Abstract
The seismic stability of multi-story structures can be characterized by an instantaneous material stiffness coefficient that represents the effect of material yielding and an instantaneous geometric coefficient that represents the effect of geometric nonlinearity (P-Δ effects). These terms, when added together, define a net stability coefficient that is related to the instantaneous eigenvalue of the structure and quantifies the susceptibility of a structure to exhibit large deformations under earthquake loading. The results of nonlinear dynamic analyses of a nine-story steel frame are used to evaluate the proposed stability coefficients as well as previously proposed methods for assessing structural stability. Data show that under earthquake loading the instantaneous material stiffness coefficient decreases due to material inelasticity while the instantaneous geometric coefficient generally increases due to the development of drift concentrations. Also data show that the column continuity has the large impacts on the structural stability and hence drift response.
