A next generation scalar intensity measure for analytical vulnerability studies

Abstract

To assess the seismic performance of a structural system within an analytical context, we need, among others, to specify a ground motion Intensity Measure (IM). The wary IM selection is undoubtedly an important step towards the successful implementation of a risk assessment, since insufficient and/or inefficient IMs can induce unwanted bias and variance in the vulnerability estimates. Supplementary issues related to practicality, necessitate the use of IMs for which ground motion prediction relationships exist, such as the elastic response spectral values (i.e. acceleration, velocity and displacement). Several past studies suggested as an improvement the use of IMs defined as the geometric mean of spectral acceleration values computed over a period range. The latter range may span between periods that are below, at or above the fundamental one. Some of these choices were proven to significantly improve both efficiency and sufficiency of the IM compared to more commonly used counterparts. This study investigates the efficiency and sufficiency of a newly developed scalar IM that combines the geometric mean IM concept with the significant duration of the ground motions. Improving the geometric mean IMs via including the significant duration of the ground motions, was driven by recent findings suggesting there is a strong tie between the collapse capacity of a structure and the ground motion duration. Hence, the performance of the proposed next generation IM is addressed in detail by means of comparisons and statistical significance tests. The testing is performed at specific levels of local engineering demand parameters that are closely related to losses, using a testbed capacity-designed steel moment-resisting frame. It was demonstrated that ground motion duration is closely related to the collapse capacity whereas its effect at lower demand levels is insignificant. Hence, the proposed IM may be employed to improve the estimates in collapse assessment studies. Nevertheless, at least for steel moment-resisting frame buildings that exhibit moderate cyclic degradation rates and sustain most losses prior to the global collapse state, the significant duration is anticipated to only minimally affect the evaluated vulnerability and consequently may be disregarded.