ensure that their adoption is appropriate (neither insufficient nor excessive).
4.3.2 Historical analysis
Knowledge of what has occurred in the past can help to forecast future behaviour and can be a useful indication of the level of safety provided by the present state of the structure. History is the most complete, life-size, experimental laboratory. It shows how the type of structure, building materials, connections, joints, additions and human alterations have interacted with different actions, such as overloads, earthquakes, landslides, temperature variations, atmospheric pollution, etc., perhaps altering the structure's original behaviour by causing cracks, fissures, crushing, movement out-of-plumb, decay, collapse, etc. The structural task is to discard superfluous information and correctly interpret the data relevant to describing the static and dynamic behaviour of the structure.
Although satisfactory behaviour shown in the past is an important factor for predicting the survival of the building in the future, it is not always a reliable guide. This is particularly true where the structure is working at the limit of its bearing capacity and brittle behaviour is involved (such as high compression in columns), when there are significant changes in the structure or when repeated actions are possible (such as earthquakes) that progressively weaken the structure.
4.3.3 Qualitative analysis
This approach is based on the comparison between the present condition of structure and that of other similar structures whose behaviour is already understood. Experience gained from analysing and comparing the behaviour of different structures can enhance the possibility of extrapolations and provide a basis for assessing safety.
This approach (known in philosophical terms as inductive procedure) is not entirely reliable because it depends more upon personal judgement than on strictly scientific procedures. Nonetheless, it can be the most rational approach where there are such uncertainties inherent in the problems that other approaches only give the appearance of being more rigorous and reliable.
Having observed the behaviour of different structural types in varying stages of damage and decay caused by different actions (earthquakes, soil settlement, etc.), and having acquired experience of their soundness and durability, it is possible to extrapolate this knowledge to predict the behaviour of the structure under examination. The reliability of the evaluation will depend on the number of structures observed and, therefore, on the experience and skills of the individuals concerned. An appropriate programme of investigation and monitoring of progressive phenomena can increase its reliability.
4.3.4 The analytic approach
This approach uses the methods of modern structural analysis which, on the basis of certain hypotheses (theory of elasticity, theory of plasticity, frame models, etc.), draws conclusions based on mathematical calculations. In philosophical terms it is a deductive procedure. However, the uncertainties that can affect the representation of the material characteristics, and the imperfect representation of the structural behaviour, together with the simplifications adopted may lead to results that are not always reliable, even very different from the real situation. The essence of the problem is the identification of meaningful models that adequately depict both the structure and the associated phenomena with all their complexity making it possible to apply the theories at our disposal.Mathematical models are the common tools used in structural analysis. Models describing the original structure, if appropriately calibrated, allow comparison of the theoretical damage produced by different kinds of action with the damage actually surveyed, providing a useful tool for identifying the causes of such damage. Mathematical models of both the damaged and the reinforced structure will help to evaluate present safety levels and to assess the benefits of proposed interventions.
Structural analysis is an indispensable tool. Even when the results of calculations and analysis cannot be precise, they can indicate the flow of the stresses and possible critical areas. But mathematical models alone are usually not able to provide a reliable safety evaluation. Grasping the key issues, and correctly setting the limits for the use of mathematical techniques, depends upon the expert's use of his scientific knowledge. Any mathematical model must take into account the three aspects described in section 3: the structural scheme, the material characteristics and the actions to which the structure is subjected.
4.3.5 The experimental approach
Specific tests (such as test loading a floor, a beam, etc.) will provide a direct measure of safety margins, even if they are applicable only to single elements rather than to the building as a whole.
4.4 Decisions and explanatory report
The judgement on a structure's safety is based on the results of the three (or four) main approaches described above (the fourth approach having a limited application). When analysis shows inadequate safety levels, it should be checked to see if it has used insufficiently accurate data or excessively conservative values. This might lead to the conclusion that more investigation is necessary before a diagnosis can be made.
Because qualitative judgements may play a role as important as quantitative data, the safety assessment and the consequent decisions on intervention should be set out in the Explanatory Report (already referred to) where all the considerations which have led to the final evaluation and decisions are clearly explained. This must take into account both the degree of accuracy and of caution underlying each decision and be based on logically consistent reasoning.
Time factors must be considered in the EXPLANATORY REPORT, because a decision to undertake immediate measures, or a decision to accept the status quo, are simply the two extremes in a scale of choices. The alternatives are often to strengthen the structure on the basis of present knowledge or to extend the research to obtain more complete and reliable data in the hope of reducing any interventions. However some deadline must be set for implementing the decisions, bearing in mind that safety is of probabilistic nature with the likelihood of damage or failure increasing the longer remedial actions is delayed.
The factors underlying the setting of a deadline will depend essentially on three types of phenomena:
- continuous processes (for example decay process, slow soil settlements, etc.) which will eventually reduce safety levels to below acceptable limits, so that measures must be taken before that occurs;
- phenomena of cyclical nature (variation in temperature, moisture content, etc.) that will produce increasing deterioration;
- phenomena that can suddenly occur (such as earthquakes, hurricanes, etc.). The probability of these occurring at any defined level increases with the passage of time, so that the degree of safety to be provided can theoretically be linked to the life expectancy of the structure (for example, it is well known that to protect a building against earthquakes for five centuries it is necessary to assume highest actions than those assumed to protect the same building for one century).
5 Structural damage, materials decay and remedial measures
5.1 General aspects
This section considers decision procedures involved in both the investigation of a structure and the selection of remedial measures to be applied. In the following paragraphs some examples of the most frequent damage and repair methods for the main structural materials are outlined, without pretending to provide an exhaustive review of the many possible solutions published elsewhere.
Structural damage occurs when the stresses produced by one or more action (see 3.4) exceed the strength of the materials in significant zones, either because the actions themselves have increased or because strength has been reduced. Substantial changes in the structure, including partial demolition, may also be a source of damage.
Manifestation of damage is related to the kind of actions and construction of material. Brittle materials will fail with low deformations while ductile materials will exhibit considerable deformation before failure.
The appearance of damage, and in particular cracks, is not necessarily an indication of risk of failure in a structure because cracks may relieve stresses that are not essential for equilibrium (for example, certain kinds of cracks produced by soil settlements) and may, through changes in the structural system, allow a beneficial redistribution of the stresses.
Damage may also occur in non-structural elements, e.g. cladding or internal partitions, as a result of stresses developed within those elements due to deformations or dimensional changes within the structure.
Material decay is brought
about by chemical, physical and biological actions and may be accelerated when
these actions are modified in an unfavourable way (for example by pollution,
etc.). The main consequences are the deterioration of the surfaces, the loss of
material and, from the mechanical point of view, a reduction of strength.
Stabilisation of the material characteristics is therefore an important task
for the conservation of historic buildings; a programme of maintenance is an
essential activity because while preventing or reducing the rate of change may
be possible it is often difficult or even impossible to recover lost material
properties.
5.2 Masonry building
The term masonry here refers to stone, brick and earth based construction (i.e. adobe, pisé de terre, cobb, etc.). Masonry structures are generally made of materials that have a very low tensile strength and may easily show cracking within, or separation between elements. Nevertheless, these signs are not necessarily an indication of danger as masonry structures are intended to work mainly in compression.
The preliminary analysis of
masonry requires the identification of the characteristics of the constituents
of this composite material: the stones (limestone, sandstone, etc) or bricks,
(fired or sun dried, etc.), and the type of mortar (cement, lime, etc.). It is
also necessary to know how the elements are bonded (dry joints, mortar joints
etc) and the way in which they are geometrically related to each other. Different kinds of tests may be used to
ascertain the composition of the wall (endoscopic tests, etc.)
Particularly attention has to be paid to large walls constructed of different kinds of material. Such walls include cavity walls, rubble filled masonry walls and veneered brick walls which have a poor quality core. Not only may the core material be less capable of carrying load but it can also produce thrusts on the faces. In this type of masonry the external leaves can separate from the core so that it is necessary to determine whether the facing and the core are acting together or separately. The latter condition is usually dangerous because the faces may become unstable.
An analysis of the distribution of stresses is useful to identify the causes of the damage. To understand the cause of damage (diagnosis) it is first necessary to determine the levels and distribution of stress, even if approximately, because they are usually very low, so that some errors do not significantly affect the safety margin. A detailed visual inspection of the crack pattern may provide an indication of load paths within a structure.
When the stresses in significant areas are close to the ultimate strength it is necessary to carry out either a more accurate structural analysis or specific tests on the masonry (flat jack test, sonic test, etc.) to provide a more accurate assessment of the strength.
In-plane lateral loads can cause diagonal cracks or sliding. Out-of-plane or eccentric loads may cause separation of the leaves in a multi-leaf wall or rotation of an entire wall about its base. Where the latter occurs, horizontal cracks at the base might be seen before overturning occurs.
Various interventions to strengthen a wall include:
- repointing of the masonry, consolidation of the wall with grout,
- vertical longitudinal or transverse reinforcement,
- removal and replacement of decayed material,
- dismantling and rebuilding, either partially or completely.
The selection of appropriate fluid mortars (lime, c
As an alternative to the consolidation of the material itself, ties made of appropriate materials can be used to improve the load-bearing capacity of the masonry.
A number of products are available for consolidation of surfaces that have no plaster to protect them. However, these products are seldom completely effective and particular attention has to be paid to possible side effects.
Typical to masonry structures are arches and vaults. These rely on their curvature and the thrust at the abutments to reduce or eliminate bending moments, thus allowing the use of materials with low tensile strength. Their load bearing capacity is excellent and it is the movement of the abutments, that introduces bending moments and tensile stresses, leading to opening of the joints and possible collapse.
The formation of thin cracks is quite normal to the behaviour of some vaulted structures.
Structural distress may be associated with poor execution, (poor bonding of units, low material quality, etc.), inappropriate geometry for the load distribution, or inadequate strength and stiffness of components that must resist the thrusts (chains, shoulders).
When the construction material has very low strength (as in structures made of irregular stones with a lot of mortar) it is possible for parts of the vaults to become detached in the zones where the compression is lower or where there are tension stresses, possibly leading to progressive collapse.
The relationship between load distribution and geometry of the structure needs to be carefully considered when loads (especially heavy dead loads) are removed or added to arches or vaulted masonry structures.
The main repair measures are based on recognition of the above points, i.e. the addition of new tie rods (usually at the spring level in the vaults, or along parallel circles in the domes) construction of buttresses; correction of the load distribution (in some cases by adding loads);
High rise buildings as towers, bell towers, minarets, etc., are characterised by high compression stresses and present problems similar to those of pillars and columns. In addition, these structures are further weakened by imperfect connections between the walls, by alterations such as the making or closing of openings, etc.
Diaphragms, horizontal tie bars and chains can improve the ability to resist gravity loads.
5.3 Timber
Wood has been used in both load-bearing and framed structures, in composite structures of wood and masonry and to form major elements of load-bearing masonry structures.
Its structural performance is affected by species, growth characteristics, and by decay. Preliminary operations should be identification of the species, which are differently susceptible to biological attack, and the evaluation of the strength of individual members which is related to the size and distribution of knots and other growth characteristics. Longitudinal cracks parallel to the fibres due to drying shrinkage are not dangerous when their dimension are small.
Durability may be affected by the methods of harvesting, seasoning and conversion, which may have been different at different times.
Fungal and insect attack are the main sources of damage. These are linked to a high moisture content and temperature. The in-service moisture content should be measured as an indication of vulnerability to attack. Poor maintenance of buildings or radical changes in the internal conditions are the most common causes of timber decay.
Contact with masonry is often a source of moisture. This may occur either where the masonry supports the timber or where timber has been used to reinforce the masonry.
Because decay and insect attack may not be visible at the surface, methods, such as micro-drilling, are available for the examination of the interior of the timber.
Chemical products can protect the wood against biological attack. For example, in floors or roofs the ends of the beams inserted into masonry walls may need to be protected.
Where either reinforcing materials or consolidants are introduced, their compatibility with the timber structure must be verified. For example steel fasteners may be susceptible to corrosion in association with some species and so stainless steels should be used. Interventions should not restrict the evaporation of moisture from the timber.
To dismantle and reassemble timber structures is a delicate operation because of the risk of damage. There is also the possible loss of associated materials that are of historical significance. However, because many timber structures were originally prefabricated, there are circumstances where either partial or complete dismantling may facilitate an effective repair.
Timber is often used to form framed and trussed structures where the main problems are related to local failure at the nodes. Common remedial measures consist in reinforcing the nodes or adding supplementary diagonal elements when it is necessary to improve the stability against lateral forces.
5.4 Iron and steel
It is necessary to distinguish between cast iron, wrought iron and steel structures. The first is not only weak in tension but may have built in stresses resulting from the casting process. This is a brittle material and if subject to tensile stresses may fracture without warning. The strength of individual members can be adversely affected by poor workmanship in the foundry.
Iron and steel are alloys and their susceptibility to corrosion depends upon their composition. Corrosion is always accompanied by an increase in the volume of material that may give rise to stresses in associated materials; for example the splitting of stone or concrete as a result of the corrosion of inserted iron bars or cramps.
The most vulnerable aspects of steel structures are their connections where stresses are generally highest, especially at holes for fasteners. Bridges or other structures subjected to repeat loading might be subject to fatigue failure.
Therefore in riveted and bolted connections it is very important to check cracks starting from the holes. Fracture analysis enables the remaining life-span of the structure to be assessed.
Protection against corrosion of iron and steel requires first the elimination of rust from the surfaces (by sand blasting, etc.) and then painting the surface with an appropriate product.
Heavily damaged and deformed iron or steel structures usually can’t be repaired. Strengthening of weak structures can often be achieved adding new elements, paying particular attention when welding.
5.5 Reinforced concrete
Reinforced and prestressed concrete are the basic materials of many modern buildings that are now recognised as being of historic importance. However, at the time of their construction a full understanding of the performance of these materials was still developing, so that they may present special problems of durability (poor cement mixes, inadequate cover to the reinforcement, etc.).
The most common problems involve the carbonation of the concrete (which hardens but also becomes more brittle), reducing its capacity to protect the steel. Reinforced concrete exposed to chlorides (either in marine locations or from road salting) is particularly susceptible to corrosion of the steel.
Corrosion of the steel results in spalling of the concrete. To consolidate a reinforced concrete element thus affected usually requires the removal of the deteriorated concrete (water jet, etc.), the cleaning of the steel, the addition of new reinforcement and the rebuilding of the surface, often using special concretes.
GLOSSARY
Action n. - Any agent (forces, deformations, etc.) which directly or indirectly produces stresses and/or strains into a building structure and any phenomenon (chemical, biological, etc.) which affects the materials of which the building structure is composed. The different categories of actions and their definitions are given in the “Guidelines”.
Adobe n. - Adobe are bricks made from clay and simply dried in the sun. Some organic materials like straw or animal excrement can be used to improve durability or reduce shrinkage.
Anamnesis n. - The account of the case history of a building including past traumas, interventions, modifications, etc. The research to acquire this information prior to examination. This is the first step prior to diagnosis. See Control, Diagnosis, and Therapy.
Architectural Heritage n.- Buildings and complex of buildings (towns, etc.) of historical value. See Building .
Brick n.- A brick is a masonry unit usually made of clay which can be fired or simply dried in the sun
Brick Masonry n.- Brick masonry is a composite structure or material made of alternating brick courses set in mortar.
Building n. - Something that is built. When used in context of these “Recommendations”, the term encompasses churches, temples, bridges, dams, and all construction works. Also referred to as Architectural Heritage.
Control n. - A standard of comparison for checking the results of an experiment. To verify and regulate the efficiency of an enacted therapy through tests, monitoring and examination. See Anamnesis, Diagnosis, and Therapy.
Conservation n. – Operations which maintain the building as it is today, even if limited interventions are accepted to improve the safety levels.
Cost Benefit analysis - Costs and benefits refer to general rather than monetary terms. Costs can be measured also in the potential loss of fabric due to the invasiveness of the therapy, and benefits can be those gained by the therapy as well as knowledge that will prove useful in the future. This term should not to be interpreted as “value engineering”.
Damage n. - Change and worsening of the structural behaviour produced by mechanical actions or/and by the reduction of the strength.
Reduction of the mechanical bearing capacity related to the breakdown of a structural system. See Decay and Structure.
Decay n. – Change and worsening of the materials characteristics produced by chemical or biological actions. Chemical deterioration related to the breakdown of the materials of which a structural system is composed. Loss of quality, wasting away, decayed tissue. See Damage.
Diagnosis n. - The act or process of identifying or determining the nature and cause of damage and decay through, observation, investigation (including mathematical models) and historical analysis, and the opinion derived from such activities. See Anamnesis, Control, and Therapy.
Examination n. - The visual part of an investigation that excludes material testing, structural analysis, structural testing, and other more sophisticated investigative techniques.
See Investigation Material Testing, Structural Analysis and Structural Testing.
Explanatory Report - A report that specifically defines the subjective aspects involved in a safety assessment, such as uncertainties in the data assumed, and the difficulties in a precise evaluation of the phenomena that may lead to conclusions of uncertain reliability.
Fabric n. - The structural and material parts that make up the building (frames, walls, floors, roof, etc.)
Fired bricks - A fired brick is ceramic material obtained by preparation, moulding (or extrusion) of raw material (clay) and subsequent drying and firing at an appropriate temperature.
Geometrical Survey - Survey sheets. Measured drawings (plans, elevations, sections, etc.) where the geometry of the building is identified.
Heritage Value - Architectural, cultural, and/or historic value ascribed to a building or site. Heritage value may have varying definitions and importance from culture to culture.
Historical Approach - Evaluation based upon historical research and past experience. See Qualitative Approach and Quantitative Approach.
Holistic adj. - Emphasizing the importance of the whole and the interdependence of its parts.
Intervention n. - The physical intrusion upon a building during a diagnosis, or its therapy.
Investigation n. - A systematic and detailed evaluation of a building that can include examination, material testing, structural analysis, and structural testing. See Diagnosis, Examination, Material Testing, Structural analysis, and Structural Testing.
Maintenance - A series of activities finalised to the conservation of the asset
Material Testing - Laboratory or field testing of materials (physical, chemical, porosity, accelerated weathering, etc.).
Mortars – A mortar is a mix of one or more binders, aggregates and water. Sometimes additivies in certain proportions are included to give the mixture appropriate consistency and workability in the fresh state and adequate physical-mechanical properties when hardened.
Multi leaf masonry – Masonry made of leaves of different constitution. (The most common is the three leaves masonry made of two external faces and an inner rubble core.)
Natural stones - Natural stones have been formed by geological processes. They consist of mixtures of minerals.
Natural stones can be grouped according to their origin into magmatic, metamorphous and sedimentary stones (sandstone, limestone, etc.). Natural stones differ by origin, if their composition has not been altered by man.
Observational Method - An increment approach to intervene or to strengthen, starting from a minimum level of intervention, with possible subsequent adoption of a series of corrective measures.
Quantitative Approach - Evaluation based on analytic or scientific methods such as testing, calculations, and mathematical modeling. See Historical Approach and Qualitative Approach.
Rehabilitation – Process to bring a building to a new use or function, without alterating the portions of the building that are significant to its historical value.
Repointing - Result of repair or restoration on a deteriorated joint. It can be homogeneous to the existing joint or made of different material (e.g. cement of polymer).
Restoration – Process of recovering the form of a building as it appeared at a particular period of time by means of removal of additional work or by replacement of missing later work.
Safety Evaluation (assessment) - Evaluation of the safety margins of a structure with regard to heavy damage, partial or total collapse. See Historical approach, Qualitative Approach, Quantitative Approach.. the opposite of safety is risk.
Strengthening – Interventions to increase the bearing capacity of a structure.
Structural Analysis - Calculations, computations, computer analysis using mathematical models.
Structural scheme – An approximate representation (or model) of the structure, different, but close to the reality.
Structural Testing - Laboratory or field testing of structures (assembly and component testing, floor loading, shaking-tables, etc.).
Structural Typology - The types of structures interpreted as regards their structural behaviour and their capacity to bear loads.
Structure n. – The part of a building which provides the bearing capacity, sometimes coincident with the building itself.
Tell-tale - A device fixed across a crack in a masonry structure to indicate movement.
Therapy - The choice of remedial measures (reinforcement, strengthening, replacement, etc.) in response to diagnosis. See Anamnesis, Control, an d Diagnosis.
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The first treatment, Preservation, places a high premium on the retention of all historic fabric through conservation, maintenance and repair. It reflects a building's continuum over time, through successive occupancies, and the respectful changes and alterations that are made.
Rehabilitation, the second treatment, emphasizes the retention and repair of historic materials, but more latitude is provided for replacement because it is assumed the property is more deteriorated prior to work. (Both Preservation and Rehabilitation standards focus attention on the preservation of those materials, features, finishes, spaces, and spatial relationships that, together, give a property its historic character.)
Restoration, the third treatment, focuses on the retention of materials from the most significant time in a property's history, while permitting the removal of materials from other periods.
Reconstruction, the fourth treatment, establishes limited opportunities to re-create a non-surviving site, landscape, building, structure, or object in all new materials.
Choosing the most appropriate treatment for a building requires careful decision-making about a building's historical significance, as well taking into account a number of other considerations:
Relative importance in history. Is the building a nationally significant resource--a rare survivor or the work of a master architect or craftsman? Did an important event take place in it? National Historic Landmarks, designated for their "exceptional significance in American history," or many buildings individually listed in the National Register often warrant Preservation or Restoration. Buildings that contribute to the significance of a historic district but are not individually listed in the National Register more frequently undergo Rehabilitation for a compatible new use.
Physical condition. What is the existing condition--or degree of material integrity--of the building prior to work? Has the original form survived largely intact or has it been altered over time? Are the alterations an important part of the building's history? Preservation may be appropriate if distinctive materials, features, and spaces are essentially intact and convey the building's historical significance. If the building requires more extensive repair and replacement, or if alterations or additions are necessary for a new use, then Rehabilitation is probably the most appropriate treatment. These key questions play major roles in determining what treatment is selected.
Proposed use. An essential, practical question to ask is: Will the building be used as it was historically or will it be given a new use? Many historic buildings can be adapted for new uses without seriously damaging their historic character; special-use properties such as grain silos, forts, ice houses, or windmills may be extremely difficult to adapt to new uses without major intervention and a resulting loss of historic character and even integrity.
Mandated code requirements. Regardless of the treatment, code requirements will need to be taken into consideration. But if hastily or poorly designed, a series of code-required actions may jeopardize a building's materials as well as its historic character. Thus, if a building needs to be seismically upgraded, modifications to the historic appearance should be minimal. Abatement of lead paint and asbestos within historic buildings requires particular care if important historic finishes are not to be adversely affected. Finally, alterations and new construction needed to meet accessibility requirements under the Americans with Disabilities Act of 1990 should be designed to minimize material loss and visual change to a historic building.
Background. In the Code of Federal Regulations, Title 36, Parks, Forests, and Public Property, Chapter I, ("National Park Service, Department of the Interior"), Parts 1 to 99, Revised as of July 1, 1998, p. 329, it states: PART 68--The Secretary of the Interior's Standards for the Treatment of Historic Properties. AUTHORITY: National Historic Preservation Act of 1966, as amended, (16 U.S.C. 470 et seq.); Section 2124 of the Tax Reform Act of 1976, 90 Stat. 1918; EO 11593, 3 CFR Part 75 (1971); sec. 2 of Reorganization Plan No. 3 of 1950 (64 Stat. 1262). Federal Register Source: Volume 60, page 35843, July 12, 1995.
History. The Secretary of the Interior's Standards for the Treatment of Historic Properties are the Secretary's best advice to everyone on how to protect a wide range of historic properties. By separate regulation, the Secretary has required the application of the Standards in certain programs that the Secretary administers through the National Park Service.They apply to all proposed development grant-in-aid projects assisted through the national Historic Preservation Fund, and are intended to be applied to a wide variety of resource types, including buildings, sites, structures, objects, and districts.
The Standards, revised in 1992, were codified as 36 CFR Part 68 in the July 12, 1995 Federal Register (Vol. 60, No. 133). The revision replaces the 1978 and 1983 versions of 36 CFR 68 entitled The Secretary of the Interior's Standards for Historic Preservation Projects. It is noted that another regulation, 36 CFR 67, focuses on "certified historic structures" as defined by the IRS Code of 1986. The Standards for Rehabilitation in 36 CFR 67 should always be used when property owners are seeking certification for Federal tax benefits.
How to Use the Standards and Guidelines. The Secretary of the Interior's Standards for the Treatment of Historic Properties with Guidelines for Preserving, Rehabilitating, Restoring and Reconstructing Historic Buildings are intended to provide guidance to historic building owners and building managers, preservation consultants, architects, contractors, and project reviewers prior to treatment. As noted, while the treatment Standards are designed to be applied to all historic resource types included in the National Register of Historic Places--buildings, sites, structures, districts, and objects--the Guidelines apply to specific resource types; in this case, buildings.
The Guidelines have been prepared to assist in applying the Standards to all project work; consequently, they are not meant to give case-specific advice or address exceptions or rare instances. Therefore, it is recommended that the advice of qualified historic preservation professionals be obtained early in the planning stage of the project. Such professionals may include architects, architectural historians, historians, historical engineers, archeologists, and others who have experience in working with historic buildings.
The Guidelines pertain to both exterior and interior work on historic buildings of all sizes, materials, and types. Those approaches to work treatments and techniques that are consistent with The Secretary of the Interior's Standards for the Treatment of Historic Properties are listed in the "Recommended" column on the left; those which are inconsistent with the Standards are listed in the "Not Recommended" column on the right.
One section of this web site is devoted to each of the four treatments: Preservation, Rehabilitation, Restoration, and Reconstruction.
Each section contains one set of Standards and accompanying Guidelines that are to be used throughout the course of a project. The Standards for the first treatment, Preservation, require retention of the greatest amount of historic fabric, along with the building's historic form, features, and detailing as they have evolved over time. The Rehabilitation Standards acknowledge the need to alter or add to a historic building to meet continuing or new uses while retaining the building's historic character. The Restoration Standards allow for the depiction of a building at a particular time in its history by preserving materials from the period of significance and removing materials from other periods. The Reconstruction Standards establish a limited framework for re-creating a vanished or non-surviving building with new materials, primarily for interpretive purposes.
The Guidelines are preceded by a brief historical overview of the primary historic building materials (masonry, wood, and architectural metals) and their diverse uses over time. Next, building features comprised of these materials are discussed, beginning with the exterior, then moving to the interior. Special requirements or work that must be done to meet accessibility requirements, health and safety code requirements, or retrofitting to improve energy efficiency are also addressed here. Although usually not part of the overall process of protecting historic buildings, this work must also be assessed for its potential impact on a historic building.
Mortar is used to bond together masonry units. Historic mortar was generally quite soft, consisting primarily of lime and sand with other additives. By the latter part of the 19th century, portland cement was usually added resulting in a more rigid and non-absorbing mortar. Like historic mortar, early stucco coatings were also heavily lime-based, increasing in hardness with the addition of portland cement in the late 19th century. Concrete has a long history, being variously made of tabby, volcanic ash and, later, of natural hydraulic cements, before the introduction of portland cement in the 1870s. Since then, concrete has also been used in its precast form. While masonry is among the most durable of historic building materials, it is also very susceptible to damage by improper maintenance or repair techniques and harsh or abrasive cleaning methods.
Because it can be easily shaped by sawing, sanding, planing, carving, and gouging, wood is used for architectural features such as clapboard, cornices, brackets, entablatures, shutters, columns and balustrades. These wooden features, both functional and decorative, are often important in defining the historic character of the building.
Metals commonly used in historic buildings include lead, tin, zinc, copper, bronze, brass, iron, steel, and to a lesser extent, nickel alloys, stainless steel and aluminum. Historic metal building components were often created by highly skilled, local artisans, and by the late 19th century, many of these components were prefabricated and readily available from catalogs in standardized sizes and designs.
European settlers used clay tile for roofing at least as early as the mid-17th century. In some cities, such as New York and Boston , clay tiles were popularly used as a precaution against fire. The Spanish influence in the use of clay tiles is found in the southern, southwestern and western states. In the mid-19th century, tile roofs were often replaced by sheet-metal, which is lighter and easier to maintain.
Evidence of the use of slate for roofing dates from the mid-17th century. Slate has remained popular for its durability, fireproof qualities, and its decorative applications. The use of metals for roofing and roof features dates from the 18th century, and includes the use of sheet metal, corrugated metal, galvanized metal, tin-plate, copper, lead and zinc.
New roofing materials developed in the early 20th century include built-up roll roofing, and concrete, asbestos, and asphalt shingles.
Changes in technology led to larger panes of glass so that by the mid-19th century, two-over-two lights were common; the manufacture of plate glass in the United States allowed for use of large sheets of glass in commercial and office buildings by the late 19th century. With mass-produced windows, mail order distribution, and changing architectural styles, it was possible to obtain a wide range of window designs and light patterns in sash. Early 20th century designs frequently utilized smaller lights in the upper sash and also casement windows. The desire for fireproof building construction in dense urban areas contributed to the growth of a thriving steel window industry along with a market for hollow metal and metal clad wooden windows.
As one of the few parts of a building serving as both an interior and exterior feature, windows are nearly always an important part of a historic building.
If features of the structural system are exposed such as loadbearing brick walls, cast iron columns, roof trusses, posts and beams, vigas, or stone foundation walls, they may be important in defining the building's overall historic character. Unexposed structural features that are not character-defining or an entire structural system may nonetheless be significant in the history of building technology. The structural system should always be examined and evaluated early in the project planning stage to determine its physical condition, its ability to support any proposed changes in use, and its importance to the building's historic character or historical significance.
Secondary spaces are often more functional than decorative, and may include kitchens, bathrooms, mail rooms, utility spaces, secondary hallways, firestairs and office cubicles in a commercial or office space. Extensive changes can often be made in these less important areas without having a detrimental effect on the overall historic character.
The greatest impacts of the 20th century on mechanical systems were the use of electricity for interior lighting, forced air ventilation, elevators for tall buildings, exterior lighting and electric heat. The new age of technology brought an increasingly high level of design and decorative art to many of the functional elements of mechanical, electrical and plumbing systems.
The visible decorative features of historic mechanical systems such as grilles, lighting fixtures, and ornamental switchplates may contribute to the overall historic character of the building. Their identification needs to take place, together with an evaluation of their physical condition, early in project planning. On the other hand, mechanical systems need to work efficiently so many older systems, such as compressors and their ductwork, and wiring and pipes often need to be upgraded or entirely replaced in order to meet modern requirements.
Some features of a historic building or site such as cupolas, shutters, transoms, skylights, sun rooms, porches, and plantings can play an energy-conserving role. Therefore, prior to retrofitting historic buildings to make them more energy efficient, the first step should always be to identify and evaluate existing historic features to assess their inherent energy-conserving potential. If it is determined that retrofitting measures are appropriate, then such work needs to be carried out with particular care to ensure that the building's historic character is retained.
It is often necessary to make modifications to a historic building so that it will be in compliance with current accessibility code requirements. Accessibility to certain historic structures is required by three specific federal laws: the Architectural Barriers Act of 1968, Section 504 of the Rehabilitation Act of 1973, and the Americans with Disabilities Act of 1990. Federal rules, regulations, and standards have been developed which provide guidance on how to accomplish access to historic areas for people with disabilities. Work must be carefully planned and undertaken so that it does not result in the loss of character-defining spaces, features, and finishes. The goal is to provide the highest level of access with the lowest level of impact.
In undertaking work on historic buildings, it is necessary to consider the impact that meeting current health and safety codes (public health, occupational health, life safety, fire safety, electrical, seismic, structural, and building codes) will have on character-defining spaces, features, and finishes. Special coordination with the responsible code officials at the state, county, or municipal level may be required. Securing required building permits and occupancy licenses is best accomplished early in work project planning. It is often necessary to look beyond the "letter" of code requirements to their underlying purpose; most modern codes allow for alterative approaches and reasonable variance to achieve compliance.
Some historic building materials (insulation, lead paint, etc.) contain toxic substances that are potentially hazardous to building occupants. Following careful investigation and analysis, some form of abatement may be required. All workers involved in the encapsulation, repair, or removal of known toxic materials should be adequately trained and should wear proper personal protective gear. Finally, preventive and routine maintenance for historic structures known to contain such materials should also be developed to include proper warnings and precautions.
preserving
Preservation is defined as the act or process of applying measures necessary to sustain the existing form, integrity, and materials of an historic property. Work, including preliminary measures to protect and stabilize the property, generally focuses upon the ongoing maintenance and repair of historic materials and features rather than extensive replacement and new construction. New exterior additions are not within the scope of this treatment; however, the limited and sensitive upgrading of mechanical, electrical, and plumbing systems and other code-required work to make properties functional is appropriate within a preservation project.
3. Each property will be recognized as a physical record of its time, place, and use. Work needed to stabilize, consolidate, and conserve existing historic materials and features will be physically and visually compatible, identifiable upon close inspection, and properly documented for future research.
4. Changes to a property that have acquired historic significance in their own right will be retained and preserved.
5. Distinctive materials, features, finishes, and construction techniques or examples of craftsmanship that characterize a property will be preserved.
6. The existing condition of historic features will be evaluated to determine the appropriate level of intervention needed. Where the severity of deterioration requires repair or limited replacement of a distinctive feature, the new material will match the old in composition, design, color, and texture.
7. Chemical or physical treatments, if
appropriate, will be undertaken using the gentlest means possible. Treatments
that cause damage to historic materials will not be used.
8. Archeological resources will be
protected and preserved in place. If such resources must be disturbed,
mitigation measures will be undertaken.
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Choosing
Preservation as a Treatment
In Preservation, the options for
replacement are less extensive than in the treatment, Rehabilitation. This is
because it is assumed at the outset that building materials and
character-defining features are essentially intact, i.e, that more historic
fabric has survived, unchanged over time. The expressed goal of the Standards
for Preservation and Guidelines for Preserving Historic Buildings is
retention of the building's existing form, features and detailing. This may be
as simple as basic maintenance of existing materials and features or may
involve preparing a historic structure report, undertaking laboratory testing
such as paint and mortar analysis, and hiring conservators to perform sensitive
work such as reconstituting interior finishes. Protection, maintenance, and
repair are emphasized while replacement is minimized.
Identify,
Retain, and Preserve Historic Materials and Features
The guidance for the treatment Preservation
begins with recommendations to identify the form and detailing of those
architectural materials and features that are important in defining the
building's historic character and which must be retained in order to preserve
that character. Therefore, guidance on identifying, retaining, and
preserving character-defining features is always given first. The
character of a historic building may be defined by the form and detailing of
exterior materials, such as masonry, wood, and metal; exterior features, such
as roofs, porches, and windows; interior materials, such as plaster and paint;
and interior features, such as moldings and stairways, room configuration and
spatial relationships, as well as structural and mechanical systems; and the
building's site and setting.
Deteriorated portions of a historic
building may need to be protected thorough preliminary stabilization measures
until additional work can be undertaken. Stabilizing may include
structural reinforcement, weatherization, or correcting unsafe conditions.
Temporary stabilization should always be carried out in such a manner that it
detracts as little as possible from the historic building's appearance. Although
it may not be necessary in every preservation project, stabilization is
nonetheless an integral part of the treatment Preservation; it is equally
applicable, if circumstances warrant, for the other treatments.
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Preservation
of the exterior of the Hale House, Los
Angeles , California ,
involved repainting the exterior walls and decorative features in
historically appropriate colors. In excellent example of the Preservation
treatment focused upon the ongoing maintenance of historic materials and
features. Photo: Before, NPS files; After: Bruce Boehner. |
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Next, when the physical condition of character-defining materials and features requires additional work, repairing by stabilizing, consolidating, and conserving is recommended. Preservation strives to retain existing materials and features while employing as little new material as possible. Consequently, guidance for repairing a historic material, such as masonry, again begins with the least degree of intervention possible such as strengthening fragile materials through consolidation, when appropriate, and repointing with mortar of an appropriate strength. Repairing masonry as well as wood and architectural metal features may also include patching, splicing, or otherwise reinforcing them using recognized preservation methods. Similarly, within the treatment Preservation, portions of a historic structural system could be reinforced using contemporary materials such as steel rods. All work should be physically and visually compatible, identifiable upon close inspection and documented for future research.
If repair by stabilization, consolidation, and conservation proves inadequate, the next level of intervention involves the limited replacement in kind of extensively deteriorated or missing parts of features when there are surviving prototypes (for example, brackets, dentils, steps, plaster, or portions of slate or tile roofing). The replacement material needs to match the old both physically and visually, i.e., wood with wood, etc. Thus, with the exception of hidden structural reinforcement and new mechanical system components, substitute materials are not appropriate in the treatment Preservation. Again, it is important that all new material be identified and properly documented for future research. If prominent features are missing, such as an interior staircase, exterior cornice, or a roof dormer, then a Rehabilitation or Restoration treatment may be more appropriate.
These sections of the Preservation guidance address work done to meet accessibility requirements and health and safety code requirements; or limited retrofitting measures to improve energy efficiency. Although this work is quite often an important aspect of preservation projects, it is usually not part of the overall process of protecting, stabilizing, conserving, or repairing character-defining features; rather, such work is assessed for its potential negative impact on the building's historic character. For this reason, particular care must be taken not to obscure, damage, or destroy character-defining materials or features in the process of undertaking work to meet code and energy requirements.
Rehabilitation is defined as the act or process of making possible a compatible use for a property through repair, alterations, and additions while preserving those portions or features which convey its historical, cultural, or architectural values.
3. Each property will be recognized as a physical record of its time, place, and use. Changes that create a false sense of historical development, such as adding conjectural features or elements from other historic properties, will not be undertaken.
4. Changes to a property that have acquired historic significance in their own right will be retained and preserved.
5. Distinctive materials, features, finishes, and construction techniques or examples of craftsmanship that characterize a property will be preserved.
6. Deteriorated historic features will be repaired rather than replaced. Where the severity of deterioration requires replacement of a distinctive feature, the new feature will match the old in design, color, texture, and, where possible, materials. Replacement of missing features will be substantiated by documentary and physical evidence.
7. Chemical or physical treatments, if appropriate, will be undertaken using the gentlest means possible. Treatments that cause damage to historic materials will not be used.
8. Archeological resources will be protected and preserved in place. If such resources must be disturbed, mitigation measures will be undertaken.
9. New additions, exterior alterations, or related new construction will not destroy historic materials, features, and spatial relationships that characterize the property. The new work shall be differentiated from the old and will be compatible with the historic materials, features, size, scale and proportion, and massing to protect the integrity of the property and its environment.
10. New additions and adjacent or related new construction will be undertaken in a such a manner that, if removed in the future, the essential form and integrity of the historic property and its environment would be unimpaired.
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Choosing Rehabilitation as a Treatment
In Rehabilitation, historic building materials and character-defining features are protected and maintained as they are in the treatment Preservation; however, an assumption is made prior to work that existing historic fabric has become damaged or deteriorated over time and, as a result, more repair and replacement will be required. Thus, latitude is given in the Standards for Rehabilitation and Guidelines for Rehabilitation to replace extensively deteriorated, damaged, or missing features using either traditional or substitute materials. Of the four treatments, only Rehabilitation includes an opportunity to make possible an efficient contemporary use through alterations and additions.
Like Preservation, guidance for the treatment Rehabilitation begins with recommendations to identify the form and detailing of those architectural materials and features that are important in defining the building's historic character and which must be retained in order to preserve that character. Therefore, guidance on identifying, retaining, and preserving character-defining features is always given first. The character of a historic building may be defined by the form and detailing of exterior materials, such as masonry, wood, and metal; exterior features, such as roofs, porches, and windows; interior materials, such as plaster and paint; and interior features, such as moldings and stairways, room configuration and spatial relationships, as well as structural and mechanical systems.
After identifying those materials and features that are important and must be retained in the process of Rehabilitation work, then protecting and maintaining them are addressed. Protection generally involves the least degree of intervention and is preparatory to other work. For example, protection includes the maintenance of historic material through treatments such as rust removal, caulking, limited paint removal, and re-application of protective coatings; the cyclical cleaning of roof gutter systems; or installation of fencing, alarm systems and other temporary protective measures. Although a historic building will usually require more extensive work, an overall evaluation of its physical condition should always begin at this level.
Next, when the physical condition of character-defining materials and features warrants additional work repairing is recommended. Rehabilitation guidance for the repair of historic materials such as masonry, wood, and architectural metals again begins with the least degree of intervention possible such as patching, piecing-in, splicing, consolidating, or otherwise reinforcing or upgrading them according to recognized preservation methods. Repairing also includes the limited replacement in kind--or with compatible substitute material--of extensively deteriorated or missing parts of features when there are surviving prototypes (for example, brackets, dentils, steps, plaster, or portions of slate or tile roofing). Although using the same kind of material is always the preferred option, substitute material is acceptable if the form and design as well as the substitute material itself convey the visual appearance of the remaining parts of the feature and finish.
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This
two-story brick commercial building--with its corner storefront--was originally
constructed ca. 1876, then remodeled in 1916 in the Craftsman style and given
a new, distinctive roofline. It served a number of uses, including a hotel,
boarding house, saloon, restaurant, liquor store, warehouse, and office
furniture showroom. The red brick walls had been painted several times over
the years. Rehabilitation work included removal of multiple paint layers
using a chemical stripper and thorough water rinse; spot repointing with
matching mortar; and appropriate interior alterations. The building is now
being used as a retail shop. Photos: NPS files. |
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Following repair in the hierarchy, Rehabilitation guidance is provided for replacing an entire character-defining feature with new material because the level of deterioration or damage of materials precludes repair (for example, an exterior cornice; an interior staircase; or a complete porch or storefront). If the essential form and detailing are still evident so that the physical evidence can be used to re-establish the feature as an integral part of the rehabilitation, then its replacement is appropriate. Like the guidance for repair, the preferred option is always replacement of the entire feature in kind, that is, with the same material. Because this approach may not always be technically or economically feasible, provisions are made to consider the use of a compatible substitute material. It should be noted that, while the National Park Service guidelines recommend the replacement of an entire character-defining feature that is extensively deteriorated, they never recommend removal and replacement with new material of a feature that--although damaged or deteriorated--could reasonably be repaired and thus preserved.
When an entire interior or exterior feature is missing (for example, an entrance, or cast iron facade; or a principal staircase), it no longer plays a role in physically defining the historic character of the building unless it can be accurately recovered in form and detailing through the process of carefully documenting the historical appearance. Although accepting the loss is one possibility, where an important architectural feature is missing, its replacement is always recommended in the Rehabilitation guidelines as the first or preferred, course of action. Thus, if adequate historical, pictorial, and physical documentation exists so that the feature may be accurately reproduced, and if it is desirable to re-establish the feature as part of the building's historical appearance, then designing and constructing a new feature based on such information is appropriate. However, a second acceptable option for the replacement feature is a new design that is compatible with the remaining character-defining features of the historic building. The new design should always take into account the size, scale, and material of the historic building itself and, most importantly, should be clearly differentiated so that a false historical appearance is not created.
Some exterior and interior alterations to a historic building are generally needed to assure its continued use, but it is most important that such alterations do not radically change, obscure, or destroy character-defining spaces, materials, features, or finishes. Alterations may include providing additional parking space on an existing historic building site; cutting new entrances or windows on secondary elevations; inserting an additional floor; installing an entirely new mechanical system; or creating an atrium or light well. Alteration may also include the selective removal of buildings or other features of the environment or building site that are intrusive and therefore detract from the overall historic character. The construction of an exterior addition to a historic building may seem to be essential for the new use, but it is emphasized in the Rehabilitation guidelines that such new additions should be avoided, if possible, and considered only after it is determined that those needs cannot be met by altering secondary, i.e., non character-defining interior spaces. If, after a thorough evaluation of interior solutions, an exterior addition is still judged to be the only viable alterative, it should be designed and constructed to be clearly differentiated from the historic building and so that the character-defining features are not radically changed, obscured, damaged, or destroyed. Additions and alterations to historic buildings are referenced within specific sections of the Rehabilitation guidelines such as Site, Roofs, Structural Systems, etc., but are addressed in detail in New Additions to Historic Buildings (see nav bar, right).
These sections of the guidance address work done to meet accessibility requirements and health and safety code requirements; or retrofitting measures to improve energy efficiency. Although this work is quite often an important aspect of Rehabilitation projects, it is usually not a part of the overall process of protecting or repairing character-defining features; rather, such work is assessed for its potential negative impact on the building's historic character. For this reason, particular care must be taken not to radically change, obscure, damage, or destroy character-defining materials or features in the process of meeting code and energy requirements.
3. Each property will be recognized as a physical record of its time, place, and use. Work needed to stabilize, consolidate and conserve materials and features from the restoration period will be physically and visually compatible, identifiable upon close inspection, and properly documented for future research.
4. Materials, features, spaces, and finishes that characterize other historical periods will be documented prior to their alteration or removal.
5. Distinctive materials, features, finishes, and construction techniques or examples of craftsmanship that characterize the restoration period will be preserved.
6. Deteriorated features from the restoration period will be repaired rather than replaced. Where the severity of deterioration requires replacement of a distinctive feature, the new feature will match the old in design, color, texture, and, where possible, materials.
7. Replacement of missing features from the restoration period will be substantiated by documentary and physical evidence. A false sense of history will not be created by adding conjectural features, features from other properties, or by combining features that never existed together historically.
8. Chemical or physical treatments, if appropriate, will be undertaken using the gentlest means possible. Treatments that cause damage to historic materials will not be used.
9. Archeological resources affected by a project will be protected and preserved in place. If such resources must be disturbed, mitigation measures will be undertaken.
10. Designs that were never executed historically will not be constructed.
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Rather than maintaining and preserving a building as it has evolved over time, the expressed goal of the Standards for Restoration and Guidelines for Restoring Historic Buildings is to make the building appear as it did at a particular--and most significant--time in its history. First, those materials and features from the "restoration period" are identified, based on thorough historical research. Next, features from the restoration period are maintained, protected, repaired (i.e., stabilized, consolidated, and conserved), and replaced, if necessary. As opposed to other treatments, the scope of work in Restoration can include removal of features from other periods; missing features from the restoration period may be replaced, based on documentary and physical evidence, using traditional materials or compatible substitute materials. The final guidance emphasizes that only those designs that can be documented as having been built should be re-created in a restoration project.
The guidance for the treatment Restoration begins with recommendations to identify the form and detailing of those existing architectural materials and features that are significant to the restoration period as established by historical research and documentation. Thus, guidance on identifying, retaining, and preserving features from the restoration period is always given first. The historic building's appearance may be defined by the form and detailing of its exterior materials, such as masonry, wood, and metal; exterior features, such as roofs, porches, and windows; interior materials, such as plaster and paint; and interior features, such as moldings and stairways, room configuration and spatial relationships, as well as structural and mechanical systems; and the building's site and setting.
After identifying those existing materials and features from the restoration period that must be retained in the process of Restoration work, then protecting and maintaining them is addressed. Protection generally involves the least degree of intervention and is preparatory to other work. For example, protection includes the maintenance of historic material through treatments such as rust removal, caulking, limited paint removal, and re-application of protective coatings; the cyclical cleaning of roof gutter systems; or installation of fencing, alarm systems and other temporary protective measures. Although a historic building will usually require more extensive work, an overall evaluation of its physical condition should always begin at this level.
Next, when the physical condition of restoration period features requires additional work, repairing by stabilizing, consolidating, and conserving is recommended. Restoration guidance focuses upon the preservation of those materials and features that are significant to the period. Consequently, guidance for repairing a historic material, such as masonry, again begins with the least degree of intervention possible, such as strengthening fragile materials through consolidation, when appropriate, and repointing with mortar of an appropriate strength. Repairing masonry as well as wood and architectural metals includes patching, splicing, or otherwise reinforcing them using recognized preservation methods. Similarly, portions of a historic structural system could be reinforced using contemporary material such as steel rods. In Restoration, repair may also include the limited replacement in kind--or with compatible substitute material--of extensively deteriorated or missing parts of existing features when there are surviving prototypes to use as a model. Examples could include terra-cotta brackets, wood balusters, or cast iron fencing.
In Restoration, replacing an entire feature from the restoration period (i.e., a cornice, balustrade, column, or stairway) that is too deteriorated to repair may be appropriate. Together with documentary evidence, the form and detailing of the historic feature should be used as a model for the replacement. Using the same kind of material is preferred; however, compatible substitute material may be considered. All new work should be unobtrusively dated to guide future research and treatment. If documentary and physical evidence are not available to provide an accurate re-creation of missing features, the treatment Rehabilitation might be a better overall approach to project work.
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In
a project at Fort Hays , Kansas , the wood frame officers' quarters
were restored to the late 1860s--their period of significance. This included
replacing a missing kitchen ell, chimneys, porch columns, and cornice, as
well as closing a later window opening in the main block. The building and
others in the museum complex is used to interpret frontier history. Photo:
NPS files. |
Most buildings represent continuing occupancies and change over time, but in Restoration, the goal is to depict the building as it appeared at the most significant time in its history. Thus, work is included to remove or alter existing historic features that do not represent the restoration period. This could include features such as windows, entrances and doors, roof dormers, or landscape features. Prior to altering or removing materials, features, spaces, and finishes that characterize other historical periods, they should be documented to guide future research and treatment.
Most Restoration projects involve re-creating features that were significant to the building at a particular time, but are now missing. Examples could include a stone balustrade, a porch, or cast iron storefront. Each missing feature should be substantiated by documentary and physical evidence. Without sufficient documentation for these "re-creations," an accurate depiction cannot be achieved. Combining features that never existed together historically can also create a false sense of history. Using traditional materials to depict lost features is always the preferred approach; however, using compatible substitute material is an acceptable alternative in Restoration because, as emphasized, the goal of this treatment is to replicate the "appearance" of the historic building at a particular time, not to retain and preserve all historic materials as they have evolved over time. If documentary and physical evidence are not available to provide an accurate re-creation of missing features, the treatment Rehabilitation might be a better overall approach to project work.
These sections of the Restoration guidance address work done to meet accessibility requirements and health and safety code requirements; or limited retrofitting measures to improve energy efficiency. Although this work is quite often an important aspect of restoration projects, it is usually not part of the overall process of protecting, stabilizing, conserving, or repairing features from the restoration period; rather, such work is assessed for its potential negative impact on the building's historic appearance. For this reason, particular care must be taken not to obscure, damage, or destroy historic materials or features from the restoration period in the process of undertaking work to meet code and energy requirements.
reconstructing
Reconstruction is defined as the act or process of depicting, by means of new construction, the form, features, and detailing of a non-surviving site, landscape, building, structure, or object for the purpose of replicating its appearance at a specific period of time and in its historic location.
3. Reconstruction will include measures to preserve any remaining historic materials, features, and spatial relationships.
4. Reconstruction will be based on the accurate duplication of historic features and elements substantiated by documentary or physical evidence rather than on conjectural designs or the availability of different features from other historic properties. A reconstructed property will re-create the appearance of the non-surviving historic property in materials, design, color, and texture.
5. A reconstruction will be clearly identified as a contemporary re-creation.
6. Designs that were never executed historically will not be constructed.
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Choosing Reconstruction as a Treatment
Whereas the treatment Restoration provides guidance on restoring--or re-creating--building features, the Standards for Reconstruction and Guidelines for Reconstructing Historic Buildings address those aspects of treatment necessary to re-create an entire non-surviving building with new material. Much like restoration, the goal is to make the building appear as it did at a particular--and most significant--time in its history. The difference is, in Reconstruction, there is far less extant historic material prior to treatment and, in some cases, nothing visible. Because of the potential for historical error in the absence of sound physical evidence, this treatment can be justified only rarely and, thus, is the least frequently undertaken. Documentation requirements prior to and following work are very stringent. Measures should be taken to preserve extant historic surface and subsurface material. Finally, the reconstructed building must be clearly identified as a contemporary re-creation.
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In
the 1930s reconstruction of the 18th century Governor's Palace at Colonial Williamsburg , Virginia ,
the earliest archeological remains of the brick foundation were carefully
preserved in situ, and serve as a base for the reconstructed walls. Photo:
The Colonial Williamsburg
Foundation. |
Guidance for the treatment Reconstruction begins with researching and documenting the building's historical significance to ascertain that its re-creation is essential to the public understanding of the property. Often, another extant historic building on the site or in a setting can adequately explain the property, together with other interpretive aids. Justifying a reconstruction requires detailed physical and documentary evidence to minimize or eliminate conjecture and ensure that the reconstruction is as accurate as possible. Only one period of significance is generally identified; a building, as it evolved, is rarely re-created. During this important fact-finding stage, if research does not provide adequate documentation for an accurate reconstruction, other interpretive methods should be considered, such as an explanatory marker.
Investigating archeological resources is the next area of guidance in the treatment Reconstruction. The goal of physical research is to identify features of the building and site which are essential to an accurate re-creation and must be reconstructed, while leaving those archeological resources that are not essential, undisturbed. Information that is not relevant to the project should be preserved in place for future research. The archeological findings, together with archival documentation, are then used to replicate the plan of the building, together with the relationship and size of rooms, corridors, and other spaces, and spatial relationships.
Closely aligned with archeological research, recommendations are given for identifying, protecting, and preserving extant features of the historic building. It is never appropriate to base a Reconstruction upon conjectural designs or the availability of different features from other buildings. Thus, any remaining historic materials and features, such as remnants of a foundation or chimney and site features such as a walkway or path, should be retained, when practicable, and incorporated into the reconstruction. The historic as well as new material should be carefully documented to guide future research and treatment.
After the research and documentation
phases, guidance is given for Reconstruction work itself. Exterior and interior
features are addressed in general, always emphasizing the need for an accurate
depiction, i.e., careful duplication of the appearance of historic interior
paints, and finishes such as stencilling, marbling, and graining. In the
absence of extant historic materials, the objective in reconstruction is to
re-create the appearance of the historic building for interpretive purposes.
Thus, while the use of traditional materials and finishes is always preferred,
in some instances, substitute materials may be used if they are able to convey
the same visual appearance. Where non-visible features of the building are
concerned--such as interior structural systems or mechanical systems--it is
expected that contemporary materials and technology will be employed.
Re-creating the building site should be an integral aspect of project work. The
initial archeological inventory of subsurface and aboveground remains is used
as documentation to reconstruct landscape features such as walks and roads,
fences, benches, and fountains.
Code requirements must also be met in Reconstruction projects. For code purposes, a reconstructed building may be considered as essentially new construction. Guidance for these sections is thus abbreviated, and focuses on achieving design solutions that do not destroy extant historic features and materials or obscure reconstructed features.
Research and Document
Researching and documenting the property's historical significance, focusing on the availability of documentary and physical evidence needed to justify reconstruction of the non-surviving building.
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Jean
Baptiste Wengler's watercolor rendering of Fort Snelling, Minnesota, in 1857
is aesthetically pleasing, but the overall view does not constitute adequate
documentary evidence for a Reconstruction. Oral histories are also unreliable
sources of documentation for treatment. Painting: NPS files. |
Undertaking a reconstruction based on insufficient research, so that, as a result, an historically inaccurate building is created. Reconstructing a building unnecessarily when an existing building adequately reflects or explains the history of the property, the historical event, or has the same associative value.
Executing a design for the building that was never constructed historically.
Investigating archeological resources to identify and evaluate those features and artifacts which are essential to the design and plan of the building. Minimizing disturbance of terrain to reduce the possibility of destroying archeological resources.
Failing to identify and evaluate archeological information prior to reconstruction, or destroying extant historical information not relevant to the reconstruction but that should be preserved in place. Introducing heavy machinery or equipment into areas where it may disturb archeological resources.
Identifying, retaining, and preserving extant historic features of the building and site, such as remnants of a foundation, chimney, or walkway.
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Two
photos illustrate the use of contemporary construction materials and
techniques within the treatment, Reconstruction. Because Reconstruction is
employed to portray a significant earlier time, usually for interpretive
purposes, substitute materials may be appropriate if they are able to convey
the historic appearance. |
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Beginning reconstruction work without first conducting a detailed site investigation to physically substantiate the documentary evidence. Basing a reconstruction on conjectural designs or the availability of different features from other historic buildings.
Building Exterior
Reconstructing a non-surviving building to depict the documented historic appearance. Although traditional building materials such as masonry, wood, and architectural metals are preferable, substitute materials may be used as long as they re-create the historical appearance. Re-creating the documented design of exterior features such as the roof shape and coverings; architectural detailing; windows; entrances and porches; steps and doors; and their historic spatial relationships and proportions.
Reproducing the appearance of historic paint colors and finishes based on physical and documentary evidence.
Using signs to identify the building as a contemporary re-creation.
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The
1778 Kershaw House, which served as British Headquarters during the
Revolutionary War, was burned by Union
troops in 1865. In the early 1970s, the house was reconstructed as part of Camden Battlefield, Camden , South Carolina .
Built expressly for interpretive purposes, it serves as an illustrative
reminder of a past event of national significance. The Standards for
Reconstruction call for any re-created building to be clearly identified as a
contemporary depiction. This is most often done by means of an exterior sign
or plaque, or through an explanatory brochure or exhibit. A guide may inform
visitors as well. Photo: NPS files. |
Reconstructing features that cannot be documented historically or for which inadequate documentation exists. Using substitute materials that do not convey the appearance of the historic building.
Omitting a documented exterior feature; or re-building a feature, but altering its historic design.
Using inappropriate designs or materials that do not convey the historic appearance, such as aluminum storm and screen window combinations.
Using paint colors that cannot be
documented through research and investigation to be appropriate to the building
or using other undocumented finishes.
Failing to explain that the building is a
reconstruction, thus confusing the public understanding. Building Interior
Re-creating the appearance of visible features of the historical structural system, such as post and beam systems, trusses, summer beams, vigas, cast iron columns, above-grade stone foundations, or loadbearing brick or stone walls. Substitute materials may be used for unexposed structural features if they were not important to the historic significance of the building.
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The
interior of the Hall of the Governor's Palace in Williamsburg , Virginia ,
was re-created in its entirety, including decorative features and finishes.
Photo: Courtesy, Colonial Williamsburg
Foundation.
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Duplicating the documented historic
appearance of the building's interior features and finishes, including columns,
cornices, baseboards, fireplaces and mantels, panelling, light fixtures,
hardware, and flooring; and wallpaper, plaster, paint and finishes such as
stencilling, marbling and graining; and other decorative materials that
accented interior features and provided color, texture, and patterning to
walls, floors and ceilings.
Installing modern mechanical systems in
the least obtrusive way possible, while meeting user need.Installing the vertical runs of ducts, pipes, andcables in closets, service rooms, and wall cavities.
Installing exterior electrical and telephone cables underground, or in the least obtrusive way possible.
Changing the documented appearance of visible features of the structural system.
Altering the documented historic floor
plan or relocating an important interior feature such as a staircase so that
the historic relationship between the feature and space is inaccurately
depicted.
Altering the documented appearance of
interior features and finishes so that, as a result, an inaccurate depiction of
the historic building is created. For example, moving a feature from one area
of a room to another; or changing the type or color of the finish.
Altering the historic plan or the
re-created appearance unnecessarily when installing modern mechanical systems.
Installing vertical runs in ducts, pipes,
and cables in places where they will intrude upon the historic depiction of the
building.
Attaching exterior electrical and
telephone cables to the principal elevations of the reconstructed building,
unless their existence and visibility can be documented.
Basing decisions for reconstructing
building site features on the availability of documentary and physical
evidence.
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The
spacious grounds at Middleton
Place , near Charleston ,
South Carolina , constitute the first
landscaped garden in America .
The molded terraces, originally constructed in the 18th century, were largely
reconstructed in the early 20th century based on extant remains and other
documentary evidence. Photo: Middleton
Place . |
Re-establishing the historic relationship between the building or buildings and historic site features, whenever possible.
Reconstructing building site features without first conducting a detailed investigation to physically substantiate the documentary evidence. Giving the building's site a false appearance by basing the reconstruction or conjectural designs or the availability of features from other nearby sites.
Changing the historic spatial relationship between the building and historic site features, or reconstructing some site features, but not others, thus creating a false appearance.
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Two
views of the Officers' Quarters at Fort
Snelling (ca.
1885-1890) not only provide information on the materials and form of the
historic block, they document the wooden walkway and other landscape
features, such as stairs, railings, and tree placement. Historical and
pictorial evidence would need to be combined with specific physical evidence
in order to make the case for Reconstruction as a treatment. Photos: NPS
files.
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Inventorying the setting to determine the
existence of aboveground remains and subsurface archeological materials, using
this evidence as corroborating documentation for the reconstruction of missing
features of the setting. Such features could include roads and streets;
furnishings such as lights or benches; vegetation, gardens and yards; adjacent
open space such as fields, parks, commons or woodlands; and important views or
visual relationships.
Re-establishing the historic spatial
relationship between buildings and landscape features of the setting.
Reconstructing features of the setting without first conducting a detailed investigation to physically substantiate the documentary evidence.
Giving the building's setting a false
appearance by basing the reconstruction on conjectural designs or the
availability of features from other nearby districts or neighborhoods.
Confusing the historic spatial
relationship between buildings and landscape features within the setting by
reconstructing some missing elements, but not others. Energy Efficiency
Installing thermal insulation, where appropriate, as part of the reconstruction.
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The
18th century Kershaw House, reconstructed in the early 1970s as part of
Camden Battlefield, Camden, South Carolina, features multiple
energy-conserving features to guard against the warm southern
climate--porches, shutters, trees, and landscape features. Photo: NPS files. |
Utilizing the inherent energy conserving
features of windows and blinds, porches and double vestibule entrances in a
reconstruction project.
Utilizing plant materials, trees, and
landscape features, especially those which perform passive solar energy
functions such as sun shading and wind breaks, when appropriate to the
reconstruction. Installing thermal insulation with a high moisture content. Using windows and shading devices that are inappropriate to the reconstruction.
Installing new thermal sash with false muntins instead of using sash that is appropriate to the reconstruction.
Removing plant materials and landscape
features which perform passive energy functions if they are appropriate to the
reconstruction.
Taking accessibility requirements into consideration early in the planning stage so that barrier-free access can be provided in a way that is compatible with the reconstruction.
Obscuring or damaging the appearance of the reconstructed building in the process of providing barrier-free access.
Considering health and safety code requirements, such as the installation of fire suppression systems, early in the planning stage of the project so that the work is compatible with the reconstruction.
Meeting health and safety requirements without considering their visual impact on the reconstruction.
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