Cable-stayed bridges subjected to near-fault vertical motion

The existence of a horizontal large velocity pulse that contains a substantial proportion of the seismic energy, often referred to as a 'fling', tends to increase seismic demand on medium-to-long period structures. Vertical ground motion is most intense and damaging in near-fault region, which openings the possibility that vertical motion may also include strong velocity pulses. An investigation of possible 'fling' features in vertical ground motion and their effects on cable-stayed bridge structures is presented in this paper. It is endeavored to identify and explain the reasons for strong velocity pulses in vertical ground motion. A set of earthquake records from recent large thrust events, which provided abundant near-fault vertical ground motion records, was selected and analyzed to identify large velocity pulses. Comparison is made with a set of normal acceleration records. Detailed numerical models of a class of cable-stayed bridges were developed, based on an existing bridge structure. The strong-motion ensemble is used for dynamic geometrically-nonlinear response history analysis of the cable-stayed bridges. Global and local response parameters were monitored and compared. The results indicate that pulse-like vertical ground motion increases moment and rotation demands along the bridge deck and should therefore be considered in seismic design and assessment.