^Earthen Building Technologies, Pasadena, California, P7707, L%4

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1 Seismic stabilization of historic adobe buildings W.S. GinelP, C.C. Uriel, Jr\ E.L. Tolles", F.A. Webster' "The Getty Conservation Institute, Marina del Rey, California, P02P2, ^Earthen Building Technologies, Pasadena, California, P7707, L%4 Abstract Many historic adobe structures in the Southwestern United States are vulnerable to destruction or damage by earthquakes. In the recent Northridge earthquake in Los Angeles, a significant number of the remaining, unretrofitted adobe structures dating to the 18th and early 19th century period were damaged extensively. Yet, it has been observed that many other adobes have performed well during large seismic events. This dichotomy exists because low strength adobe, when used in the form of thick walls, develops cracks, but these do not necessarily lead to catastrophic building collapse. The concept of seismic retrofitting based on improving the stability of the adobe buildings, rather than the more conventional criterion of strength, seems to be a valid approach. The objective of such a concept is to prevent walls from overturning by restricting the relative displacement of blocks formed following cracking thereby enabling the structure to dissipate energy by friction and rocking without catastrophic collapse. Shaking table tests of model adobe buildings have been carried out. Various types of simple retrofitting measures were evaluated that were designed to be relatively noninvasive and respectful of the historic fabric of the building. These measures consisted of thin, flexible horizontal or vertical straps applied to both sides of walls and small diameter steel rods inserted within the walls, which were used in conjunction with a thin wood bond beam. The intent of these measures was to provide overall structural continuity rather than strength improvement. The results of the tests involving seven 1:5 scale model buildings clearly demonstrated the effectiveness of the stability-based techniques. Wall height-to-thickness ratios were varied from 5 to 11 and maximum table displacements of ± 38 cm (prototype domain) were used. Some retrofitted models were able to withstand twice the displacement that resulted in collapse of an unretrofitted model.

2 54 Dynamics, Repairs & Restoration 1 Introduction Many historic adobe buildings in high seismicity areas have been destroyed by earthquakes. Yet, some that date back to the Spanish Colonial period in the American Southwest and in Latin America, although damaged, have been repaired and are still in use. Traditionally, minor damage to adobes has been accepted as being unavoidable in these cultures and existing adobe structures that have survived are authentic in the sense that traditional means and materials have been used for repairs. However, many of the currently accepted seismic repair and retrofitting techniques involve the use of both materials that are physically and aesthetically incompatible and architectural alterations that are inconsistent with maintaining the historic fabric and authenticity of the building. The methods are generally invasive and frequently ignore the fact that the structure and every other part of the adobe - down to individual bricks - are historic artifacts whose modification or loss contribute to the diminuation in value of the building, (Kimbro [1], Alva [2]). Adobe is considered to be a highly hazardous building material by architects and engineers who are involved in retrofitting design. Using strength and ductility as fundamental survival criteria, it has been assumed that adobe buildings should not be able to resist the damaging effects of large seismic events. Yet, many adobes, although cracked, have not collapsed catastrophically and therefore a retrofitting design approach based on achieving stability rather than strength should be considered. A stability-based design approach accepts the fact that although adobe has no material ductility, other characteristics of adobe buildings can be used to develop the needed structural ductility that is important in determining the seismic performance of the building. An important parameter in this regard is the relationship between the adobe wall thickness and the associated height-to-thickness ratio, SL (Bariola [3]). In the context of the work reported here, values of S^ associated with estimated seismic effects on adobe walls are given in TABLE 1. TABLE 1: Seismic Performance of Walls Relative Wall Thickness Very Thick Thick Moderate Thin Very Thin s. < >12 Seismic Effects Overturning unlikely Overturning difficult. Typical of historic buildings in California Some inherent stability but susceptible to overturning during large ground motions. Simple retrofit measures can stabilize. Likely to be unstable. Requires invasive retrofitting methods to stabilize. Unstable. Difficult to stabilize.

3 Dynamics, Repairs & Restoration 55 For thin-walled adobe buildings, collapse is imminent once the walls have cracked and therefore a thin-walled building has little structural ductility. For moderate to thick-walled adobe buildings, cracks result in the formation of adobe blocks whose relative displacements can become very large and both in-plane and out-of-plane collapse (overturning) become the principal failure modes. Failure in this sense refers to an adobe wall that cannot support itself or other loads. Thin walls that are non-load-bearing are most susceptible to overturning, but moderate to thick, load-bearing walls are more stable because displacements are limited by increased interblock friction and by connections to the roof or floor systems. Gabled adobe walls are particularly vulnerable to overturning because SL is usually large and the walls are not load-bearing. 2 Seismic Stability Improvement Techniques. A stability-based retrofit system is intended to prevent wall overturning by limiting the relative displacement between adjacent cracked wall segments and by improving the overall structural continuity of the building. A key feature of this concept is that movement across cracks and rocking should not be prevented so that seismic energy can be dissipated as friction at the cracked interfaces. If a retrofitting measure is too strong or too stiff, failure of the structurally weak adobe will occur adjacent to the retrofit. To validate the stability-based retrofit concept, several improvement techniques have been studied and their effectiveness evaluated during shaking table tests of scale model adobe structures [4]. The retrofit measures tested are listed in TABLE 2 for models without roofs. TABLE 2: Retrofit Measures Tested Retrofit Measure Bond Beam Strap Strap Center Core Crack Tie Application Direction Horizontal Horizontal Vertical Vertical Local Notes Flexible beam attached to the wall tops. Used with vertical straps and center cores Flexible, continuous strap applied to both sides of a wall. Flexible strap applied on both sides of wall and passing through the bottom of the wall and over the bond beam. Small diameter steel or fiberglass rods inserted in the wall and attached to the bond beam. Flexible cord applied across existing cracks. These and other techniques, such as partial wood diaphragms and anchors into the gable walls, were applied to a model with a gabled roof and attic structure.

4 56 Dynamics, Repairs & Restoration The ranges of relative effectiveness of particular retrofits are shown in Figure 1 as a function of earthquake severity. STIFF BOND BEAM _ : START OF EFFECTIVENESS : BUILDING COLLAPSE ^~ FLEXIBLE BOND BEAM HORIZONTAL STRAPS FLEXIBLE BOND BEAM & VERTICAL STRAPS MODERATE EARTHQUAKE SEVERITY HIGH Figure 1. Retrofit Effectiveness A stiff bond beam may have an effect on elastic performance but may be so stiff that the connection between the bond beam and the adobe fails during moderate ground motions. This is especially true when there is no positive connection between the bond beam and the adobe. A more flexible bond beam that is well anchored to the walls may have negligible effects on the elastic behavior but is more than stiff enough to have a substantial effect on the post-elastic performance. The walls must be cracked and begin to move beforeflexible,horizontal straps become effective. After damage has developed, these straps can greatly decrease the risk of collapse by providing some restraint against excessive outof-plane movements and by providing continuity between cracked sections of the walls. Vertical straps and a flexible bond beam do not change the elastic behavior or the behavior when damage is small, but the straps greatly reduce the relative displacements of the cracked wall sections and prevent overturning. 3 Model adobe building tests The models tested were constructed on a scale of 1:5 and were fabricated from molded adobe bricks and adobe mortar. The models, whose dimensions were 148 cm square by 58 cm high, were rigidly attached to a concrete base that was mounted on the shaking table. Each wall of the models was penetrated by a door and a window.

5 Dynamics, Repairs & Restoration 57 The straps used were 3 mm wide, woven, flat nylon strips that were tied at 30 cm intervals through the wall with 1 mm nylon cord ties. The wood bond beam was 30 mm x 6 mm thick, and was attached to the top of the wall using 75 mm long, coarse thread, sheet metal screws. Center core rods, 1.5 mm diameter, were inserted vertically in the wall, through the bond beam, and attached to the beam with epoxy adhesive. The shaking table was computer-controlled and each model was subjected to a series of table motions (Tests I to X) with increasing displacements based on the N21E component of the 1952 Taft earthquake record. Each succeeding displacement and acceleration was 30% larger than the previous one up to a maximum of ± 38 cm and 0.58 g in the prototype domain. Various combinations of retrofit measures and SL were tested on each of six model adobe structures and on a seventh model that was constructed to simulate an actual building. This building was configured to include gabled end walls, an attic floor joist system with a partial floor diaphragm, tampanco walls that extended 15 cm above the attic floor level, and a rafter-type roof system. A summary of the model configurations is shown in TABLE 3. TABLE 3: Model Building Retrofit Configurations Model SL Walls S, E Retrofit Upper horizontal strap Upper and lower horizontal straps Bond beam, center cores Bond beam, vert. & horiz. straps Bond beam, center cores Bond beam center cores and straps Upper horizontal straps Upper and lower horizontal straps Control - no retrofit Bond beam and vertical straps Bond beam and local ties Upper and lower horiz. & vert, straps Upper and lower horiz. straps and single vertical strap Control - no retrofit 4 Summary of Test Results Models Without Roof The shaking table motion was in the east-west direction so that the east and west walls were out-of-plane. In general, wall cracking was first observed during Test m or IV at an acceleration of 0.23 g and a displacement of ± 7.5 cm (both in the prototype domain). Listed in TABLE 4 are the important observations regarding the behavior of each model.

6 58 Dynamics, Repairs & Restoration TABLE 4. Results of Tests of Models 0-6 Test No. V vn vm DC X Accel. g Displ. cm ± 13 ± 19 ±25 ±32 ± 38 Mode All SL Behavior Collapse of east, south and west walls Collapse - unretrofitted control Collapse of east wall Continued cracking Collapse Remained standing after 3 repetitions of Test X Partial collapse of east wall Remained standing The results of these tests clearly established the effectiveness of the stability-based retrofit techniques for improving the seismic performance of adobe walls. Although the slenderness ratio has an effect on the relative stability of the adobe walls, thin walls that would otherwise have collapsed were stabilized by the use of the straps and bond beams. Figure 2 shows the condition of the unretrofitted Model 5 after Tests III and V, and Figure 3 is that of retrofitted Model 4 after Test X. Figure 2. Unretrofitted Model 5 After Tests III, and V Figure 3. Retrofitted Model 4 After Test X

7 Dynamics, Repairs & Restoration 59 Model 7 With Roof The performance of Model 7 was very satisfactory through Test X and was consistent with the results of the previous unroofed model tests. Without the retrofits, substantial damage would have occurred during Tests VI or VII. The roof diaphragm was stiff enough to prevent out-of-plane collapse of the highly vulnerable east and west gable walls. Horizontal cracks occurred at the attic floor line and large permanent displacements were observed in the gable end walls during Tests VIII, IX, and X, but collapse did not occur. Additional ties through the vertical straps could have limited these displacements. Horizontal straps were effective in preventing deterioration of the piers under the windows and they prevented diagonal cracks, which originated at the window corners, from widening and causing corner loss. The ties between the attic floor joists and the walls, and the pins and screws that anchored the roof to the gable walls and the roof to the bearing plates, respectively, were responsible for maintaining the continuity of the structure. Photographs of Model 7 before the tests and after Test X are shown in Figures 4 and 5. Figure 4. Model 7 Before Tests Figure 5. Model 7 after Test X

8 60 Dynamics, Repairs & Restoration 5 Conclusions Historic adobe structures that are endangered by virtue of their location in an area of high seismic risk, can be most effectively retrofitted using techniques based on stability rather than strength improvement criteria. Because the post-cracking behavior of adobe building determines their ultimate stability, conventional elastic analytical techniques for estimation of building behavior and design of retrofits are not appropriate. The stability-based approach considered the inherent properties of historic adobe construction. The design of retrofit measures takes into account the location where cracks and blocks will occur and how to limit block displacements to prevent catastrophic structural, failure. In general, the techniques investigated can be implemented in adobe buildings without extensive invasion of the historic building's fabric. A continuation of these studies by tests of larger scale models for simulation of gravity loading effects is in progress. The work reported in this paper is based on research performed under the Getty Conservation Institute Seismic Adobe Project (GSAP), References 1. Kimbro, E., Conservation principles applied to seismic retrofitting of culturally significant adobe buildings, pp in Proceedings of the 7th Int. Conf. on the Study and Conservation of Earthen Architecture, Silves, Portugal, October Alva, A. B., Earthquake damage to historic masonry structures, Conservation of Building and Decorative Stone, ed. J. Ashurst & F. G. Dimes, Vol. 2, pp , Butterworth-Heinemann, London, Bariola, J. B. Dynamic stability of adobe walls, Doctoral Thesis, University of Illinois, p 275, Tests were performed at the John A. Blume Earthquake Engineering Laboratories, Stanford University, Palo Alto, California.