Analysis Of Collapse Resistance Of Offshore Rigid Frame-Continuous Girder Bridge Based On Time-varying Fragility

The key to ensure the good service performance of offshore bridge in the whole life cycle is to reduce the material durability damage caused by special bridge site environment and prevent the continuous collapse of bridge structure under the earthquake.At present,the idea of multi-level seismic design is widely used in the world,and most countries have also added provisions to relevant codes and standards to prevent continuous collapse of structures.However,in the bridge engineering field,theory and method of anti collapse analysis of bridge structures are not systematically summarized,therefore,how to put forward a set of anti collapse analysis method and quantitative criteria of bridge structure under the earthquake has become one of the hot topics in the seismic research of bridge engineering.In this paper,a 4-span offshore rigid frame-continuous girder bridge is taken as an example to analyze the collapse resistance of the bridge structure considering the time-varying durability damage of materials in the whole life cycle,the main research contents and conclusions are as follows:(1)The degradation law of all material performance parameters in the life cycle of bridge is obtained by considering the coupled effects of concrete carbonation,the chloride ion corrosion and the damaged concrete repair of the pier;the time-varying damage index limits of bridge components under different damage states in the whole life cycle are obtained by the component section analysis software-XTRACT;and a FE model of example is established to analyze its collapse resistance based on the time-varying durability damage of materials,time-varying bond slip effect and time-varying damage index.The results show that the elasticity modulus of concrete after carbonation increases by 0.6%,the yield strength of longitudinal reinforcement and stirrup decreases by 2.5%and 7%respectively when 1~#pier has been in service for100 years;and the damage indexes of the components decreased with the service life of the bridge.(2)Taking peak acceleration as the index of ground motion intensity,and selecting enough records of ground motion to reduce its discreteness;what's more,the fragility analysis process of bridge components,first and second order limit methods for fragility analysis of bridge system and the quantification criteria of fragility,importance and anti collapse robustness coefficient are analyzed based on the theoretical fragility and reliability theory,in order to put forward a set of collapse criterion and anti collapse analysis method which are suitable for the example bridge.(3)The time-varying fragility law of bridge components and its system in the whole life cycle is discussed;based on the collapse criterion proposed in this paper,the anti collapse performance of the example under different working conditions is analyzed.The results show that the fragility of bridge components and bridge system increases with the service life of the bridge,the failure probability of bridge system is higher than that of any component,and the failure probability interval of bridge system obtained by the second-order limit method is smaller,but it is very sensitive to the arrangement order of components,particularly the lower bound failure probability of the bridge system,which should be sorted according to the principle of large to small based on failure probability.(4)By considering the main shock and main aftershock,the time-varying fragility coefficient,time-varying importance coefficient and time-varying robustness coefficient curves under different algorithms are obtained based on the fragility analysis of bridge components and bridge system under the complete failure state,seismic response analysis and the robustness quantification criteria proposed in this paper in the service period.The results show that both the fragility coefficient and importance coefficient increase with the service life of the bridge,while the robustness coefficient decreases with the service life of the bridge,and the corresponding parameters are improved by different optimization algorithms;In the whole life cycle,the robustness coefficient under main aftershock is obviously smaller than that under main shock,therefore,during the service period,the main aftershock has a significant impact on the collapse resistance of the bridge structure.