Data Utilization Report California Strong Motion Instrumentation Program, December 1, 2007.
This study evaluates current Nonlinear Static Procedures (NSPs) specified in the FEMA356, ASCE-41, ATC-40, and FEMA-440 documents using strong-motion data from reinforced-concrete buildings. For this purpose, three-dimensional computer models of five reinforced concrete buildings – Imperial County Services Building, Sherman Oaks Commercial Building, North Hollywood Hotel, Watsonville Commercial Building, and Santa Barbara Office Building – are developed. When appropriate, springs at the building’s base are included to account for the soil-structure interaction effects. These buildings are selected because they were strongly shaken, several deformed beyond their linear-elastic range, during past earthquakes and their recorded motions are available. The recorded motions are interpolated to obtain motions at non-instrumented floors. These motions are used to derive seismic demands – peak roof (or target node) displacement, floor displacements, story drifts, story shears, and story overturning moments. The pushover curves are developed from nonlinear static analysis of computer models of these buildings and various demands estimated from the NSP methods. A comparison of peak roof (or target node) displacements estimated from the NSPs with the value derived from recorded motions shows that: (1) the NSPs either overestimate or underestimate the peak roof displacement for several of the buildings considered in this investigation; (2) the ASCE-41 Coefficient Method (CM), which is based on recent improvements to the FEMA-356 CM suggested in FEMA-440 document, does not necessarily provide a better estimate of roof displacement; and (3) the improved FEMA-440 Capacity Spectrum Method (CSM) generally provides better estimates of peak roof displacements compared to the ATC-40 CSM. However, there is no conclusive evidence that either the CM procedures (FEMA-356 or ASCE-41) or the CSM procedure (ATC-40 or FEMA-440) lead to a better estimate of the peak roof displacement when compared with the value derived from recorded motions. A comparison of the height-wise distribution of floor displacements, story drifts, story shears and story overturning moments indicates that the NSP provides: (1) reasonable estimate of floor displacements; (2) poor estimate of drifts in upper stories due to its inability to account for higher mode effects; (3) very poor, and possibly unreliable, estimates of story shears and story overturning moments. A comparison of pushover curves from various computer programs using different modeling assumptions led to significantly different pushover curves. This indicates significant sensitivity of pushover curves to modeling assumptions which may potentially lead to different results and conclusions from the NSP.
Civil and Environmental Engineering
Jointly Funded By California Strong Motion Instrumentation Program, California Geological Survey and California Survey of Conservation