Inelastic structural responses due to bidirectional seismic excitations

A study has been made of the seismic response ofinelastic structural columns subjected to bidirectionalground excitations. The prime concern of this study is toexamine the significance of including the work hardeningproperties in the resistance function of the structuralmembers under biaxial excitation conditions. Literaturecontains no information as to the effect that work hardeninghas on the inelastic dynamic response, specifically under bidirectional excitations.For this purpose, inelastic columns are modelled assingle mass two degrees of freedom models whose resistancefunction is one of the following types: (1) elasto-plastichysteretic type; and (2) hysteretic type with work hardeningfeatures. In each model, the inelastic forces interaction iseither taken into account or ignored. The system parametersdeemed significant are: (1) the amount of work hardeningassumed in the post - yield range as denoted by the workhardening coefficient; (2) the natural period of vibration ofthe system; and (3) the severity level of base excitation.Response parameters of interest are: (1) displacementductility demand; and (2) energy input.It is found that the significance of work hardening isfrequency dependent. For stiff structural elements, inelasticresponse is sensitive to the amount of work hardeningassumed, and special care should be given to the selection ofthe work hardening coefficient in the used model. For suchstructures ignoring work hardening features may seriouslyov~restimate the response, especially under severe excitationconditions. In such cases, inelastic biaxial interaction isalso significant. For flexible structural elements, includ-ing work hardening has little bearing on the inelasticresponse and hence it is suggested that the simple elasto-plastic type of behavior can be used in such cases.