Earthquake Duration Effect on Structural Reliability

The duration of ground motion has been shown to have a significant effect on the level of damage sustained by engineered structures during moderate to severe earthquakes. In this technical note, the authors present and apply an approach that makes use of the fundamental structural period, yield strength, and their regressive relationship with the number of reversed deformation cycles during a specified duration. This approach is applied in order to quantify the effect of earthquake duration on structural reliability using (1) a low-cycle damage limit state and (2) an ultimate strength limit state. The theory of order statistics was combined with an extreme value distribution model of the deformation response peaks in order to provide full coupling of the seismic hazard with the structural response. A Monte Carlo simulation framework was utilized to estimate the reliabilities of a suite of elastoplastic oscillators representative of structures having significantly varying heights, ductility, and yield strengths. The low-cycle damage limit state functions were written in terms of a special case of the Park-Ang damage model for elastoplastic oscillators and were expressed in terms of the oscillator characteristics and the four largest peaks of the deformation response. It was concluded based on the mean structural reliability indices for a suite of nonlinear systems that earthquake duration has a significant enough effect that it should be considered in seismic reliability analyses that focus on either low-cycle damage or on ultimate strength.