NONLINEAR SEISMIC ANALYSIS OF MULTI-SPAN SIMPLY SUPPORTED VIADUCT

Abstract

During the past earthquakes, it has been observed that bridges failed due to unseating of bearings, failure of bearings or pounding besides failure of piers in flexure or shear. It is essential to carryout non-linear modeling of the bridge in order to study the seismic response of a bridge/viaduct. An extensive literature review of the current engineering practice and code criteria for bridge modeling and analysis was carried out to guarantee consistency with the proposed modeling guidelines and recommendations. A stick model was used to simulate a doubly curved viaduct having 8 simply supported spans @ 30 m c/c with different pier heights. It was subjected to the N-S component of the El Centro earthquake of May 1940. Three models have been used: a conventional stick model, an improved stick model and a 3D finite element model. The improved stick model is obtained by making use of body constraints. The expansion joint was modeled using a non-linear gap element. The difference between the first eight frequency of improved stick model and the finite element model was 10%. The effect of curvature is to increase the shear forces and bending moments in some piers by 35%. However, the improved stick model did not predict torsion in piers satisfactorily. A 3-span simply supported viaduct was modeled using finite element package SAP2000 with due consideration of impact and separation, nonlinearity of the bearings and the pounding between the adjacent spans and cyclic yielding of materials. Discrete zero-length spring elements are used to model bearings. The friction force between the superstructure and the support is modeled by linear Coulomb friction element. Various states of damage of bearings are represented by the variation of shear capacity. Pounding effect is modeled using a compression only gap element. The nonlinear behavior of reinforced concrete pier is represented by a hysteresis fiber model. The geometric nonlinearity of RC pier due to the P-A effect is also considered. The pounding has significantly effects on acceleration, displacement and axial forces of decks and an increase, in acceleration up to 750% at the instant of pounding was observed. 50% of the bearings are found to excessively damage from damage assessment study of bearings. The difference between linear and nonlinear response were about found to be varies between 25% to even up to 500%, which accounts the importance of nonlinear analysis..