Fragility Analysis Of Offshore Isolated Bridge In The Whole Life Circle Based On The Material’s Degradation

With the maritime sovereignty and the offshore resource being paid more attention, the offshore engineering such as the sea crossing bridge is of great importance for resource explorations, promoting regional economic development and maintaining sovereignty. The sea crossing, as the life line system, is the significant hinge of economic ties between coastal cities. The isolation design is adopted to reduce the earthquake disaster. While the seismic performance evaluation for offshore bridge under the frame work system of the whole life cycle design is not sufficient. In this paper, with the corrosion law of the reinforced concrete and the isolation bearing under marine environment, the seismic performance of offshore isolated bridge under the whole life cycle was evaluated from the three aspects including determining the damage indexes, the vulnerability comparison between offshore isolated and non-isolated bridges and the fragility of offshore isolated bridge based on the whole life cycle period. It contained as follows:1. The deterioration law of the reinforced concrete and the isolation bearing: the corrosion analysis process of chloride ion against to the reinforced concrete structure under the whole service period for the bridge engineering was summarized combing the basic theory of chloride ion corroding the reinforced concrete structure and referring to the research results by the predecessors including the process of chloride ion transferred in the concrete covering layer, the corrosion effect on the steel and the decreed of concrete cracking. And the performance degradation rule of the rubber isolation bearing under the marine environment and the aging effect was inducted.2. The seismic performance level and damage index research on the offshore isolation bridge: the seismic performance was divided into five levels including the fully running state, running state, basis running state, life safety state and collapse prevention state.Combing with the unique constructs of offshore isolation bridge, the pier top drift rate was selected as the pier damage index and the bearing shear strain as the bearing damage index.The Pushover method was carried out to analyze and quantize the damage index for the offshore isolation bridge pier stud. Besides, the damage index was estimated by the method of IDA. The result showed that it was reasonable to determine the damage index of offshore isolation bridge by the method of Pushover.3. The comparison analysis based on the seismic performance of the offshore bridge fragility: ‘The seismic oscillation-isolation structure’ sample model was generated combining the random sample which is generated by the Latin hypercube sampling method and the stochastic seismic motion model. The nonlinear time history analysis for the sample model was carried out by the IDA method, the result of which could provide the seismic response of the bridge pier and the isolation bearing. The Probabilistic Seismic Demand Model(PSDM)was built up by regression analysis. Then the seismic fragility curves of components were analyzed by using the damage indexes of bridge pier and the isolation bearing. The system vulnerability curves were plotted by the grace method in order to estimate the seismic performance for the whole structure. Finally the finite element model was set up for the non-isolated bridge. To verify the isolation rubber bearing with excellent vibration isolation effect, the seismic fragility curves of was used to estimate the seismic performances of non-isolated and isolated bridges.4. The fragility analysis of offshore isolation bridge based on the material performance degradation: the finite element model of offshore isolation bridge under different service periods. And the seismic oscillation-isolation structure sample was reset to analyze the vulnerability of the sample model. The fragility curves of the components and the whole bridge structure were obtained, and the vulnerability curves of probability in different service periods was compared, the seismic performance was estimated in the whole life period,all which providing some reference and guidance for the seismic reinforcement of the offshore isolation bridge.