Accurate Methods for Elastic Seismic Demand Analysis of Reinforced Concrete Walled Buildings

Concrete walls are used commonly as part of the lateral-load resisting system for buildings in regions of high seismicity. Damage to walled buildings in recent earthquakes has highlighted the seismic vulnerability of these systems. In recent years, most research addressing the seismic design of walls has employed experimental testing and focused on detailing of boundary element reinforcement to improve wall deformability. The research presented here employs numerical modeling and focuses on determining appropriate moment and shear demands for use in design. Previous research by the authors employed experimental data to develop a computationally efficient model that provides accurate simulation of flexural wall response, including compression- and tension-controlled flexural failure modes that have been observed in the laboratory and field. The research presented here used this model to develop validated expressions of the shear demand and the moment envelope. To do so, the earthquake response of idealized walled buildings, ranging in height from six to thirty stories, was numerically simulated. Initially, a series of walled buildings was designed using current U.S. code requirements, with moment and shear demands determined using both the equivalent lateral force (ELF) procedure and elastic modal response spectrum analysis (MRSA). Nonlinear dynamic analyses of these code-compliant buildings were conducted using a set of far-field ground motions scaled to various intensity levels. The results of the nonlinear analyses indicated the shear demands developed during earthquake loading exceed the design demands that were calculated using the elastic analysis methods. This could be expected to result in walls developing undesirable failure modes and exhibiting reduced deformation capacity. Because nonlinear analysis is not practical for design of many walled buildings, the nonlinear analysis results were used to (1) develop new procedures for determining the seismic shear demand to ensure flexure-controlled response and (2) identify moment envelopes for use in design that ensure flexural yielding is isolated to locations identified by the engineer. A suite of walled buildings was designed using the new recommendations. Analyses show that use of these new procedures results in a shear demand/capacity ratio less than 1 and controlled flexural hinging. In addition, the response modification coefficients (i.e., R-factors) were revisited; the results show that lower R-factors are needed for walled buildings with planar or asymmetric walls to achieve acceptable collapse risk.