SEISMIC ASSESSMENT AND REHABILITATION OF UNREINFORCED MASONRY BUILDINGS IN THE USA

Transcription

1 SEISMIC ASSESSMENT AND REHABILITATION OF UNREINFORCED MASONRY BUILDINGS IN THE USA Daniel P. ABRAMS Willett Professor of Engineering University of Illinois Urbana-Champaign United States ABSTRACT Engineering procedures developed in the last decade in the United States for performancebased seismic assessment and rehabilitation of unreinforced masonry buildings are summarized. As a preface to these procedures, common damage patterns observed following destructive earthquakes are given, followed by a brief discussion of decision-making processes for mitigating losses due to damage to masonry construction. Common rehabilitation methods are discussed as they apply to these engineering procedures for enhancing not only lateral seismic strength but also deformation capacity, and thus overall performance. Within this context, results of recent research are presented which was done to evaluate the effectiveness of these rehabilitation options. 1. INTRODUCTION Performance issues are indeed relevant for unreinforced masonry (URM) construction since such buildings are often relied on for critical or essential facilities. For example, many firehouses, hospitals or school buildings in Mid-America, constructed of unreinforced masonry, will be needed for support of emergency services in the event an earthquake occurs. Consequences of their poor performance can be severe to a community reliant on them for public health and safety. Thus, rehabilitation methods increasing performance will ultimately result in reduced consequences not only for individual building structures but systems which

2 72 SÍSMICA º Congresso Nacional de Sismologia e Engenharia Sísmica rely on them. Enhanced benefits of retrofit across regions (i.e. communities, networks, or other systems of stakeholder interest) for critical building systems may justify rehabilitation costs in many cases where benefit-cost of retrofit for individual structures may not be justified. Many times retrofit implies adding strength. The concept of more is better has traditionally been the premise of seismic mitigation. Thus, the words strengthening and rehabilitation have often been used interchangeably. However, this is not necessarily always true. In some instances, rehabilitation can consist of increasing nonlinear deformation capacity, even though strength may be decreased. In the FEMA 356/357 Prestandard and Commentary for Seismic Rehabilitation of Buildings both strength and deformation capacity are implicitly considered when estimating seismic capacity. Several options to increase deformation capacity and/or strength are given in this paper to illustrate these concepts. 2. LIKELY SEISMIC DAMAGE TO UNREINFORCED MASONRY CONSTRUCTION Consequences resulting from damage or collapse of unreinforced masonry buildings can in many cases exceed the costs of mitigation. Prevalent with this form of construction, apart from life safety concerns, economic losses due to loss of functionality can be significant, particularly if the structure is relied on for vital services to a regional system. Such consequences can be avoided through prudent application of rehabilitation techniques that will enhance not necessarily seismic strength, but also nonlinear deformation capacity. Unlike traditional strengthening methods, new strategies for improving ductility can result in enhanced performance of building systems, which in turn will have an impact on regional loss reduction. Common damage patterns for URM construction observed following earthquakes can be simplified in the following four categories. a) damage or collapse of out-of-plane walls b) shear or flexural cracking of in-plane walls c) loss of anchorage of walls to floor or roof diaphragms d) damage or collapse of corners Each of these damage mechanisms can include a complex interaction of multiple effects. Out-ofplane wall damage can result from inertial loads exceeding capacity, excessive deflections imposed on them from floor or roof diaphragms, or some combination of each. As a minimal retrofit solution, walls should be anchored to floor or roof diaphragms to transfer inertial loads to horizontal elements, and to protect walls from falling out from imposed diaphragm displacements. In-plane damage can be a result of tensile cracking normal to bed joints resulting in rocking, shear sliding along bed joints, excessive toe compressive stress causing crushing, or diagonal cracking through masonry units from excessive shear stress. Through judicious engineering for design or rehabilitation, masonry shear walls can be proportioned to behave with respect to one of these four mechanisms. If so, the first two mechanisms are preferred because they are displacement controlled as opposed to the second two mechanisms that are related to stresses or forces.

3 Daniel P. ABRAMS 73 Interactions of in-plane and out-of-plane forces can result in damage or collapse of corners of masonry buildings. In addition to biaxial effects, corners, acting as rotational restraints to horizontal diaphragm deflections can attract torsional forces that can lead to diagonal cracking. Numerous examples can be shown where corners of unreinforced masonry buildings fall out during earthquakes. Response and performance of URM building systems is usually modeled with simplified analytical models that often do not reflect these complicated mechanisms, yet serve to allocate the proper proportions of lateral strength. However, retrofit strategies seldom rely on assigning partial solutions that implicitly consider enhancement of inelastic deformation capacity for a particular building system. This theme is the subject of on-going research at the University of Illinois at Urbana-Champaign described herein. 3. MINIMIZING DAMAGE THROUGH PERFORMANCE-BASED MITIGATION Damage can be minimized through proper mitigation practice that relies on acknowledgement of the mechanisms and damage patterns addressed above. However, before an engineering assessment is done, the following questions need to be resolved between the building owner and the engineer or architect that will influence the decision-making process regarding whether rehabilitation should be done or not. Should an investment be made in rehabilitation? Will the reduction in risk be commensurate or less than the engineering and construction costs, or will the costs exceed the anticipated benefits? What levels of performance are desired by the owner of the building? Should the building be safe against loss of life or against collapse? Does the building need to be occupied immediately following an earthquake, or do operations within the structure need to be maintained? Are there critical non-structural components (electrical, plumbing, heating, ventilating, partitions, etc.) or vital contents that must be protected against damage? Where is the building located with respect to expected seismic intensities, governing building codes, accepted performance levels, or integration within regional economic systems? Rehabilitation techniques near the earthquake source can differ from those used at some distance from a fault. Seismic regulations differ depending on location, thus seismic capacity can vary. Once decisions have been made regarding these issues, then a practitioner can prescribe rehabilitation measures that will meet a specified level of performance for seismic demands that will be the most economical for the desired constraints. The remainder of this paper discusses research supporting such performance-based concepts.

4 74 SÍSMICA º Congresso Nacional de Sismologia e Engenharia Sísmica 4. CHARACTERIZING INELASTIC DEFORMATION CAPACITY The FEMA 356/357 Prestandard and Commentary for Seismic Rehabilitation of Buildings is a set of guidelines that restricts damage to three basic levels of performance. These are shown on the normalized force-deflection curve for a typical masonry building system in Figure 1. Because inelastic deformations often occur across the first story in such a system (one with door and window openings where first-story piers rock or slide), performance is expressed in terms of the lateral drift across the first story. From experimental testing of building systems (Abrams, 1997), first-story drifts of approximately 0.1% are known to result in minor extents of cracking which correspond to the immediate occupancy (IO) performance limit state. As an approximation, peak lateral-load strength is associated with a drift equal to about 0.6% which by default is assumed to correspond with the life-safety (LS) limit state. A loss of strength is permitted before reaching the collapse-prevention (CP) state at about 0.8% drift. These drift estimates are approximate and vary with the structural configuration as well as definitions of acceptable performance. 1.5 Base Shear / Weight IO LS CP V b y Q E Q CE i First Story Drift, % Figure 2: Equal displacement rule. Figure 1: Performance índices. The simplest analysis according to the FEMA Guidelines is a linear static approach where lateral displacements are approximated in terms of member forces based on the equal displacement rule (i.e. elastic and inelastic displacements are the same for structures beyond a given period range as shown in Figure 2). With this assumption, as noted in Equation 1, elastic forces, Q E, will be equal to the inelastic force, Q CE, times the ductility factor, m, which is defined as the deflection at a given limit state divided by the yield deflection. An additional factor, κ, is applied to the capacity to account for how well the structural system is know (it is equal to 1.0 for full knowledge). mκq CE Q E (1) The distinction between the capacity-demand equation above and traditional seismic procedures, is that conditions for capacity and demand must be met at the member or

5 Daniel P. ABRAMS 75 component level rather than the system level. Thus, ductility factors, m, must be determined for each masonry pier or wall in the structural system. To do this, the NEHRP Guidelines have taken information from testing of various masonry components as depicted in Figure 3. Q CE Force y i Deflection Figure 3: Definition of ductility for masonry structural components. The apparent yield deflection, y is determined as the quotient of the strength divided by the elastic stiffness in accordance with Equation 2. The m factor for each performance level is then the quotient of the deflection at that limit state divided by the yield deflection as given in Equation 3. Q CE y = k mi = i y (2) (3) Whereas the NEHRP Guidelines provide methods for estimating strength and deformation capacity of existing URM walls and piers, they do not provide such information for components rehabilitated via different means. This is largely because such research data is scant. However, an existing project of the Mid-America Earthquake Center has investigated strength and deformation of components rehabilitated via different means. The study has explored the worthiness of various options in terms of the principles presented above. Summaries of each rehabilitation method are given in the next section.

6 76 SÍSMICA º Congresso Nacional de Sismologia e Engenharia Sísmica 5. DEFORMATION CAPACITY OF VARIOUS REHABILITATION OPTIONS 5.1. Behavior of existing unrehabilitated walls and piers As discussed previously, behavior of an unreinforced masonry wall or pier can be dominated by shear or flexure in either a deformation or force-controlled manner. Members that are taller than they are wide and subjected to some vertical compressive stress are prone to rocking as demonstrated with the force-deflection curve presented in Figure 4. Behavior is essentially nonlinear, but elastic, since the deflection and the force return to zero at nearly the same time. In this mode, a member can possess considerable nonlinear deformation capacity though the hysteretic energy dissipation is low. In accordance with the NEHRP Guidelines, the m factor for this type of action is quite large, exceeding ten in some cases. Recognizing that the capacity as given in Equation 1 is multiplied by this m factor for comparison with the demand force, implies that rocking is a highly desirable mode. A masonry member performing with this behavior should not be rehabilitated to increase its strength at the cost of decreasing its deformation capacity. Furthermore, masonry members acting in other than a rocking mode could be altered in such a way as to promote rocking. For example, the height of window openings may be enlarged so that the aspect ratios of adjacent piers will be increased. Figure 4: Shape of forcedeflection curve for rocking pier Figure 5: Shape of force-deflection curve for pier with bed-joint sliding mechanism Displacement, inches A second deformation-controlled action for masonry shear walls or piers is sliding along mortar bed joints. Behavior in this mode, depicted in Figure 5 taken from Erbay and Abrams (2002), is robust in terms of hysteretic energy dissipation as well as inelastic deformation capacity. As long as the gravity compressive stress remains, the lateral strength of the wall or pier will be maintained through a simple friction mechanism. Some strength deterioration may occur as a result of grinding of mortar particles and thus a reduction in the coefficient of friction, but overall the behavior is acceptable. Because large strength and ductility factors are possible with this mechanism, little or no rehabilitation may be necessary.

7 Daniel P. ABRAMS Behavior of strengthened members One common theme of rehabilitation is to increase lateral strength of masonry walls or piers by parging on a thin coating of ferrocement, casting shotcrete over reinforcement, or infilling door or window openings with new masonry. Each of these methods can increase strength though deformation capacity may be sacrificed. Measured force-deflection behavior of a URM pier (similar to that with behavior shown in Figure 4) strengthened with a ferrocement surface coating is presented in Figure 6. For this case, a thin (25mm) coating of cement plaster was parged over a mesh of steel hardware cloth (widely available at local stores). A slight increase in strength is noted, but once the wire reinforcement fractures, behavior reverts to that of a rocking pier. Thus, this retrofit method does not have appreciable benefits for increasing the capacity term on the left side of Equation 1. Figure 6: Shape of forcedeflection curve for pier with ferrocement coating. Figure 7: Shape of force-deflection curve for pier with shotcrete overlay. Applying shotcrete over a grid of reinforcing bars is essentially the same as constructing a reinforced concrete wall adjacent to a masonry wall. Behavior of the wall or pier is governed by the reinforced shotcrete as depicted in Figure 7. Inelastic behavior is robust with a significant increase in both strength and deformation capacity. Large crack widths in the masonry and shotcrete along the wall base, resulting from yield of vertical reinforcement in the shotcrete, limit the performance of the element. This is a highly appropriate rehabilitation method in terms of increasing capacity per Equation 1, though expensive and intrusive. Infilling door or window openings with new masonry will indeed increase the cross-sectional area of a wall and apparently the shear strength according to formulas given in current building codes. However, damage observed after earthquakes, has shown that cracking often occurs in or near to the infilled opening, limiting deformation capacity. This is a result of incompatible moduli or inadequate shear transfer between old and new masonries.

8 78 SÍSMICA º Congresso Nacional de Sismologia e Engenharia Sísmica Reinforced cores are another option for strengthening unreinforced masonry walls. A vertical core is drilled through the height of a wall and then filled with a grout to bond a conventional reinforcing bar, thus effectively causing the wall to behave as a reinforced masonry component. This method is advantageous for increasing lateral strength and deformation capacity provided that sufficient anchorage of reinforcement to the grout, and of the grout to the masonry, is provided. This procedure is unobtrusive since the coring and grouting procedure can be done externally from the roof of a building, however, the coring process can be expensive Behavior of members rehabilitated to enhance nonlinear deformation capacity The opposite of infilling openings with new masonry as discussed in the previous section, is to enlarge the openings to increase the deformation capacity of the adjoining piers. This was discussed previously as a method for promoting rocking behavior by increasing the height-towidth aspect ratio of a pier. In such case, nonlinear deformation capacity will increase more than the lateral strength of a wall or pier will reduce. As a result, the left-hand side of Equation 1, expressing capacity, will increase despite the fact that material is being removed. Behavior mechanisms in piers or beams of a masonry shear wall perforated with openings can be altered by adhering fiber-reinforced polymer (FRP) strips to the surface of a masonry wall. Several configurations of strips are possible to provide strengthening to resist excessive tensile or shear stresses that would otherwise limit strength. Behavior of piers strengthened with vertical strips to increase flexural strength is shown in Figure 8. The strength was increased as much as five times over a counterpart part pier that was unrehabilitated and susceptible to rocking. Due to delamination of the FRP at the peak strength, nonlinear deformation capacity of the rehabilitated pier was limited. As a single element, strengthening of a pier in this manner would not necessarily be of benefit in terms of increasing capacity as defined with the left-hand side of Equation 1. However, selective strengthening of critical piers or beams with this procedure may have merit for redirecting undesirable nonlinear mechanisms. For example, strengthening a weak pier between two openings with FRP strips may result in overall rocking of the entire perforated wall. As long as inelastic deformation capacity is not required of an individual pier, this method can result in greater lateral strength and deformation capacity of the system. Additional research is being done by the MAE Center (Orton, et. al. 2001) on the concept of selective retrofit using FRP strips to validate this concept. Figure 8: Shape of force-deflection curve for pier strengthened with FRP strips.

9 Daniel P. ABRAMS 79 One other method for increasing deformation capacity is post-tensioning of piers. This method, in effect, increases the vertical compressive stress and thus promotes rocking. Piers are core drilled with the same procedure as used for reinforced cores. Unbonded tendons are draped into cores and anchored at their base with grout. Tendons are then tensioned by pulling them at the top of a wall. This procedure results in a nonlinear, but elastic type of response as shown in Figure CONCLUDING REMARKS Research described in this paper helps to demonstrate the need for a new perspective on an old problem rehabilitation of unreinforced masonry construction. New approaches for performance-based rehabilitation, developed for building systems other than masonry, have proven to be applicable for masonry as well. Enhancement of strength and deformation capacity of critical masonry components through various rehabilitation procedures has been discussed, in a conceptual manner to illustrate this new approach. The coming of performance-based engineering for rehabilitation of masonry brings an ever increasing need to better understand nonlinear deformational properties of this form of construction in its existing or rehabilitated state. Through new knowledge, more safe and economical retrofit schemes can be prescribed which will have an appreciable effect at reducing consequences from future earthquakes. 7. ACKNOWLEDGMENTS Research on seismic rehabilitation of unreinforced masonry construction as discussed herein was supported by the Mid-America Earthquake Center headquartered at the University of Illinois at Urbana-Champaign. The MAE Center is supported by the National Science Foundation through the Earthquake Engineering Research Centers Program under award number EEC Appreciation is extended to graduate research assistants, Shaun Franklin, Omer Erbay and Jaret Lynch for their assistance with the experimental work presented in this paper. Much of the content of this paper was presented in the Proceedings of International Conference on Advances and New Challenges in Earthquake Engineering Research (ICANCEER), Harbin, People s Republic of China, August 15-17, REFERENCES Abrams, D.P. (1997). Response of unreinforced masonry buildings. Imperial College Press Journal of Earthquake Engineering, 1:1, Abrams, D.P. (2001). Performance based engineering concepts for unreinforced masonry building structures. Journal of Progress in Structural Engineering and Materials, Wiley Interscience, 3:1,

10 80 SÍSMICA º Congresso Nacional de Sismologia e Engenharia Sísmica Erbay, O.O., and D.P. Abrams (2002). Seismic rehabilitation of unreinforced masonry shear walls, Proceedings of the Seventh National Conference on Earthquake Engineering, Boston. Orton, S.L., D.P. Abrams, and J. Hayes (2001). Performance of rehabilitated unreinforced masonry building systems, Proceedings of 9 th Canadian Masonry Symposium, University of New Brunswick.