historic structures: the role of the structural engineer ... · historic structures: the role of...

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C H A P T E R 1 Historic Structures: The Role of the Structural Engineer

THE STRUCTURAL ENGINEER AS A PRESERVATIONIST

S Services of the structural engineer may include planning, estimating, mak-ing structural evaluations, conducting feasibility studies, designing, and

consulting. Adaptive use, the practice of renovating or rehabilitating a struc-ture for a use other than that for which it was fi rst designed, involves, for the structural engineer, several opportunities to be of service. Although the fi nal construction cost may include as little as 5 to 12 percent of the total project cost for structural work, the structural engineer may play a pivotal role in determining whether a project will be economically feasible. A structural eval-uation will indicate whether structural elements are reused, reinforced, or replaced.

Often, adapting a building to a new use requires extensive structural changes to be made in order to make the building conform to current needs and code requirements. A common challenge for the structural engineer is how to construct, within a historic structure, an elevator shaft so that upper fl oors may be accessible. Seismic retrofi t is an important concern facing historic structures in areas where retroactive seismic building code requirements are in force. The services provided by a structural engineer associated with moving a

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historic building might include a structural evaluation to determine whether the structure can be moved intact, partially disassembled, or relocated in sec-tions. Structural reinforcement or the design of a new foundation might also be required. Registered land surveyors are the most qualifi ed to provide accurate base measurements when the extensive documentation of a historic structure is required.

REHABILITATION GUIDELINES

All structural engineers should obtain a copy of The Secretary of the Interior ’ s Standards for Rehabilitation . Under these standards, rehabilitation means “ the process of returning a property to a state of utility, through repair or alteration, which makes possible an effi cient contemporary use while preserving those portions and features of the property that are signifi cant to its historic, archi-tectural, and cultural value. ” Minimum alteration of the building, its environ-ment, and its distinguishing architectural qualities is required for a project to qualify as a “ certifi ed rehabilitation ” benefi ting from the provisions of the tax act. Archeological resources must be protected, as well as signifi cant historical, architectural, or cultural material. An understanding of the historical signifi -cance of a building must be obtained to enable the engineer to provide an acceptable solution to a particular design problem while following the “ secre-tary ’ s standards. ”

The guidelines for applying The Secretary of the Interior ’ s Standards for Rehabilitation recommend recognition of the “ special problems inherent in the structural system of historic buildings, especially where there are visible signs of cracking, defl ection, or failure. ” In addition, “ stabilization and repair of weakened structural members and systems when damaged or inadequate ” are recommended. “ Historically important structural members ” are to be replaced “ only when necessary. ” 1

BUILDING CODES AND HISTORIC STRUCTURES

The structural engineer must make a realistic judgment when applying modern building code live - load requirements to a historic structure. For example, how much snow can adhere to a steeply pitched slate or tin roof? How does one rationalize the successful service of a hundred - year - old church roof struc-ture when the forces obtained from the frame analysis appear too great for the

2 Historic Structures: The Role of the Structural Engineer

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connections involved? As in the review and evaluation of any structure, the engineer must make a judgment, based on available information, of the safety of such a structure. Should the engineer call for the reinforcing of a timber fl oor structure with steel beams when it is safe in bending and shear but exceeds defl ection requirements when loaded with a required minimum live - load based on occupancy?

The 2006 North Carolina State Building Code, which is the Standard Building Code with North Carolina amendments, includes several provisions for historic structures. In Section 1009, “ Historic Buildings for Public Display or Exhibition ” and Section 1010, “ Historic Buildings for Adaptive Use, ” the defi nition of historic buildings is as follows:

General (a) Historic buildings means buildings designated as historic properties. (1) By the state historic preservation offi cer acting on behalf of the North

Carolinian Historical Commission in accordance with the provisions of G.S. 121.8 and NCAC 46.0600.

(2) Or by a local historic properties commission constituted in accordance with G.S. 160A.399.2 subject to review and approval by the Building Code Council. 2

Included in the code are provisions regarding repairs, additions, sprinkler systems, means of egress, and the moving of historic buildings.

In recent years several codes have adopted the innovations and principles of the New Jersey Rehabilitation Subcode commonly know as the New Jersey Rehab Code which was fi rst published in 1997. Chapter 34, Existing Struc-tures in the 2003 International Building Code also includes Section 3407 enti-tled Historic Buildings states that, “ The provisions of this code relating to the construction, repair, alteration, addition, restoration, and movement of struc-tures, and change of occupancy shall not be mandatory for historic buildings where such buildings are judged by the building offi cial to not constitute a distinct life safety hazard. ” 2

THE STRUCTURAL EVALUATION

Structural engineers have been reluctant to become involved with historic preservation projects, often because of the potential liability imposed on the engineer. By merely providing a structural evaluation of a historic structure, the engineer may become “ the engineer of record ” for a building constructed

The Structural Evaluation 3

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with “ primitive ” methods and materials. Many historic structures were not rigorously analyzed, but proportioned by the eye of an experienced builder or simply built as was the custom.

If the engineer becomes involved in even a small portion of a historic pres-ervation project, there must be adequate compensation for the assumed liabil-ity. The engineer must be completely satisfi ed that a structure meets the loading requirements for its intended occupancy, as well as all external forces.

Structural evaluations usually include a determination of the ability of a fl oor or roof system to support these service loads. An existing structure might have to be monitored in order to obtain data for such an evaluation. Testing programs may have to be designed to aid in determining the strength of com-ponent materials or complete assemblies. Methods may consist of destructive or nondestructive testing of component materials or load tests of structural members such as beams or assemblies such as trusses. It will likely be neces-sary to adapt current testing methods for fi eld use on historic structures.

Accurate fi eld measurements are essential in defi ning the structure and its condition. Surveying methods have been successfully utilized in determining the stiffness of defl ected beams and trusses. A topographic plan of the fl oor surface of a historic structure will yield a useful visual representation of an irregular fl oor if the contour interval is small. Irregular fl oors in a historic structure could be caused by movement in the supporting soils, timber decay or shrinkage, or defl ection of structural components. The ability to interpret the response of a structure to background vibrations and induced vibrations has made vibratory testing a valuable historic preservation tool. X - ray, liquid penetrant, nuclear particle density meters, and ultrasonic techniques are being used to evaluate various construction materials.

The correct interpretation of masonry cracks may yield accurate informa-tion regarding the location and amounts of settlement or thermal movement. Monitoring such cracks is possible with “ telltales ” such as glass slides epoxied to the wall surface on each side of a fi ssure. Even slight movement can be detected by using such a strategy. Accurate monitoring of cracks is possible with calibrated telltales accurate to within one millimeter and electronic strain gauges.

Load testing a historic structure may be the only reasonable way to justify conditions or materials that are diffi cult to analyze. In designing a load test, the engineer must call for the application of realistic loads carefully applied.

Great care should be taken before applying twice the design live load, as required by many building codes, to a historic structure. For timber structures it may be unrealistic to apply full live load plus an increase for a period such as 24 or 48 hours, when the structure actually will never reach that service loading for


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that length of time. Applying a known, safe load to a historic structure is an excellent method for determining the stiffness characteristics of various materi-als. This is of special value when evaluating timber structures. It is important to realize that because of the variation in the strength characteristics of timber, a load test of one member of a structure may not be indicative of the true capacity in other areas of the building.

For any material, before a load test is undertaken, the engineer must be cer-tain that all lateral bracing and slenderness requirements are satisfi ed. A prelimi-nary analysis must be performed to ensure that the structure will not be loaded past the elastic limit or further to destruction. The application of strain gages and other instrumentation is highly desirable in monitoring a load test. Through the use of monitoring techniques, testing, measurements, observation, and structural calculations, an accurate interpretation of the structural capability of a historic structure can be presented in a carefully written report.

MATERIALS RESEARCH

Research may yield information useful in the evaluation, rehabilitation, or ren-ovation of the historic structure. Original plans, construction photographs, and written or oral accounts may provide clues to the original design or con-struction methodology. Old textbooks or materials handbooks may provide design methods and design strengths of various materials.

FIGURE 1-1 This load test utilized water to determine the average stiffness of a series of recycled joists.

Materials Research 5

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The structural engineer must be familiar with the properties of materials such as timber, steel, cast iron, wrought iron, stone, brick masonry, terra cotta, and reinforced concrete. Patented fl oor or roof systems composed of various materials might appear only in manufacturer ’ s literature, with little or no design information available.

THE EVALUATION OF CONCRETE

Reinforcing steel in older concrete buildings may be square, round, or hexago-nal in cross - section with various types of deformations and exhibiting various physical properties. Early reinforcing steel was produced as plain and deformed steel in structural, intermediate, and hard grades. Structural grade was nor-mally used unless specifi ed otherwise. Structural engineers should obtain a copy of CRSI Engineering Data Report, no. 11, titled “ Evaluation of Reinforc-ing Steel in Old Reinforced Concrete Structures. ” 3

The fi rst specifi cations for reinforcing steel were developed in 1910 by the Association of American Steel Manufacturers. In 1911, the American Society for Testing and Materials adopted standard specifi cation A15 for billet steel concrete reinforcing bars. Minimum working stresses and yield strengths for these and other early specifi cations are presented in the CRSI report.

The most diffi cult problem in evaluating historic reinforced concrete struc-tures is determining the size and location of the reinforcing steel. Various instru-ments now available may be used for such purposes, but should be verifi ed by exposing the reinforcing steel in noncritical locations to visual inspection.

Development lengths, bending and cutoff details, and effective depths must be determined. The material properties of both the steel and concrete should be determined by testing. Samples of reinforcing steel suitable for test-ing can usually be obtained without affecting the structural adequacy of an existing structure if the locations are carefully selected. A preliminary struc-tural analysis aids in locating areas of low stress suitable for sampling. Nonde-structive load testing of complete fl exural members can be employed to verify calculated defl ections. Accurate methods are still needed to aid engineers in evaluating the effects of voids, cracks, and deteriorated reinforcing.

BRICK MASONRY RESTORATION

Lime - sand mortar, commonly used in structures located above water level, is of great importance in the repair and restoration of historic buildings. The structural engineer interested in historic preservation should be familiar with masonry restoration specifi cations.


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Mortar for face brick should match the original mortar in color, texture, density, and porosity. It should have strength equal or less than that of the original mortar. New mortar should have hardness equal to or less than that of the original brick, as determined by testing. The color of mortar used for repointing should be matched to the original by matching the color of original aggregates and mortar components as closely as possible. An archeological search may uncover ingredients of the original job mixed mortar, such as oyster shells, in the soil strata at the site in the builder ’ s trench, which contains other construction debris.

The density and strength of historic brick units are a function of their position in the kiln and how well they were fi red. Salmon brick, which are lightly burned, were typically reserved for the center of a wall and the harder, better burned, brick used as exterior face brick. Because of the extreme vari-ations in their strength and durability, the use of salvaged brick should be discouraged.

There are manufacturers who can match old brick very accurately and several manufacturers who are making bricks by the old methods. These bricks and a compatible lime mortar design mix are what is required to match the brick masonry of historic buildings. Repointing brick masonry is a waterproof-ing procedure and not a solution for structural problems. The repointing process is a critical procedure that should be done in carefully selected areas with great care. 4

Materials engineers should become familiar with the components of his-toric mortars, and in that light review the methods for sampling and testing of masonry. The Brick Institute of America, Technical Notes on Brick Construc-tion , no. 39A, reviews procedures for testing brick prisms. 5 The standard ASTM methods of tests for masonry assemblages are especially applicable to the testing of historic masonry because of the possible variation in the mortar and brick strengths. The performance of historic mortar and brick can be eval-uated in this way, not as individual components, but as they would perform together in the wall. Of course, obtaining suitable undisturbed samples for testing can be a problem with fragile materials.

Once a replacement brick is selected and the original mortar approxi-mated, prism testing of replacement masonry should yield information regard-ing allowable stresses that may be used in design. Mortar analysis and mix design should be accompanied by strength tests that can be evaluated by a materials testing engineer. There has been a tendency in the fi eld of historic preservation to select brick and mortar so that they merely “ look ” right when placed alongside original masonry. The structural engineer can best determine that strength characteristics of replacement masonry materials are as compatible as the color and texture.

Brick Masonry Restoration 7

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The use of grinders, sandblasting, sanding discs, or other abrasives normally are not permitted in the cleaning of brick. Irrevocable damage has been done to historic masonry by abrasive cleaning. For example, removal of the outer surface of old brick may expose the more porous inner portion of the brick, which may lead to spalling due to moisture penetration and subsequent freezing.

TIMBER DESIGN AND HISTORIC PRESERVATION

Often, the proper analysis and evaluation of a historic structure requires that the structural engineer have extensive timber design experience. Many historic structures in the United States are timber framed. Masonry construction is generally used for foundations, exterior wall support, and building enclosure. A thorough knowledge of the physical and mechanical properties of wood is necessary.

Many historic structures were constructed of green timber because of the considerable time required to air dry large timbers. In the seasoning process, timber gives off or takes on moisture from the surrounding atmosphere with changes in temperature and relative humidity until it attains a balance relative to the atmospheric conditions. Historic structures have had time to reach this point of balance, known as the equilibrium moisture content.

FIGURE 1-2 The entrance gates for Camden (Railroad) Yards in Charleston, South Carolina, were restored and the stucco was renewed.


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Moisture content is the weight of the water contained in wood, expressed as a percentage of the weight of the oven - dried wood. As wood loses moisture, the water in the cell cavity evaporates fi rst. The condition at which the water in the cell cavity has been evaporated but the cell wall is still saturated is known as the “ fi ber saturation point. ” This point is usually assumed to be at approximately 30 percent. When the moisture content is reduced below this point, shrinkage will occur. 6

Builders of heavy timber structures usually made allowances for shrinkage in the design of members and connections. The amount of shrinkage may be calculated using tables that give amounts of radial, tangential, and volumetric shrinkage from green to the oven oven - dried moisture content for various spe-cies. A moisture meter is an important tool for the structural engineer. What has been misinterpreted as defl ection or settlement in historic structures may be due to the across - the - grain shrinkage of large timber girders that were installed in a green condition and subsequently dried to low moisture content.

An increment borer can be used by the structural engineer to obtain core samples 0.2 of an inch in diameter, which can be used to determine the species, the number of growth rings per inch, the oven - dried weight, the moisture con-tent, and the specifi c gravity of the wood sampled.

Often, certain parts of a structure will indicate a moisture content that is considerably higher than the equilibrium moisture content determined by the dry bulb temperature and relative humidity. Usually, close contact of timber with moisture - containing masonry or earth will cause elevated moisture con-tent at the bearing points of timber purlins, joists, beams, columns, or trusses. The moisture is most readily absorbed through end grain. Once the moisture content rises above 20 percent, decay will probably occur. Strength - reducing effects of decay and termite infestation commonly occur at support points. Repairing these areas of high shear is a challenge to the structural engineer involved in historic preservation.

The structural engineer evaluating the heavy timber frame of a historic structure should be intimately familiar with the causes and signifi cance of checking and the structural considerations.

The structural engineer evaluating a timber structure should become famil-iar with the grading rules of the various species of wood that he may encounter. A familiarity with timber - grading rules and the strength reduction properties of various natural growth characteristics can be obtained from The Wood Handbook of the U.S. Forest Products Laboratory. 7 Obviously, the purpose here is not to transform the engineer into a grading - rules expert, but to provide suffi cient knowledge so that the engineer is comfortable in assigning a particu-lar grade to the timber framing under consideration in order for the engineer

Timber Design and Historic Preservation 9

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to assume reasonable design values to be used in the analysis. Certainly, in historic structures, species and density play a most signifi cant role in determin-ing the strength of a particular timber. Of course, the timber ’ s history is critical also. Wood that appears to be of a superior grade may have been subjected to overstressing, cyclic loading, elevated temperatures, or other environmental conditions. The true capacity of a historic timber is often less than one would expect based on a visual inspection.

The testing of representative samples to destruction is the surest way to determine reasonable design values. Results from such tests can be used to establish appropriate design values in a simplifi ed manner or through statisti-cal analysis of the results.

Certainly, the old - growth, dense, clear timber found in many existing struc-tures built before World War II, in North America, should be judged on the quality of the wood and not on published design values assigned for timber cut today. In spite of the insistence of the forest products industry that today ’ s timber is in every way equal to previously cut old - growth timber, I have observed a consistent difference in the density of today ’ s timber used for con-struction and that of the past.

FIGURE 1-3 Cutting through the fl oor joists at Market Hall in Charleston, South Carolina, revealed the material to be dense southern pine.

The past practice of excessively notching fl oor or roof joists into carrying members must be reviewed by the structural engineer using the end - notched beam formulae presented in various timber design manuals and textbooks. 8

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A joist might be perfectly adequate in bending and defl ection and be critical in horizontal shear at a notched support. This condition may be easily remedied by installing custom - sized nailed joist hangers to transfer vertical forces from the joist to the supporting member.

THE TIMBER TRUSS COMPUTER MODEL

Slippage, rotation, shrinkage, or the lack of continuity in a timber joint is dif-fi cult to allow for in a computer analysis of a timber truss. Multiple - chord trusses will invariably appear stiffer when analyzed, even when all joints are free to rotate in the computer model. Structural engineers are aware that it is very diffi cult to produce a true hinge or a true fi xed joint in the actual struc-ture. Joints in timber trusses may act somewhere between the two, causing a very different distribution of forces than produced by the analysis. How does one model a half - lapped and notched joint in an indeterminate frame? What about the problem of describing the intersecting member of a multiple chord truss where half of the member section in each direction passes through a joint and all pieces nailed together with wrought iron nails?

HISTORIC HIGHWAY BRIDGES

Highway bridge inspection, rehabilitation, and replacement programs have involved many structural engineers in the evaluation of older, sometimes his-toric, highway bridges. Many times these bridges are found defi cient because of deterioration of structural elements, the increased magnitude of service loads, inadequate lane width, or a geometry or confi guration of the highway or the bridge itself that makes the passage of vehicles unsafe at normal speeds.

The structural engineer plays a pivotal role in the determination of possible methods to retain a historic bridge structure. Many two - lane bridges have been converted to one - lane bridges with addition of an adjacent span to carry traffi c in the opposite direction. The lacing and cover plates of built - up members and the eyebolts and pinned connections of older steel truss spans provide the structural engineer with the opportunity to study design details that are uncommon today.

Repairs and reinforcement can be made to deck, abutments, and super-structure. When this cannot be justifi ed, truly historic spans may be removed from service by rerouting traffi c to a replacement span. Many historic bridges have been successfully adapted for reuse for pedestrians in areas such as parks and downtown redevelopment projects.

Historic Highway Bridges 11

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DISMANTLING HISTORIC RIVETED STEEL STRUCTURES

There is a method for carefully dismantling a historic riveted steel structure. Rivets may be removed by drilling a pilot hole into the center of the rivet head, reaming the head to the same diameter of the rivet shank, and chipping the head off by means of a chisel or cutting tool held in a pneumatic hammer. The remaining portion of the rivet may then be driven from the hole with a drift pin and sledgehammer.

Although this method is not as fast as torch cutting and resplicing by weld-ing, it preserves the original confi guration of connections, does not require splice plates, and does not subject the steel to excessively high temperatures. Field rivets can usually be identifi ed by their heads, which may not be as well formed as shop rivets.

Field splice locations can be determined so that a structure can be disman-tled in much the same way it was fi rst erected.

FIGURE 1-4 The presence of fi eld bolts provide an opportunity to dismantle a historic bridge in the same way it was assembled.

Reassembly in the original manner of construction is possible, with rivets, if a source for rivets, rivet heaters, and other tools can be secured and if steel workers experienced in riveted construction are available.

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HISTORIC STRUCTURES PROVIDE RESEARCH OPPORTUNITIES

Historic structures can provide a “ laboratory ” for engineering research. They provide an opportunity to evaluate the longevity of materials and the result of various construction practices. The causes of masonry deterioration and tim-ber decay can be observed. The causes and consequences of masonry cracks can be determined. Failure modes of various materials may be observed, as well as the effects of moisture penetration and thermal movement.

Creep deformations in materials such as timber or concrete can be studied, as well as other time - related properties such as fatigue strength. Someday, we may understand better the effects of load duration because of historic struc-tures research. This research will provide useful information to designers of contemporary buildings.

CONCLUSION

As the movement towards the preservation, restoration, rehabilitation, and adaptive reuse of historic structures expands, structural engineers will fi nd themselves playing an ever - increasing role. To be successful, they must apply their engineering knowledge and skill in a sensitive manner providing a safe environment while preserving the signifi cant historic, architectural, and cul-tural value of historic structures and places.

REFERENCES

1. U.S. Department of the Interior, The Secretary of the Interior ’ s Standards for Rehabilitation and Guidelines for Rehabilitating Historic Buildings (Washington, DC: U.S. Government Printing Offi ce, 1979).

2. The North Carolina Building Code Council and the North Carolina Depart-ment of Insurance, North Carolina State Building Code , vol. I (Raleigh: The North Carolina Department of Insurance, 2006).

3. Concrete Reinforcing Steel Institute, Engineering Date Report, no.11, “ Evaluation of Reinforcing Steel in Old Concrete Structures, ” (Chicago: Concrete Reinforcing Steel Institute: 1981).

4. Harley J. McKee, F. A. I. A., Introduction to Early American Masonry: Stone, Brick, Mortar and Plaster, National Trust for Historic Preservation and Columbia University, Washington Preservation Press, 1973.

References 13

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5. Brick Institute of America, Technical Notes on Brick Construction, No. 39A (City: Publisher: 1975).

6. American Institute of Timber Construction, Timber Construction Manual 2nd ed. (New York: John Wiley and Sons, Inc.: 1974).

7. U.S. Department of Agriculture, Forest Service. Forest Products Labora-tory , Handbook, no. 72, Wood Handbook (Washington, DC: U.S. Govern-ment Printing Offi ce, 1974).

8. National Forest Products Association, National Design Specifi cation for Wood Construction (Washington D.C.: National Forest Products Association: 1977).

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historic structures: the role of the structural engineer ... · historic structures: the role of...

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