Abstract
Ground motions at sites close to a fault are sometimes affected by forward directivity, where the rupture energy arrives at the site in a form of a very short duration pulse. These pulses impose a heavy demand on structures located in the vicinity of the fault. In this research, a probabilistic seismic demand analysis (PSDA) for a self-centering bridge is carried out. The bridge columns consisted of unbonded, post-tensioned, concrete-filled, fiber-reinforced polymer tubes. A bridge model was developed and non-linear time history analyses were performed. Three different methodologies that used spectral accelerations to predict structural responses were used, and a time-domain approach was used for PSDA. In addition to the three approaches, a time-domain PSDA methodology was also used. The results of the PSDA from the four approaches are compared, and the advantages of using the time-domain methodology are discussed. The results of the PSDA showed that for a site located very close to the fault (6 km in this study), earthquakes having a magnitude (Mw) as small as 6.5 can be significantly hazardous because the periods of pulses generated by small magnitude earthquakes coincide with the periods of the bridge. Since small magnitude events occur with greater frequency than large magnitude events, they can have important contributions to risk.
Highlights
Current practices in the seismic design of structures involves the determination of earthquake loads using seismic hazard analysis, which is commonly performed within the framework of probabilistic seismic hazard analysis (PSHA), and these loads for structural design are used within the design framework dictated by building codes [1]
An alternative approach is to use performance-based seismic design (PBSD), which refers to a framework that allows for structural and non-structural design decisions to be made in terms of the mean annual rate of exceedance of decision variables (DV) such as dollar value of loss, death toll due to collapse, or downtime under different seismic demands
For the non-linear time history analyses of the bridge, the ground motions were extracted from a database compiled by Sehhati et al (2011) that included a set of 27 Forward directivity (FD) and 27 non-forward directivity (NFD) ground motions
Summary
Current practices in the seismic design of structures involves the determination of earthquake loads using seismic hazard analysis, which is commonly performed within the framework of probabilistic seismic hazard analysis (PSHA), and these loads for structural design are used within the design framework dictated by building codes [1]. PBSD accounts for all sources of uncertainty in computing exceedance curves for the DV These computations involve, among other steps, the computation of the mean annual rate of exceedance of parameters that characterize structural response through engineering demand parameters (EDPs). The computation of these exceedance rates is known as probabilistic seismic demand analysis (PSDA) [1]. PSDA was applied to a self-centering bridge that was affected by near-fault ground motions. The paper introduces a time-domain methodology for computing structural responses of the bridge to the pulse-like ground motions resulting from forward directivity effects.
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