Study of the Ground-Motion Spectral Shape Effect on Structural Collapse Capacity

One of the concerns of assessing structural collapse performance is the appropriate selection of ground motions for use in the nonlinear dynamic collapse simulation. For this purpose, the selected earthquakes must be stronger than the earthquake which used in the initial design of structure. For modern buildings which have the ductile behavior against earthquake, ground motions that cause collapse are expected to be rare high-intensity motions associated with a large magnitude earthquake. The researches have shown that rare high-intensity ground motions have a peaked spectral shape that should be considered in groundmotion selection and scaling. They have shown that spectral shape, in addition to ground-motion intensity, is a key characteristic of ground motions affecting structural response. One method to account for this spectral shape effect is through the selection of a set of ground motions that is specific to the building’s fundamental period and the site hazard characteristics. The using of this method for each building faces challenges as need to selecting specific sets of ground motions. The selection of a specific ground motion set for each building is not practical; nor is it desirable because the goal is to generalize the collapse assessment results across seismic design categories. Thus, another method was presented in which uses a general ground-motion set, selected without regard to ε values, and then corrects the calculated structural response distribution to account for the target epsilon expected for the specific site and hazard level. To provide a more practical method for adjusting the collapse capacity, a simplified version of the second method presented that can be used to determine the appropriate adjustment factors for the collapse capacity distribution without requiring the computation of the epsilon values for each record and then performing a regression analysis. The development of simplified method causes that reduced computation. The collapse capacity results of three different methods were compared by several examples and the results of past research. The results show that the first two methods produce nearly identical results, with the predictions of the mean collapse capacity differing by only. Comparison of the Simplified Method and Method 2 indicates that the results are very close to each other, so that the full regression analysis results yield 0.257, which agrees very well with the simplified value of 0.254. The corresponding mean collapse capacity from Method 2 is 2.63 g as compared to the simplified value of 2.83 g. This difference of about 8% is reasonable for most applications, particularly in contrast to the alternative of neglecting the spectral shape effects. The calculated dispersion from Method 2 is 0.35, which is about 10% lower than the slightly conservative value of 0.40 used in the simplified method. This paper indicates all methods that modified the resulting collapse capacity by the spectral shape effects. Also, these methods have been compared and introduced the best method by the examples provided.