Inelastic behavior of asymmetric buildings under lateral loads

Buildings subjected to a large intensity ground motion often deform well into the inelastic range. In particular, asymmetric buildings undergo coupled lateral and torsional deformations that can lead to large deformations and ductility demands of the lateral load resisting elements. Lateral-torsional coupling is well understood within the limits of elastic deformations but is still far from being fully understood when significant inelastic deformations take place. A relationship between a system's asymmetry and its inelastic deformations is yet to be developed. The effects of plan-asymmetry; caused by stiffness, mass, and/or strength eccentricities; on the inelastic behavior of two one-story building models are studied. One model uses nonlinear springs to replace lateral load resisting elements that provide resistance in one direction. Elements with significant resistance in two directions are modeled using two orthogonal springs coupled using interaction equations. This model, although simple, does not provide accurate predictions of the inelastic deformations. The second model is more realistic since it uses finite elements to model the behavior of lateral load resisting elements that provide resistance in one or in two directions. The nonlinear incremental solution is obtained using the tangent stiffness method. One conclusion is that asymmetric building systems can be classified into three groups. Asymmetric building systems with stiffness eccentricity close to the strength eccentricity undergo rotations that are larger than linear elastic values but remain moderate in magnitude. Ductility demands and lateral displacements are also moderate. Asymmetric building systems with strength eccentricity smaller than stiffness eccentricity are the most favorable systems because their inelastic rotations decrease after first yield and converge to low plastic limits dependent mostly on the strength eccentricity. These systems are characterized as systems that yield mostly in the direction of loading. Asymmetric building systems, with stiffness eccentricity smaller than the strength eccentricity, can undergo inelastic rotations much higher than linear elastic levels. The latter are caused by large relative decreases in the torsional stiffness compared to the lateral one.