Static Nonlinear Analysis Methods For Estimating Seismic Performance For Bridges

With the deepening of understanding of seismic damage to bridges and the development of the computer technology, nonlinear analysis and calculation for bridges gradually replace linear analysis theory. Nonlinear time history analysis (RHA) is considered to be the most accurate method for bridges elasto-plastic analysis, but it isn't widely used because of its complex technology, complicated calculation and miscellaneous result processing, and the accuracy of RHA depends a great extent on the input ground motion. Recently, with the development of performance-based seismic design thought, pushover analysis procedure has been accepted and gradually used for estimating the seismic performance for bridges because of its simple concept and easy calculation. All the classic pushover analysis procedures are based on the invariant force distributions, but none of the invariant force distributions can account for the contributions of higher modes to response, or for a redistribution of inertia forces after structural yielding and the associated changes in the vibration properties of the structure. So the classic pushover analysis procedures are just suitable for the beam bridge with low piers, not for the beam bridge with high piers and not for cable-stayed bridge. To overcome these limitations, several improved pushover analysis procedures are proposed, and an improved pushover analysis procedure which is applicable to cable-stayed bridge is put forward. The main research work covers the following aspects:1. The adaptive spectra-based pushover analysis (ASPA) procedure has been proposed by Gupta which accounts for the effect of higher modes and overcomes the shortcomings of the classic pushover analysis procedures. While it provides better estimates of seismic demands, it is conceptually complicated and computationally demanding for routine application in structural engineering practice. In this paper, an improved adaptive spectra-based pushover analysis (IASPA) procedure is proposed by using the thought of neglecting the coupling of the N modes in modal pushover analysis procedure (MPA). And the seismic performance determined by IASPA is compared with pushover analysis using two force distributions in FEMA-274 and nonlinear RHA procedures. The results indicate that IASPA procedure provides more superior accuracy in estimating seismic performance on bridge structures because that it accounts for the effect of higher modes and for a redistribution of inertia forces after structural yielding. And because of its neglecting the coupling of the N modes, it is more convenient for structural engineering practice than ASPA method. 2. Three piers in different height are investigated by the IASPA procedure and nonlinear RHA method. And the effects of higher modes to seismic performance of the piers in different height are studied. The results indicate that higher modes have little effects on medium or low piers but great effects on higher piers. The medium or low piers can be equivalent to single freedom systems and be estimated by the classic pushover analysis procedures. But neglecting the inertia force of pier body and the effects of higher modes will lead to big errors for piers higher than 40m.3. The results of the classic pushover analysis procedure based on force for estimating the seismic performance of the bridge depend on the selection of the lateral load pattern. Earthquake damage, experiment and theoretical analysis all indicate that the deficiency of transfiguration capacity and capacity of energy dissipation is the main reason of structure collapse. The degree of damage of the structures in large earthquake relates to their displacement responses and deformability. So it is more reasonable to control structural behavior by displacement. By studying the change of mode participation coefficient and displacement pattern of a six-story-frame in pushover process, in this paper, an adaptive displacement-spectra-based pushover analysis procedure (DASPA) is put forward which accounts for the effect of higher modes and for a redistribution of inertia forces because of structural yielding and the associated changes in the vibration properties of the structure.4. Pushover analysis is carried out for two bridges of different height by DASPA procedure and other two classic pushover methods. And the seismic performance determined by the three pushover analysis methods is compared with nonlinear RHA method. The results indicate that DASPA procedure provides more superior accuracy and reasonability than other two classic pushover methods. Because DASPA is based on displacement directly, and the behavior of the structure in the earthquake is controlled by displacement in the whole processing, it omits the process of transformation the internal force to deformation to estimate the seismic performance of the structure. So it has the simpler calculating process and computational attractiveness for routine application in structure engineering practice.5. Cable-stayed bridge belongs to a typical multi-degree-of-freedom system which has complicated vibration characteristic. The seismic performance of the cable-stayed bridge can't be estimated by classic pushover analysis methods because of the great effects of higher modes on it. But DASPA procedure can be used to estimate the seismic performance for cable-stayed bridge because it considers the contributions of higher modes to response and the changes in the vibration properties of the structure after structural yielding. But seismic performance determined by DASPA for cable-stayed bridge depends on the loading direction because of the special bridge type for cable-stayed bridge. In this paper, a modified DASPA procedure (MDASPA) is put forward that the post-yield displacement pattern is modified to be close to practice. The seismic performance determined by DASPA and MDASPA is compared with nonlinear RHA procedure. The results indicate that the MDASPA procedure provides more superior accuracy in estimating seismic performance for structures compared with DASPA procedure.