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Diagnosis and Prognosis of-Stnlctural integrity NEAL FITZSIMONS and JAMES COLVILLE Present deficiencies in historic masonry monuments and buildings may be generally classified as structural, aesthetic, or both. Decisions related to pres- ervation and/or restoration of such structures depend on accurately assessing the existing structural condition and identifying the cause of the deficiency. This paper presents a general methodology for the diagnosis and prognosis of structural integrity in masonry structures. Basic steps in this procedure include preliminary evaluation, on-site investigation, off-site research, preliminary analysis, laboratory and field tests, and structural evaluation. The paper also outlines a deficiency correction process, identifies major areas of needed re- search, and makes specific recommendations that can be implemented re- gardless of the structure under study. Any discussion of the structural integrity of historic structures is com- plicated by the variety of construction materials and types of construc- tion and architecture encountered. Adding to the complexity is the extreme range of condition and age of historic structures. Associated with existing condition and age are differing concepts of adequacy. Thus, structural problems in the Tower of Pisa or in castle ruins in Scotland, for example, are related to preservation of the existing struc- ture, while in other, more modern historic structures the objective may be restoration to the as-built condition. Finally, a successful I.e., Neal FitzSimons is Principal Engineering Counsel, Kensington, Maryland. James Col- ville is Professor of Civil Engineering, University of Maryland, College Park. 233
234 CONSERVATION OF HISTORIC STONE BUILDINGS accurate) diagnosis and prognosis of structural integrity, upon which the identification of proper remedial action hinges, is greatly influenced by limited availability of resources, missing historical information, and limited material sampling. In view of these complications the main purpose of this paper is to present a general methodology for diagnosis and prognosis of structural integrity in masonry structures. In particular, certain basic procedures and guidelines are presented along with specific recommendations and approaches that can be applied regardless of the particular structure under study. Most of the material presented is drawn from past ex- periences of the authors and from a recent report to the National Bureau of Standards by the senior author.) This report also contains a com- prehensive bibliography of pertinent publications. GENERAL REVIEW OF PROBLEM That a great majority of historic and other older structures are of masonry construction is, in itself, testimony to the durability of ma- sonry materials, which include burned-cIay units, concrete masonry units, and natural stones bonded together with a cementitious material such as mortar. Such construction materials can be manufactured lo- cally and, properly used, provide aesthetically pleasing structures. Ra- tional methods of design of load-bearing masonry structures, based on modem engineering principles, however, have been developed only recently. Before World War II, masonry design was based on a few empirical rules developed from experience gained over hundreds of years of construction dating back to Roman times. Present deficiencies in historic masonry monuments and buildings may be classified as structural, aesthetic, or both. Structural deficien- cies may result from a number of factors. In relatively new construc- tion, problems may generally be attributed to faulty design, including the use of improper or incompatible materials or poor workmanship. In older construction that has survived years of use with limited dam- age from normal foundation settlement and gradual deterioration, faulty design can be dismissed as the principal reason for recently accelerating decay. Rapidly increasing deterioration of basically sound original `de- signs may be attributed to such factors as excessive loading, accident, human action, and/or significantly increased environmental exposure. Excessive loading includes any loading greater than that considered in the original design, such as sonic booms and vibration resulting from traffic. Accident could include seismic disturbances and other natural phenomena. Human action includes increased tourism, vandalism, arid
Diagnosis and Prognosis of Structural Integrity 235 improper remedial treatment. Finally, recent escalations in environ- mental pollution acid rain, for instance also cause significant de- t-erioration. Assessing the structural condition of an existing building may be a difficult problem, depending on the extent of damage. Even more com- plex in many cases is identifying the cause of the damage, which is essential to the development of appropriate corrective action. The in- vestigator's basic objective is to arrive at a course of action that will reestablish structural adequacy, and eliminate or minimize the cause of the existing damage without creating new hazards. Various problems are encountered in the preservation and/or res- toration of historic masonry structures. Many of these lie outside the traditional scope of the art and science of engineering. Thus, basic questions relating to the validity of solutions that alter the natural environment, including removal of the structure, protective coatings and enclosures, concealed structural appendages, or partial replace- ment with simulations, must be addressed. More technical problems development of structural and architectural restoration criteria, iden- tification of structural analysis methodologies, research to establish appropriate nondestructive testing methods, and research to categorize and identify deficiencescan be resolved with proper resources and support. Obviously, cooperation and contributions in the development of needed knowledge and philosophy will be needed from a variety of disciplines. THE DIAGNOSIS The evaluation of existing structural integrity is only one phase in the overall rehabilitation process, which also includes social, economic, and political considerations. A flow diagram of the evaluation process (Figure 1) should be followed in this process. Major elements of the diagram are described below. Preliminary Evaluation Plan The preliminary evaluation plan involves establishing procedures for preparing a project dossier, including methods of organizing informa- tion and of documenting and recording data obtained in the diagnostic activities. However, concurrent with organization of the diagnostic effort, pertinent available information concerning the structure should
236 | On-Sit , | | I nvestigation | CONSERVATION OF HISTORIC STONE BUILDINGS Plan | Prel imi nary Evaluation Preliminary | Analysis I r Laboratory Tests _ Final Analysis | Field | Tests r _ Strut tural _ Ev~l~tinn _ _. Report on Structural I ntegrity FIGURE 1 Flow diagram of evaluation process. Off-Site Research 1 Additional Investigative Research
Diagnosis and Prognosis of Structural Integrity 237 be collected, including relevant maps, plans and drawings, photos, and other documents. Site Investigation A first step in site evaluation is a detailed visual examination of the structure to assess its overall condition and to seek broad clues con- cerning the severity and possible causes of the damage. In practice, marufestations of structural deficiencies may be obvious or difficult to detect. Attention should be concentrated on examining rectilinearity of the structure and its apertures. Any rotation or trans- lation of the structure should be investigated. Distress in the major structural components as evidenced by cracking, spelling, or bowing should be noted. Deficiencies providing access to water should be sought, since water penetration can have serious deleterious effects on both structural and nonstructural masonry components. Many of these external deficiencies can be detected by visual examination. More pre- cise observations, if necessary, may be obtained using a variety of measuring and/or surveying instruments. If possible, each deficiency should be categorized as one of the fol- Towing types: distress, deterioration, damage, or defect. Distress results from loadings in excess of the structure's design capacity. Deterioration is defined as the result of erosion of structural capacity by environ- mental attack. Damage is the result of extraordinary Toads. A defect is a variation from the intended structural plan that is serious enough to affect structural capacity. In practice it may not be possible to classify the deficiency easily; many deficiencies result from an inter- action of these effects. Randomly located deficiencies complicate the diagnosis. Therefore, from the beginning of the investigation, it is important to identify any patterns of deficiencies. Because cracking is a significant indication of potential structural problems, the location, magnitude, and extent of cracking should be carefully noted and studied. In masonry construction in general, points of high shear and low moment should be identified and examined, because joints, bearings, and connections are normally the most vulnerable ~n(l critical areas of a structure. Testing Alternatives Visual site investigation, along with a review of available documents, is intended to permit identification of deficient portions of the struc-
238 CONSERVATION OF HISTORIC STONE BUILDINGS sure. Further investigation is necessary if it is prudent to validate ob- servations and assumed or calculated data. In normal situations where the condition of the construction ma- terials is suspect, it is possible to remove samples from several areas of interest and perform laboratory tests of the properties of the ma- terials. In historic buildings this may not be possible, and in situ, nondestructive measurements and testing will be required to diagnose structural damage. For example, standard surveying techniques can be used in the investigation of most foundation problems. Differential levels and distances accurate to one minute of angle and 0.01 It (3.048 mm), respectively, are precise enough for most situations. Special equipment con be used to measure vibrations in the structure if they are considered a potential source of distress. Crack sizes can be mea- sured using the simple monocular reticule and feeler gauges. Judgment is required in determining the type and extent of such measurements and the number, size, and location of material samples for further study. If the cause of the damage is considered to be water penetration and permeance, it may be necessary to use special nuclear instruments to discover the source and extent of the unwanted water and moisture. Testing alternatives may also include nondestructive, full-scale testing of portions of the structure. Such tests can be short-term or long-term. Short-term loading tests are generally more feasible and may consist of static or dynamic load tests. Dynamic tests may be further subdi- vided into impact and cyclic tests. With the exception of Tong-term tests involving implanted instrumentation, the scope of full-scale structural tests is usually very limited. Regardless of the testing to be performed, specific objectives of the tests must be determined well in advance. In general, test objectives may be classified according to the type and condition of the structure new, extent, distressed, deteriorated, damaged, defective, or repaired. A research project is finder way to assess the applicability of four nondestructive test (NDT) methods currently used for evalution of soil, rock, and concrete to the evaluation of masonry structures.2 The four NOT methods are: harness, mechanical pulse velocity, ultrasonic pulse velocity, and dynamic response vibration techniques. Analytical Techniques The strength, stiffness, and stability of a structure must be determined from the data gathered through investigation and testing. This infor- mation, at best, will be incomplete. Analysis of a structure is based
Diagnosis and Prognosis of Structural Integrity 239 on known geometry and material properties and characteristics, from which allowable stresses are estimated. The analysis produces values of allowable loads or factors of safety with respect to design loads. Often, funds, time, or circumstances lead to analyses based on incom- plete data. In such cases certain statistical techniques may prove help- ful when combined with real but incomplete data. The basic components in masonry construction wfl} be the masonry units, mortar, ties, and reinforcements. Masonry units vary widely in strength and durability. This is also true of mortars, which, until fairly recently, were lime-based-with no Portland cement. The compressive strengths of mortars can be extremely variable, ranging from less than 100 psi to more than 3,000 psi. Brick units, depending on their new materials and manufacturing processes, can have ranges in compressive strength from 1,500 psi to more than 16,000 psi. Structural tiles have similar variations, with compressive strength varying from 1,100 psi to more than 10,000 psi. Concrete block units, although highly con- trolled in recent years, will also display significant ranges in properties, as will stone units. Some structures may have been designed using the rule-of-thumb approach, and an analysis using modem techniques may indicate con- siderable latent capacity. Inclucing nonstructural elements in the analysis may also reveal significant influences on structural stability. Structural Evaluation Final structural evaluation should include a description of existing deficiencies along with a discussion of the causes of such deficiencies. The present condition of the structure should be defined, with de- scnptions of both sound and deficient sections. Finally, the conclusion should be presented along with substantiating documentation con- . . coming t :~e as-is capacity. THE PROGNOSIS After the diagnosis of structural integrity, an equally difficult but nec- essary part of the overall process is to determine recommended re- medial action. The necessity of such action and the type of action recommended depend on accurate prognosis of structural adequacy with and without corrective action. These prognoses wiD depend on the combined effects of reducing vulnerability once improving the struc- ture. The remedial plan, which includes the definition of tasks and
240 CONSERVATION OF HISTORIC STONE BUILDINGS time and cost estimates, should have these two objectives clearly be- fore it. A satisfactory solution may involve either or both of them. A typical deficiency-correction process is illustrated in Figure 2. Elimination of the cause of the problem may be a sufficient solution. Elimination of the effect of the problem may not be a lasting solution. Minimizing vulnerability to the cause is a valid objective. But finally, minimizing or eliminating both cause and effect provides the optimum solution. Care is needed if part of the solution is to create a new environment Detection of Deficiency Definition of Problem I. r Diagnosis f Causes | Determinatio ~ of Remedies | ~ . _ _ Decision f Remedy 1 Elimination of Causes 1 Correction of Deficiency FIGURE 2 Deficiency-correction process.
Diagnosis and Prognosis of Structural Integrity for the structure. Changes in ambient internal moisture and temper- ature may create more problems than are solved. In particular, new conditions resulting from improving the structural capacity may in themselves lead to secondary problems that hopefully will be minor. All changes should be carefully evaluated with respect to their effect on the structure's foundations. Addition of members, walls, materials, etc., to improve the capacity of the superstructure may cause redis- tributions of loading with subsequent detrimental effects on the sub- structure. All major modes of foundation failure, subsidence, rotation, and translation should be considered in the final report. Finally, after completion of remedial work, it is important to monitor the pe~fo~ance of the structure penodically to assess the impact of the intervention on its physical life. 241 REFERENCES 1. FitzSimons, N. Structural Evaluation Guide for Building Rehabilitation. Report submitted to National Bureau of Standards, July 18, 1979. 2. Noland, J.L., and Atkinson, R.H. An Evaluation of Non-destructive Test Methods Applied to Masonry. Proceedings, Conference on Research in Progress in Masonry Con- struction, ~arch, 1980, Marina Del Ray, California.