Research Note» Seismic Response of a Multi-Span Pile and Deck Structure to Near and Far Field Horizontal Excitations

During the past decades, there has been an ever increasing demand for construction of new wharves and port facilities in Iran. This is partly because of the recent rapid demographic, social and economic developments, increase in offshore oil and gas activities and also long coastlines available in the country. Wharves، Piers and jetties are lifeline structures, which provide a cost effective means for transporting large quantities of goods and raw materials into and out of a region. Harbor structures also play a central role in management of an abnormal condition such as an earthquake. For long structures such as wharves and jetties, transient ground deformations inducedby seismic wave passage and effects from the spatial difference in ground motion become important. On the other hand, if a port facility is located close to the seismic source, it will become subject to the near field pulse type excitations. An analytical approach has been used to address the problem. Linear and non-linear response of simplified structural models to near-fault and far-fault strong ground motions is investigated. The model approximates a structure by an equivalent oscillator consisting of a rigid beam supported by several rigid mass-less columns. Bi-linear rotational springs connect the columns to the deck and the base of the structure. The system has been studied under two pulses type (fault-normal pulse and fault-parallel pulse) and 108 near field and far field earthquake excitations. In this study closed form solutions for the dynamic response of multi span pile and deck structures have been developed. Material non-linearity, large deformations and P   effects are taken into consideration. The central difference method is employed to solve the nonlinear equations of motion. The model is then used to analyse the structure under incident pulse type near-field excitations. Effects from strain-hardening, columns height, the seismic source distance, magnitude of the excitation, type of the excitation (pulse or random), have been investigated and compute the reduction factor and compare with the common design rules. In general it has been found that the classic design criteria for the strength reduction factors provide non-conservative results in a number of occasions.