6. Simplified Seismic Evaluation of Structures Using Adaptive Pushover Analysis

A simplified procedure for evaluating the seismic performance and retrofit of existing low to mid-rise reinforced concrete buildings was first presented by Bracci et al. (ASCE J Struct Eng 123(1):3–10, 1997). The procedure is derived from the well-known Capacity Spectrum Method (Freeman, Prediction of response of concrete buildings to severe earthquake motion. In: Douglas McHenry international symposium on concrete and concrete structures, SP-55. ACI, Detroit, pp 589–605, 1978) and is intended to provide practicing engineers with a relatively simple methodology for estimating the margin of safety against structural failure. The procedure consists of constructing a series of seismic story demand curves from modal superposition analyses wherein changes in the dynamic characteristics of the structure (natural frequencies, mode shapes and linear viscous damping) at various response phases ranging from elastic to full failure mechanism are considered. Then these demands are compared to the lateral story capacities as determined from an independent adaptive pushover analysis, where the distribution of lateral forces is based on stiffness dependent story shear demands. The adaptive pushover analysis forms a critical aspect of the methodology which relies on a reasonable estimate of the failure modes of the structure. The proposed technique is applied to a one-third scale three story reinforced concrete frame model building that was subjected to repeated shaking table excitations, and later retrofitted and tested again at the same intensities. This study shows that the procedure can provide reliable estimates of story demands vs. capacities for use in seismic performance and retrofit evaluation of structures.