Seismic Vulnerability Analysis For Highway Bridges In Wenchuan Region

The aim of this dissertation is to construct a complete seismic vulnerability analytical process for highway bridges in the Wenchuan region. Based on seismic records and damaged bridge data collected from the Wenchuan Earthquake, empirical seismic vulnerability models for the bridges in the Wenchuan region were developed, and a more accurate and perfect analytical seismic vulnerability models for typically simply supported girder bridge were also developed. In the study, the suitability of different ground motion intensity measures was discussed, and the seismic risks of highway bridge system were evaluated. In the dissertation, the main work focused on following aspects:In Chapter2, a basic flowchart of seismic risk analysis was introduced and seismic risk assessment of transportation network system was reviewed. The summary and analysis of research history and status for bridge seismic vulnerability models were carried out.In Chapter3, general situation and main features of the Wenchuan Earthquake were introduced, basic information of the highway bridges in the Wenchuan region and damage data from the earthquake were classified. By comparing two different calculation methods for estimating ground motion intensity, a modified Wenchuan Earthquake strong motion attenuation model was developed for estimating ground motion parameter PGA at various bridge sites. Two-parameter lognormal distribution functions were assumed and used to develope fragility functions. Combined with the surveyed earthquake damage data, two different statistical models were used to estimate the parameters based on maximum likelihood method. According to the type and line style of bridges, the surveyed bridge samples were categorized and different vulnerability models for sub-sample bridges were established. Results from goodness of fit test were introduced. The results showed that calculated fragility functions could not be rejected under the significance level of10%.In Chapter4, basic data of typically simply supported girder bridge in the Wenchuan region was introduced and nonlinear finite element model of the bridge structure was built by OpenSees. In the structural parameters, material constitutive, simulation method of the bridge principal components including main girder, bearing, pier, cap-beam, foundation, abutment etc were analyzed in detail. The records recorded from the Wenchuan Earthquake, after baseline correction, frequency filtering, truncation were proceeding for the ground motion records, were chosen as ground motion input for nonlinear dynamic transient analysis of the bridge. Taking the Shifangbajiao records as an example, nonlinear seismic response analysis of the bridge example was carried out, the excitation responses of the principal bridge components were recorded, and the results were analyzed and compared.In Chapter5, a basic theoretical framework of probabilistic seismic demand analysis was introduced first. The uncertainty of bridge parameters was analyzed and a set of finite element model samples of bridges were produced by Latin hypercube sampling method. Nonlinear dynamic transient analysis was preformed using the Wenchuan Earthquake ground motions in which seismic peak responses of the bridge components were recorded and the probabilistic seismic demand modals of the bridge principal components were built. Furthermore, the most popular ground motion intensity measures and the evaluation criterion were introduced. The seismic demands of bridge components were analyzed by principal component method for reduction of dimensions. By using the regression analysis method, a probabilistic relationship between ground motion intensity and composite seismic demand was established. On this basis, different ground motion intensity measures were evaluated and compared.In Chapter6, the limit states of bridge components at each bridge damage state were analyzed and determined first. Based on logarithmic normal distribution assumption, components seismic fragility curves at each bridge damage state were generated based on probabilistic seismic demand modals developed in last chapter. By adopting the reliability theory and calculation, a lower and upper bounds of bridge system vulnerability were obtained. Furthermore, extended incremental dynamic analysis for bridge examples were developed by considering uncertainty of bridge parameters. Employing two approaches based on EDP-criterion and IM-criterion, the discrete probabilistic data points for different intensity measures were calculated, and seismic fragility curves of components and system for each bridge damage state were generated by using the least squares regression. Finally the fragility results by different methods were compared, and the causes of difference and improvement measures were analyzed and discussed.In Chapter7, the seismic risk assessment of highway bridge system in Wenchuan region was carried out. The basic theory of the seismic hazards was summarized. The seismic activity and fault parameters was analyzed, a set of earthquake scenarios by different magnitudes were produced. Reconstruction cost of categorical bridges and damage ratio to different damage states were determined. Combined with the fragility functions of categorical bridges in Chapter3, mean value and variance of seismic loss of the highway bridge system in Wenchuan region by each scenario were estimated, risk curve of the system loss was generated.Chapter8gave a summary for this dissertation. The main creative point and some research work that needs to be carried out in the future were pointed out.