New Measure for Severity of Near-Source Seismic Ground Motion

In order to estimate seismic hazards, there is a need for measures that properly characterize the severity of ground motions. Such measures are used along with the fragility of structures to estimate the probability of failure. Structures subjected to severe seismic ground motions undergo inelastic response. Proper measures of ground motions are required to be well correlated with responses of interest. In this paper, it is shown that modeling a near-source ground motion, by a simple pulse form that matches well with the main velocity pulses of the ground motion, dose not lead to a reliable prediction of the inelastic response of structures having periods of less than about 1 s. An equivalent rectangular acceleration pulse, called the significant peak ground acceleration (SPGA), is defined. Compared to other available measures, the SPGA correlates significantly better with the inelastic response of structures (having periods of less than about 1 s and displacement ductility of at least two). Subsequently, using the main characteristics of structures, a relationship between the SPGA and the response of reinforced concrete (RC) structures is developed. It is shown that the relationship reliably predicts the inelastic response of the structures to pulse-type ground motions. Finally, the efficiency of the SPGA in estimating the fragility of an RC structural wall is compared to that of elastic response spectrum. Given the fact that in the United States about 99.8% of office buildings are less than ten stories, the SPGA and the corresponding demand models for estimating the response of such structures can considerably improve the seismic design and assessment of structures.