Research On Rational Seismic System Of Small And Medium-span Girder Bridges Based On Fragility

Small and medium-span bridges have many advantages such as convenient construction,simple structure,and reasonable economy,and are widely used in bridge construction,accounting for about 87% of the total number of bridges built in China,but earthquakes often happens in China,and the existing earthquake damage data show that the number and severity of earthquake damage on small and medium-span bridges are more prominent than those on grand bridges and great bridges under strong earthquakes.Moreover,with the promulgation of the National Seismic Mitigation Plan and the development of the seismic isolation technology,the adoption of new seismic isolation measures on small and medium-span bridges will be a major development trend in the future bridge construction in China.As one of the most advanced theories in the seismic design of structures,the performance-based structural seismic design is reflected in the mainstream seismic design codes of various countries to varying degrees.The full-probability-based structural seismic performance assessment framework,as an important part of this design idea,can not only take into account various uncertainties in the bridge design,construction and service process,but also intuitively quantify the seismic performance of the structure from a probabilistic point of view by combining structural damage characteristics.Based on this,this paper further improves the definition of reasonable seismic system for small and medium-span bridges,which are widely distributed and have serious seismic damage,and proposes a set of reasonable seismic system evaluation process for small and medium-span bridges in China,and quantifies the influence of various uncertainties on their seismic reasonableness and seismic capacity analysis.The main works are as follows:(1)The concept of reasonable seismic system for small and medium-span bridges was further refined,and the evaluation process of reasonable seismic system for small and medium-span bridges applicable to China was proposed by combining the full-probability based structural seismic performance evaluation framework.Moreover,the analysis principles of seismic fragility in this framework were systematically described.(2)Taking a 4×25 m concrete continuous girder bridge as the engineering background,and based on Open SEES finite element software,refined nonlinear dynamic analysis models of engineering bridge ductility system and the seismic isolation system were established.The seismic reasonableness and seismic capacity of the above two types of seismic systems were evaluated using the reasonable seismic system evaluation process,and the influence law of the design parameters of the main seismic elements,boundary constraints and other factors on the reasonableness of the bridge seismic system was focused on.The selection principles of seismic resisting systems for small and medium-span bridges in different intensity zones were proposed.(3)To study the influence of uncertainties in seismic waves,modeling parameters and seismic capacity of components on the evaluation of structural seismic performance during the design,construction and service of bridges,the Latin hypercube sampling method was used to sample several types of uncertainties with high sensitivity at the structural level,material level and boundary level in the modeling process,and 100 seismic-model pairs were established with 100 random combinations of seismic waves.After that,the analysis was carried out by establishing 7 analysis conditions and combining the vulnerability analysis theory to establish different components and system vulnerability curves of the engineering bridge.The results show that the uncertainty factors will lead to change the crest values of structural seismic response,which will not only affect the seismic rationality of the structural seismic system and cause some members to fail to meet the established reasonable damage state,but also make the overall damage probability of the structure increase and reduce the seismic capacity of the structure.