Status and Needs for Historic Materials and Building Structures From the Construction Industry Point of View, U.S. Perspective.

Transcription

1 Status and Needs for Historic Materials and Building Structures From the Construction Industry Point of View, U.S. Perspective. By: Edmund P. Meade, P.E., Principal and Director of Preservation, Robert Silman Associates, PC, New York, NY, USA. Paper Presented at: Institute of Theoretical and Applied Mechanics of the Czech Academy of Sciences (ÚTAM), Praha (Prague, Czech Republic) 13 July 2006 Abstract: This paper presents an introduction to the basic research and best practice needs of the structural engineering profession as this field of work is applied to historic structures. The perspective of the author is that of a licensed professional engineer practicing in the United States. 1. Introduction. The application of structural engineering towards historic buildings has often been relegated to a secondary level of concern and respect by the engineering profession. Technical progress in the field of structural engineering, greater awareness of the cultural, economic, and environmental importance of existing buildings, and the natural increase in the number of existing buildings have resulted in a broadening of the importance of structural engineering as applied to existing buildings and historic construction assemblies. In many parts of the world engineers whose practice involves evaluation and alteration of existing buildings have gained recognition of the value of their work. Each of these developments has exposed the need for basic and applied research of engineering techniques, the need to determine which engineering methodologies are most effective, and the need to educate engineers and their clients of the value of engineering for existing structures. This paper is presented in the spirit that it may further the international dialogue between engineers and their colleagues. 2. Research into Best Existing Practice on Historic Materials and Structures. Our discussion can begin with awareness that the structural engineering as a scientifically based discipline has spread from the most advanced materials and assemblies (iron, steel, reinforced concrete and structures such as tall buildings) to that of seemingly more simple materials and assemblies. Engineering mechanics and design has also begun to be applied to that of historic materials and structures. 19

2 In our discussion of the more widespread application of structural engineering it is important to review what are considered some of our current Best Practices as a profession. When reviewing each of the following Best Practices consider them in answering these questions: 1. What efforts should engineers undertake to assess and protect the structural performance of a building? 2. What information do engineers not have that is essential to reviewing the performance of a building? 3. What analytical techniques do engineers need to properly review the performance of a building? 4. What is the effect of engineers work on the building and the environment? In nationally and internationally prominent documents (including various Charters and Practice Guidelines) there are Professional Standards that must be complied with for preservation and/ or alteration of historic buildings. These documents provide guidance that includes these principals: Best Practice scenarios require recognition of the value of traditional building materials. This principal compels preservation of the building fabric whenever possible and requires tailoring of the structural engineering intervention to preserve fabric even at the cost of more complex repairs. Best Practice scenarios include a complete visual (i.e., naked eye) assessment of the building complex, the specific building in question, and the localized conditions of concern with respect to that building. We must examine all of the existing, earlier documentation available about the building and we must scrutinize the physical site to see if it brings any information to our evaluation of the structure. One frequent result of this high level of investigation (in cooperation with a preservation architect, materials specialists, and a historian) is a Historic Structures Report. Depending upon the emphasis and needs of the building owner more developed reports and design recommendations may be developed. For example, if a building is being temporarily stabilized the initial reporting on the structure can be made to emphasize the types of bracing, shoring, netting, or other protective measures that can be used to help preserve the building from undergoing further deterioration or movement. After finishing a brief, initial visual examination of a building, the design professional should then undertake more complete assessment of the physical condition of the historic materials that make up the building. This often takes the form of making close-up observations of significant portions of the building, undertaking probes to reveal hidden portions of structural systems, and/ or conducting nondestructive tests. The next stage in the evaluation process is perhaps the most fraught with the pitfalls of insufficient information and inadequate techniques. The stage in question is one whereby we analyze the structural state of the materials and buildings. The next stage in the process involves the selection of appropriate repair materials and designs. This work includes the development of structural repair designs that are compatible with the other aspects of the work (e.g., architectural conservation, water/ moisture management, Mechanical/ Electrical/ Plumbing modifications, security/ fire detection alterations, etc.). 20

3 While we must specify repairs that respect Historic Preservation and Sustainable Design Standards, it is also critical to examine the effectiveness of actual repair efforts in the field. This fieldwork must be done to control the quality of the work during construction and to directly observe failures and successes. 3. Summary of Basic Research and Best Practice Needs: The previous sections of this paper briefly outlined the process that can be considered an idealized or best case scenario of observation, analysis, design, and implementation. The next questions must therefore be: 1. How can we perform our better? Can we perform our work more efficiently? 2. How can we improve our analysis techniques and our design efforts? 3. Can we preserve historic structures with less impact on the environment? 4. Can we preserve historic structures in such a way that they the preservation treatment lasts longer? To achieve answers to the above questions we must enhance our basic research and our best practice guidelines for structural engineering as applied directly to historic buildings. We should also ask ourselves what real-world experience leads to improved professional practice. Asked generically: What TASKS must engineers perform to improve the quality and appropriateness of our work? The following list discusses some of the several TASKS that engineers must embark on. This list is divided between areas of basic RESEARCH and areas requiring improvement in our BEST PRACTICE methods). Improve our understanding of the history of traditional building materials (this can be considered part of basic RESEARCH). Improve the visual/ site evaluation process (BEST PRACTICE). Improve the Material Condition Assessment process and methods (RESEARCH). Advance the understanding of analysis methods (learning the limitations and the value of analysis) (BEST PRACTICE and RESEARCH). Continue research in developing and testing repair materials (RESEARCH). Extend collaborative research and design work between structural engineers and each of the other design team members (BEST PRACTICE). Continue to respect Historic Preservation and Sustainable Design Standards; define these Standards as they may apply to changing conditions (BEST PRACTICE). Educate Building Owners to permit Design Team to perform field-testing, Non- Destructive Evaluation (NDE), probes, and detailed field examination during construction (BEST PRACTICE). 4. Historic Context of Preservation Engineering. To finish setting the context of how to implement new Research and improve our Best Practice on Historic Materials and Structures we must review two further questions: 1. What Engineering Techniques have engineers used in the Past? 2. What is the Work Context for Preservation Engineers? 21

4 In very broad terms building design and construction has experienced the following changes: From earliest times construction techniques were based on the experience of the actual builders. Often considered part of a Primitive Building Tradition these techniques may have used simple analytical methods, but by combining basic beam and arch methods these builders created the first significant structures for human habitation. With the rise of Greek, Roman, and other ancient civilizations, there were Classical Design Techniques; for example, the work of Vitruvius. After the Fall of Ancient Rome in the western world, there was some Loss of Classical Design Experience. New heights of construction were reached with the experience of Gothic and other Medieval builders. The Rise of the Scientific Method brought a discipline of Hypothesis, Testing of Theory, and Experimentation. (Galileo, Newton, Hooke, et al.) This research, theory, and testing of materials contributed to the development of a Theory of Mechanics for Structural Design. Rise of the Aesthetic of Preservation and Engineering (Viollet le Duc, et al.) Extensive Material Testing and Experimentation in Physics and Chemistry. Transition from Hand Analysis to Slide Rule Assisted Design to Solid State Personal Computers. Rise of Building Codes, Material Standards, Modern Materials, and Finite Element Analysis (FEA). FIRST CASE HISTORY AND IMAGES: Historic Speedwell is located in Morristown, New Jersey, US. The structure in question, called the Factory Building, was built in the late 18 th Century and modified for various uses up to The Factory Building is associated with the inventor Samuel F.B. Morse. He perfected and demonstrated the Electro-Magnetic Telegraph in the period 1837 to 1838 in this building. The structure is currently undergoing structural and architectural restoration and will serve as a museum dedicated to the work that Morse performed on this site. The building had experienced significant rot and insect damage to its primary wooden timbers (including sill plates, posts, and beams). The building needed to be made safe for continued occupancy as a historic site. Only removal of heavily deteriorated structural members was considered. Unfortunately the areas of deterioration were largely hidden. Interior architectural finishes covered some members; others were protected by exterior walls that could not be removed until construction began. The scope of the removal and replacement/ repair work had to be predicted prior to the start of construction (to obtain accurate bids for construction and to minimize the potential for costs to change during construction). The architect and the structural engineer recommended to the Owner that a series of nondestructive and semidestructive tests be performed to evaluate the condition of the timber in the building. The tests that were performed included resistance drilling and moisture content measurement of numerous wood members supporting the structure. These techniques did successfully and accurately determine where the deterioration of the structural elements was located. This permitted a more timely and careful restoration effort to be undertaken. 22

5 Illustration 1.1. Image of the south exterior elevation (prior to restoration) of The Factory building at Historic Speedwell, Morristown, New Jersey, US. Illustration 1.2. Image of a portion of the first floor interior face of the south exterior wall. Note the presence of heavy wood timbers framing the floors and walls. 5. Needs Regarding Visual Evaluation, Nondestructive Evaluation/ Assessment, and Development of Project Engineering Documents: In the United States, structural engineers need to pursue the following research and best practice goals to improve our efforts and results in preserving historic buildings: Research for advanced techniques for evaluation of historic wood and masonry construction--especially condition assessment of connections. Increase training of engineers in visual assessment. Educate Building Owners to support Nondestructive Evaluation (NDE) and Assessment for select buildings (e.g., buildings that have limited or no existing structural drawings, unusual structural framing systems or geometry, and/ or unusual types of structural deterioration/ distress). Educate Building Owners to support Material Testing of their structures. Develop faster and more economical ways to complete Material Testing on historic building materials. Part of this effort should be the development of a comprehensive and readily accessible database of previous material testing results (from around the U.S. and around the world). Develop ways to speed transfer of information from field data to working drawings (especially for NDE data). 6. Needs Regarding Stabilization, Investigation, and Design: In the United States, structural engineers have needs regarding stabilization efforts, investigation methodologies, and design procedures: Provided improved private, institutional, and governmental preparation to respond to emergency and/ or disaster conditions that may affect historic structures. Train individual structural engineers to respect historic materials during such emergency conditions. Create a core group of well trained preservation engineers who can lead the 23

6 teams of structural engineers who may have less training in working on historic structures; these engineers could be dispatched anywhere in the U.S. and to other parts of the world as conditions require. Educate average structural engineers about traditional materials and historical construction methods. As a group of professionals learn to balance Restoration of Historic Materials with Construction of New Buildings. There are multiple effects of building new structures on existing buildings, on the natural environment, and on our use of energy. SECOND CASE HISTORY AND IMAGES: Low Library was completed on the Columbia University campus in New York, New York, U.S., in From shortly after completion cracks were observed in the masonry dome that crowns the building. The condition, state of stress, and level of any deterioration were unknown at the start of this investigation. The building serves as the office for the president of the university and as the symbolic heart of the Morningside campus. The university wanted a structural assessment of the dome completed. This assessment required the careful measurement of the interior and exterior of the dome (using laser scanning techniques), the creation of an architectural three-dimensional model, the creation of a three dimensional structural finite element analysis model, the completion of nondestructive investigations, and the installation of electronic monitors (to measure movements in the building). The nondestructive investigations yielded important information about the construction details of the dome and the extents of open joints in the masonry. The finite element analysis model resulted in a much clearer understanding of the state of stress in the building and the relationship between these stresses and the observed cracks in the dome. As a result of this investigation it was determined that no significant repairs to the dome were required. Illustration 2.1. Image of the south exterior elevation of Low Library, Columbia University, New York, New York, US. Illustration 2.2. Image of a portion of the finite element analysis model of the dome of Low Library. 24

7 7. Needs Regarding Measuring, Monitoring and Modeling: In the United States, structural engineers have the following needs regarding measuring, monitoring, and modeling of historic buildings: Identify material properties more quickly and cheaply. Especially critical needs are the development of in-situ measurement techniques that do not require destructive removal of large amounts of historically significant materials. Develop efficient ways to transfer laser-scan data from architectural survey to structural analysis software. Important research goals include the data format of the typical laser survey architectural models and compatibility of that format with advanced finite element structural analysis software packages. Concerted industry efforts must be made to help automate much more of this process. When using finite element structural engineering analysis a great deal of effort is concentrated on coding of material properties, material cross sections, and introduction of structural loads into the computer models. Getting useful stress and movement analytical results requires getting each of these input elements to work together to correctly. Additional research is also needed to determine where or when linear versus non-linear analysis is the better approach. One of the most important limitations of such advanced analysis is the ability to perform this work in an economical manner; private resources for this type of analysis are typically fairly limited. Develop less expensive and more rapidly deployable monitoring systems (for structural movement, temperature, relative humidity, etc.). Part of the research related to creating and using electronic monitoring systems must also include how to incorporate the data from these systems into the computer structural analysis programs (see above item for reference to these analysis programs). THIRD CASE HISTORY AND IMAGES: Craftsman Farms was completed in Parsippany, New Jersey, U.S. in It was designed and built by the architect and furniture designer Gustav Stickley. The building was built as his private home and it was intended to be the center of a compound of nearby buildings where his associates and workers could create works of architecture, art, and furniture that were in keeping with Stickley s artistic philosophy. The building had experienced over 90 years of exposure to the elements and localized areas of wood deterioration were visible on the exterior of the building. This deterioration included damage caused by wood destroying insects and fungi. It was critical to understand the extent of this deterioration in order to make sure that all areas of damage were sufficiently treated and to keep track of the cost of such repairs. The architect and structural engineer worked with a wood scientist to examine the most significant existing structural elements using semi-destructive evaluation techniques to determine which building members could be kept and re-used. This assessment permitted the structural engineer to create repair details that helped to minimize loss of historic fabric and to reduce the need for future repairs. The building investigation techniques also included electronic monitoring to determine how much an interior wood wall was moving with respect to time. This wall was unusually built and had some alterations over time that needed to be evaluated. 25

8 The project made use of sustainable design standards and material selection techniques to replace original materials that could not be re-used, construction documents. The building has been successfully restored and is now continuing its role as a museum of the work of Gustav Stickley and the Arts and Crafts artistic movement. Illustration 3.1. Image of the exterior of Craftsman Farms. Illustration 3.2. Image of a contour map of one of the interior timber walls of the building. Note that the wall is not planar or plumb. 8. Needs Regarding Design: In the United States, structural engineers need to pursue the following research and best practice goals to improve the efficiency and accuracy of our design efforts: Increase training of engineers to reduce damage caused by improper structural and/ or architectural repair designs. Reduce cost of real-time data acquisition and transfer of data regarding movement (or other properties being monitored) to allow for incorporation into analysis and design of structural repairs. Consider alternative designs for repairs to historic structures. Make this a mandatory part of the design process for public and private projects. Realize that there will probably always be UNIQUE or almost unique buildings (and that these structures may require specialized analysis and design procedures). Encourage private, institutional, and governmental Owners to create databases to record information regarding previous conditions assessments, previous repairs, and all documents. These databases can be either electronic or low tech paper-based systems. Identify limitations of modern computational abilities and determine how best to respond to these limitations (e.g., more research, more conservative or limited repairs, and/ or plain humility). Identify weaknesses of individual investigative techniques and determine how to improve the effectiveness of these techniques. Establish Peer Review of Investigations, Analysis, and Design as an important aspect of more projects (not just singular structures). This will afford a greater sharing of 26

9 information between colleagues and increase the experience base of numerous design practitioners. Encourage dissemination of this gathered experience at professional conferences, web-bases seminars, and traditional classroom instruction. Support dialog and collaboration between Private Practice Designers and Academic Researchers and Educators. FOURTH CASE HISTORY AND IMAGES: St. Thomas Church, located in New York, New York, US, was completed in 1913 to the designs of Cram, Goodhue and Ferguson. The walls of the building are made of load bearing masonry. On the north and south sides of the nave of the church there were long extant open joints in several mortar joints. The structural cause of these cracks was unknown. The building had been monitored for signs of movement. The goal of the structural engineering assessment was to determine the cause of the observed open joints. The investigative methods included the use of laser survey techniques to measure and create a three-dimensional architectural model of a portion of the nave. This model was then transferred to separate structural software programs and modified to create a structural model of that portion of the nave. The final result of the modeling and analysis was to determine that there was no long-term threat to the stability of the building. Long term restoration efforts to the building are continuing. Illustration 4.1. Image of the exterior of St. Thomas Church in New York, New York, US. Illustration 4.2. Image of a portion of the finite element model for the structural analysis of the nave vaulting and adjacent walls of St. Thomas Church. The red highlighted elements are the stone ribs that cross the nave ceiling. 27

10 9. Needs Regarding Evaluation: In the United States, structural engineers need to pursue the following research and best practice goals to improve our efforts in the evaluation of historic buildings: A. Develop new, cost-effective close-up investigative techniques (to get to remote or otherwise difficult to reach portions of buildings (e.g., facades of tall buildings or ceilings and walls of large interior spaces). B. Encourage development of new condition recording methods (e.g., tablet computer based systems). C. Develop new ways to structurally analyze buildings. Systematically evaluate whether Finite Element Analysis is worthwhile or required for certain categories of buildings. Would simpler analytical techniques be more economical to complete and just as useful to the engineer in evaluating the structural stresses in buildings? What simplifying analysis assumptions can be made in each class of buildings? Perhaps Associations of Design Professionals (e.g., ASCE, APT, MIA, etc.) could develop Best Practice Guidelines for different types of structural engineering investigations. D. Provide support for fundamental research into historic building materials to determine how existing buildings function from the structural engineering perspective. This research effort can help to minimize some conservative engineering appraisals and reduce the amount of intervention or alteration of historic fabric. While seemingly aimed at just helping to protect older, historic structures, this fundamental research should also be able to illuminate how newer structures also function in the presence of forces such as wind, snow, seismic events, etc. These research efforts can help to produce more economical structures with more modern materials and/ or more affordable buildings in portions of the world where resources are more limited and may not permit introduction of large amounts of structural steel or reinforced concrete/ masonry. 10. Summary: Research and Best Practice Improvement on Historic Materials and Structures: Areas to Develop: In summary structural engineers need to pursue each of the following research and best practice goals to improve our efforts and results in preserving historic buildings: Research and Evaluate the Material Properties, Construction Details, and Performance of Historic Structures. Include in this research detailed reviews of their Behavior and the History of Construction of such Materials, Details, and Structures. Further develop In-Situ Non-Destructive Evaluation and Semi-Destructive Material Evaluation Techniques. Develop more advanced, more economical, and more rapid Condition Assessment and Documentation Techniques. Critique and research structural analysis methods (for all types of historic building materials and all types of historic buildings). More comprehensively Research and Test Repair Materials. Develop Databases of this information so that they are available around the world. Continue Collaboration between disciplines and between Academy and Field Practitioners. 28

11 Update and further define Historic Preservation and Green or Sustainable Design and Construction Standards. Encourage evaluation of the value of the embodied energy of existing buildings (versus the energy required to construct new buildings). Educate building owners about the value of Non-Destructive Evaluation, Probes, Testing, and Detailed Field Work during Construction. Educate structural engineers about the techniques required to examine, measure/ monitor, analyze, and design repairs for historic structures. Educate the general public about the cultural, scientific, and environmental benefits of working on preserving historic buildings. 11. Conclusion: In the United States our knowledge of how to systematically evaluate and structural repair historic buildings is evolving. Continuing the successful application of techniques from other parts of the world will require devoting resources to basic research and improvements in the best practice guidelines currently in place. If the necessary effort is applied to these needs, the U.S. will see greater application of these historic preservation, analysis, design, and construction techniques. These techniques will then become more economical to apply and more appropriate structural intervention efforts will be applied to preserve existing buildings. 29