Earthquakes worldwide have clearly demonstrated the vulnerability of reinforced concrete members to degradation in shear strength when subjected to cyclic loading. Such degradation can lead to significant damage to the structure and, possibly, even collapse. With the advancement of performance-based earthquake engineering, where the response of the structure must be traced through all levels of damage, there is a significant need to accurately define the deformation capacity and shear strength for such members. This symposium publication represents an effort from researchers across the globe trying to address this challenging problem. Although at the time of publication there are some methodologies that can be used in performance-based earthquake engineering, there is a significant need for improved methods better suited for these types of applications. Furthermore, one of the concerns often expressed by researchers is that test data used in the past to develop and calibrate existing models consisted of relatively small data sets. This problem is compounded by differences between experimental studies in aspects such as the type of load history used, the manner in which deformations were recorded during tests, and the definition of displacement and strength at failure. The recent development of the PEER column database, hosted by the University of Washington, provided a valuable resource to overcome some of these problems. It presented researchers with a larger pool of data, which included the full hysteretic response of every column in the data set. Although this represented a very significant step forward, efforts of this kind should continue to improve the ability of researchers to calibrate and evaluate models for shear strength and deformation capacity. A joint technical session was organized by Joint ACI-ASCE Committees 441, Reinforced Concrete Columns, and 445, Shear and Torsion, during the American Concrete Institute’s Fall 2004 Convention in San Francisco, CA. The goal of the technical session was to showcase recent developments in this area, with the hope that continued discussion will lead to improved models that are suitable for performance-based engineering. Note: The individual papers are also available. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP236

An analytical model that couples the flexural and shear responses of reinforced concrete structural walls is proposed. The proposed modeling approach involves incorporating RC panel behavior into a macroscopic fiber-based model. Results obtained with the analytical model are compared with test results for a slender wall and four short wall specimens. A reasonably good lateral load-displacement response prediction is obtained for the slender wall. The model underestimates the inelastic shear deformations experienced by the wall; however, shear yielding and coupled nonlinear shear-flexure behavior are successfully represented in the analysis results. The model captures accurately the measured responses of selected short walls with relatively large shear span ratios (e.g., 1.0 and 0.69). Discrepancies are observed between the analytical and experimental results as wall shear span ratios decrease (e.g., 0.56 and 0.35). Better response predictions can be obtained for walls with low shear span ratios upon improving the model assumptions related to the distribution of stresses and strains in a wall.

The new provisions in the 2004 Canadian code for flexural displacement capacity of concrete walls, and the new provisions for seismic shear design of slender concrete walls are presented. To facilitate explanation of the seismic shear provisions, general expressions for shear design are first presented, and the non-seismic shear design provisions in the Canadian and ACI 318 building codes are briefly reviewed. According to the new seismic shear design provisions presented here, the maximum shear force and concrete contribution depend on the inelastic rotation demand in the plastic hinge, and the compression stress (critical crack) angle used to determine the quantity of horizontal reinforcement depends on the axial compression stress applied on the wall. The 2004 Canadian code provisions generally require more horizontal reinforcement than the ACI 318 provisions except when inelastic rotational demand is small and axial compression stress is large; however, the Canadian provisions permit significantly higher shear stress for high-strength concrete walls. The new provisions can be used to design concrete walls given the expected level of drift demand or, as demonstrated in this paper, can be used to estimate drift capacity of walls accounting for the significant influence of shear.

Past RC panel tests performed at the University of Houston show that reinforced concrete membrane elements under reversed cyclic loading have much greater ductility when steel bars are provided in the direction of principal tensile stress. In order to improve the ductility of low-rise and mid-rise shear walls under earthquake loading, shear walls have been designed to have steel bars in the same direction as the principal tensile direction of applied stresses in the critical regions of shear walls. This paper presents the test results of shake table tests on two shear walls and two shear walls under reversed cyclic loading. In the specimens under shake table tests, steel bars were provided at angles of either 90 degrees or 45 degrees to the horizontal. In the reversed cyclic tests, one-half of the steel bars were placed at an angle of 45 degrees to the horizontal in the low-rise shear wall and at an angle of 65 degrees to the horizontal in the bottom portion of the mid-rise shear wall. Based on the experimental results, the tested shear walls with reinforcement oriented close to the principal tensile direction of applied stresses have greater ductility than that of the conventional shear wall.

A model is presented to quantify the reduction in shear strength caused by repeated load reversals in the inelastic range of response of reinforced concrete members. The monotonic shear strength is calculated by the superposition of components related to arch-action, truss-action, friction, and the shear strength of the uncracked compression zone. The reduced shear strength is calculated as a function of the initial monotonic strength, the deformation of the member, the axial load, and the amount of transverse reinforcement used for confining the concrete. An analysis of experimental results showed that the reduction in shear strength in members subjected to load reversals was caused by progressive reductions in the strength of the arch mechanism, the compression zone component, and the truss component. The theoretical model and the test data indicate that contributions from the friction and the shear strength of unconfined concrete should be neglected in this case.