Research On Simplified Analysis Method Of Seismic Response For Continuous Girder Bridge Based On Pushover Analysis Method

Considerable Research is under way throughout the world to establish performance based seismic design theory which is treated as the future development direction in the United States and Japan at the forefont of earthquake resistant design of structures. Performance based seismic design theory is expected to be goal of bridge earthquake resistant design practise in 21th century. A simple and effective analysis method to determine seismic response of structures is needed firstly in the comprehensive application of performance based seismic design theory.Because modifications of earthquake resistant specifications focus on performance based seismic design theory the work presented in this dissertation is concerned with pushover analysis method (POA), capacity spectrum method (CSM) and modal pushover analysis (MPA) based on Unit 5 of north approach of Sea-Bridge.The main research contents are as follows:A fiber model nonlinear beam-column element based on flexibility approach is studied systematically. Fiber model nonlinear beam-column element based on flexibility approach and the stress-strain relation of steel and concrete used in this dissertation are described respectively.The elastic pushover analysis with return period about 500 years and the nonlinear pushover analysis with return period about 2500 years are carried out to three different type models which are based on Unit 5 of north approach of Sea-Bridge as the engineering background. Then the effects of the monitoring point and lateral load pattern on the calculating accuracy of POA are studied extensively. For comparison purposes time history analysis, which is considered to be the most accurate and reliable method of seismic analysis, is also performed. It can be concluded that: In the transversal direction POA with lateral load pattern 1 and monitoring joint selected method B or C predicts well seismic response of structures; while the magnitude of error depends on structural characteristics (natural period, modal participating mass ratios, soil-pile-structure interaction and pier-to-girder isolation bearings) shears and bending moments at the base of pile caps are overestimated significantly for the fundamental period less than 2s when the elastic POA is performed in the transversal direction; if plate type elastomeric pad bearing is adopted in the pier-to-girder connection only transversal elastic POA is enough.Basic step, basic theory and step-by-step procedures of CSM are summarized; Studies on the three type relationships between yield strength factor and ductility adopted by Chopra indicate that the inelastic design spectrum is a reliable approach to estimate the earthquake-induced deformation of yielding systems. And basic steps of improved CSM proposed by Chopra and improved CSM proposed in this dissertation are described respectively; improved CSM proposed by Iwan is introduced. Comparative with CSM developed by ATC-40 it appears that an alternative representation of improved CSM proposed by Iwan for the optimal equivalent linear period and damping parameters could result in a marked improvement in the accuracy of response results based on nonlinear SDOF systems for a seires of periods and ductilities.With the further study on CSM and the relationships between yield strength factor and ductility the simplified CSM based on elastic POA and nonlinear POA is proposed respectively; based on Unit 5 of north approach of Sea-Bridge as the engineering background three different type models are analyzed. Through the elastic seismic response analysis with return period about 500 years and the nonlinear seismic response analysis with return period about 2500 years the calculating accuracies of typical CSM (ATC-40 CSM, Iwan CSM, improved CSM and simplified CSM proposed in this dissertation) are compared and lead to the following conclusions: In the longitudinal seismic response analysis simplified CSM B (Simplified CSM to elstic seismic analysis), which is better in all methods mentioned above, predicts well seismic response of piers on which the fixed bearing or plate type elastomeric pad bearing is adopted, but provides a bad estimate to piers on which the sliding bearing is adopted; in the transversal elastic seismic response analysis simplified CSM proposed in this dissertation provide a good estimate of seismic response to the structure whose fundamental period is greater than 2s; in the transversal nonlinear seismic response analysis simplified CSM B proposed in this dissertation is a simple and effective method; In all methods mentioned above accuracy of CSM proposed by Iwan is relatively low; the calculating accuracy of simlified CSM A and improved CSM in this dissertation is identical.With the considerable research on MPA simplified MPA based on elstic POA and nonlinear POA, as well as the MPA applicable to bridge, is proposed respectively; The elastic seismic response analysis with return period about 500 years and the nonlinear seismic response analysis with return period about 2500 years are carried out to three different type models which are based on Unit 5 of north approach of Sea-Bridge as the engineering background. With the time history results as the baseline the calculating accuracy of typical analysis methods (MPA based on ATC-40 CSM, MPA based on Iwan CSM, MPA based on Chopra improved CSM, simplified CSM and simplified MPA proposed in this dissertation) are compared and lead to the following conclusions: In the longitudinal seismic response analysis the calculating accuracy of simplified MPA is better in all methods mentioned above; Because the middle piers are in worse forcing status commonly simplified CSM B provides a good estimate of structural seismic response to satisfy design requirement; in the transversal seismic response analysis simplified CSM B predicts better girder displacements in all methods mentioned above, and the accuracy of all methods to assess the other seismic response depends on the dynamic characteristics of structures; In all methods mentioned above the accuracy of MPA based on Iwan CSM is relatively low; Accuracy of simlified CSM A and MPA based on improved CSM in this dissertation is identical.