Publication

Abstract : Directivity of fault ruptures modifies the ground motion in the direction of rupture (forward directivity) to have large amplitude shorter duration pulses, and the ground motion opposite to the rupture propagation (backward directivity) to have long duration low amplitude pulses, compared to the ground motion at locations perpendicular to the direction of rupture at similar rupture distances (Rrup). Directivity is prominent in the fault-normal component of ground motion. Multiple extents of directivity scenarios (4 scenarios; 1 symmetric bilateral, and 3 directivity scenarios of different unilaterality) of magnitude 7 on a vertical strike-slip fault (analogous to the San Andreas fault in Southern California) are simulated at a few racetrack locations, of which one forward and one backward directivity station are included. Three racetracks (Rrup 5 km, 10 km, and 15 km) are considered within the near-field where NGA West-2 relations suffer for limited data. The simulated ground motion is applied as base excitation to 2-span seat abutment pre-1970 Californian skew bridges of different skew angles (0°, 15°, 30°, 45o, and 60°), with bridges oriented parallel to the fault and perpendicular to the fault. Probabilistic seismic demand models (PSDMs) are computed for each skew angle at each rupture distance and scenario. Collapse fragility curves are derived from PSDMs, and the median and dispersion are compared with those of HAZUS prescribed values.