Evaluation of Nonlinear Static Procedures Using Strong-motion Records of Buildings

The objective of this investigation is to evaluate the FEMA-356 Nonlinear Static Procedure (NSP) and a recently developed Modal Pushover Analysis (MPA) procedure using recorded motions of four buildings that were damaged during the 1994 Northridge earthquake. For this purpose, the motions at non-instrumented floors are “derived” from the motions at instrumented floors by using cubic spline interpolation procedure. Analytical models of the four buildings are developed using the computer program D2DX and calibrated by matching the computed vibration periods and the “elastic” periods identified form the recorded motions. Accuracy of the computer model is evaluated by comparing the computed displacement histories with the recorded motions. Finally, the displacement and drifts from the FEMA-356 NSP and the MPA procedures are compared with the values “derived” from the recorded motions. It is found that the FEMA-356 NSP typically underestimates the drifts in upper stories and overestimates them in lower stories when compared to the recorded motions. Among the four FEMA-356 distributions considered, the “Uniform” distribution led to the most excessive underestimation or overestimation indicating that this distribution may be unnecessary. Furthermore, FEMA-356 distributions failed to provide accurate estimates of story drifts for buildings that satisfied the FEME-356 criterion for detecting the presence of higher mode effects indicating the need to carefully re-examine this criterion. The MPA procedure, in general, provides much-improved estimates of the response compared to the FEMA-356 NSP. In particular, the MPA procedure, unlike the FEMA-356 NSP, is able to capture the effects of higher modes. The error noted in few responses from the MPA procedure appears to be due to limitations associated with application of the modal combination rule, which is developed for response spectrum type applications, to peak responses from a single ground motion. For a building that exhibits dominant effects of “soft” first story, such as the Sherman Oaks 13-story building, however, neither the MPA procedure nor the FEMA-356 NSP led to reasonable estimate of the response; the MPA procedure provided reasonable estimates for the Los Angeles 19-story building that exhibited “soft” first story but this effect did not dominate the overall behavior of the building.