Optimization And Seismic Fragility Analysis Of Concrete-filled Steel Tubular Truss Composite Girder Bridge Structure

With the continuous advancement of economy and society,accompanied by the rising living standards,the demands for building structures have transcended mere practicality,safety,and cost-effectiveness.There is now an increased emphasis on the aesthetic aspects of architectural designs.Consequently,lightweight,high-strength,and visually appealing steel tubular truss structures have captured the attention of engineers.The technique of infilling hollow steel tubes with concrete to enhance the performance of steel tubular trusses has emerged accordingly.Although extensive research has been conducted on steel tubular trusses and concrete-filled steel tubular members,the investigation of concrete-filled steel tubular truss composite beam structures has been initiated relatively recently,with only a few scholars undertaking quasi-static studies.Furthermore,there is a limited body of literature focusing on the seismic performance of such structures,necessitating further comprehensive exploration.This research aims to investigate the performance optimization and seismic behavior of three-chord concrete-filled steel tubular truss composite girder bridge structures incorporating reinforced concrete slabs.The primary objectives and findings of this research are as follows:(1)A method for finite element modeling of reinforced concrete truss combination beams using the Open SEES software is presented in this study.The accuracy and effectiveness of this modeling approach were verified through a comparison between the numerical analysis results and available experimental test data,focusing on three key aspects: span-deflection load curves,damage modes,and vertical deflection.Subsequently,an extensive parametric analysis was conducted to investigate the effects of 15 major influencing factors,including material strength,geometric arrangement,steel tube dimensions,and reinforced concrete slab dimensions,on the flexural performance of the steel tube concrete joist combination beam.Valuable insights for the practical design of the combined beams are provided,and the findings can serve as a reference for structural engineers.(2)The strength of concrete-filled steel tubular truss composite girders was analyzed by employing the Plackett-Burman design approach to conduct sensitivity analysis and identify the eight significant factors.Subsequently,the influence of these significant factors on the strength of composite girders was further investigated using the response surface methodology(RSM)with Box-Behnken design.By utilizing the response surface technique with ensured accuracy,a modified formula was proposed to calculate the flexural strength of composite girders.The newly proposed formula incorporates the influence of brace mechanical properties.Excellent agreement between the predictions obtained from the modified formula and the experimental results for various failure modes was observed.(3)A mathematical model for the many-objective optimization design of concretefilled steel tubular truss composite girders was established.Totally,six optimization objectives were considered in the model,including flexural yield strength,effective flexural stiffness,steel consumption,structural weight,structural cost,and carbon emissions in production.Additionally,relevant geometric and structural constraints were evaluated.Among various algorithms for many-objective optimization,the twoarchive evolutionary algorithm was selected based on an evaluation of diversity and convergence.An example was designed using the many-objective optimization approach,and the design results were compared with the specification design results to validate the feasibility and rationality of the optimization design.(4)A continuous and multi-span bridge finite element model was established based on the outcomes of the many-objective optimization design.Nonlinear time history analyses were conducted using incremental dynamic analysis,considering a total of 60 selected ground motions.Fragility analysis and seismic risk assessment were performed at both the component and whole bridge levels,considering bidirectional seismic action.The findings from the fragility analyses and risk assessments indicate that the probability of damage to the entire bridge is higher compared to individual bridge components under bidirectional seismic action.Among the components,the bearing exhibited the highest probability of damage,followed by the pier,and finally the superstructure.Furthermore,the results of the seismic risk assessment suggest that the probability of severe damage or collapse of the bridge during the 50-year design reference period is less than 5%.This implies that the bridge exhibits good earthquake resistance performance.