Security Assessment Based On Time-dependent Reliability For Continuous Rigid Frame Bridges

Reliability is an important indicator of the safety of bridge structure.This important indicator not only can guide the design of bridge structure,but also can guide the assessment,maintenance and reinforcement of bridge structure in the operation phase.Owing to the corrosive environment and aging materials,the resistant force of the bridge structure will gradually decrease over time.With time goes by,the load of the bridge structure also changes,especially the growing vehicle traffic and increasing shaft weight in the car in recent years.Therefore,the structural reliability is a changeable variable.If the change of reliability of the bridge is not taken into account in the assessment of bridge's safe state,the reliability of the structure will be overestimated to bring catastrophic consequences.Large-scale bridge structure is often a complex high-order statically indeterminate structure.Reling on the assessment of component does not accurately reflect the overall reliability and safety of the structure.At present,it is not yet mature to achieve the bridge structure safety assessment by using structural system reliability,which urges to carry out deep and systematic research.This paper starts with the time-varying load of bridge structure and calculate the load model of bridge vehicle with WIM data.Then select the appropriate deterioration model to establish the time-varying resistance model.Finally,conclude a safety state evaluation method for continuous rigid frame bridge based on time-dependent system reliability,according to the load characteristics of continuous rigid frame bridge with improved global critical intensity-branch-bound method.The main contents and research results of this paper are as follows:(1)Long-term collection of vehicle load information is used and the probability statistics program is written based on mathematical statistics and K-S test method to establish a probability model including such parameters as the bridge lane flow,speed,weight,distance.At the same time,analyzing the stable probability model to establish the demanding minimum sample size(2)According to the obtaining lane traffic,vehicle speed,vehicle weight and vehicle distance probability model,the first step is generating random traffic to simulate the real vehicle bridge process based on the Monte Carlo method.Then,calculate the total weight probability model of the bridge and the maximum probability model of the total weight of the fleet.At the last,the maximum probability model of vehicle load effect is computed with finite element analysis in the design period is obtained.(3)Time-varying model of strength deterioration of concrete proposed by Niu Ditao and prestress loss time-varying model proposed by Zhang Zanpeng was used to establish the resistance time-varying model of structural members.Then,the method of calculating the time-dependent reliability of components is proposed.(4)According to the structural characteristics of continuous rigid frame bridge,the global critical intensity branching-bound method is improved in three aspects.The first aspect is eliminating the effect of dead load of searching for major failure units in depth,the second is improving the calculation method of residual strength of components,and the third is simplifying calculation.A search method for failure mode of continuous rigid frame bridge is proposed.And the functional function expression of the failure mode is derived.Finally,a method for calculating the reliability of continuous rigid frame bridge is established.(5)It is applied to the reliability calculation of time-varying system of a continuous rigid frame bridge with The proposed method of vehicle load model and global critical intensity branching-bound method.The results of safety assessment of the bridge show that the reliability of the bridge is 4.87 and the residual service life is 78 years.