Research On Fire Performance And Its Improvement Methods Of Curved Steel Box Girders Exposed To Fire

In order to achieve the national double-carbon goal,respond to the call of national green transportation construction,and comprehensively promote the development of country with powerful transportation,the proportion of steel structure bridges in the construction of transportation infrastructure is increasing.Curved steel box girders are widely used in the construction of large overpasses and viaducts due to its lightweight structure,convenient construction,green environmental protection,and great durability.However,bridge fire accidents have occurred frequently in recent years,posing serious threat to the safety and durability of steel structure bridges.Curved steel box girders are more sensitive to fire and high temperatures compared to straight girders.Once fire accidents occur,curved steel box girders are prone to torsion,overturning,or even continuous collapse.Bridge fire accidents may interrupt multi-directional traffic,reduce logistics support,and cause extremely serious economic losses and adverse social impacts.Currently,there is very little research on fire safety of curved steel structure bridges.This article focuses on the fire performance and improvement methods of curved steel box girders.The methods of actual bridge research,theoretical analysis,model test and numerical analysis are used.The evolution mechanism of fire performance of curved steel box girders exposed to fire is comprehensively studied.Considering the support characteristics of actual bridges,a fire resistance test method was proposed to achieve the bending and torsion coupling effect of curved steel box girders.The entire fire process tests of simply supported steel box girders and continuous steel box girders under bending and torsion coupling loading are conducted.The large-deformation theoretical analysis method of curved steel box girders under fire is proposed.High-temperature response laws and failure mechanism of curved steel box girders under fire are studied.The influence factors of fire performance were analyzed.Fire resistance improvement methods of curved steel box girders are proposed.The research results can provide guidance and reference for the improvement of fire resistance and fire resistance design of steel structure bridges,and comprehensively promote the full life construction and safe operation of steel structure bridges.The main research contents are as follows:(1)The fire resistance test method bending torsion coupling of steel box girders is proposed.The fire resistance test of single-box twin-chamber steel box girders under complex bending and torsion coupling loading was carried out.The lateral eccentric load is applied on test girders to realize the stress state of bending-torsion coupling.The section configuration and loading modes are taken as the research variables of fire tests.The temperature distribution of the section of double-chamber steel box girders under fire and the high-temperature mechanical behavior as well as failure modes under bending-torsion coupling state are studied.The influence of steel top flange on heat transfer mode and mechanical behavior of single-box twinchamber steel box girders under high temperature is studied.The enhancement effect of longitudinal stiffeners on the fire resistance of structures is researched.The effect of loading evacuation on structural response of steel box girders during fire exposure is analyzed.The numerical simulation model is established for verification,and relevant parameter analysis is further carried out.(2)Considering the support characteristics of actual bridges,the fire resistance test of single-box twin-chamber continuous steel box girders under different fire scenarios is carried out.Considering stress state of bending-torsion coupling and performance degradation of the middle rubber support under high-temperature environment,fire response and failure modes of continuous steel box girders are studied by taking the single span fire and the middle support fire as the research conditions.The post-fire performance of the rubber support is tested and evaluated.The numerical simulation model under fire is established for verification,and the internal force redistribution and failure mechanism of continuous steel box girders exposed to fire are further analyzed.(3)The theoretical analysis of large deformation of curved steel box girders exposed to fire is carried out.The basic structure is a simply-supported statically-determinate curved girder subjected to uniform load.Based on the principle of minimum potential energy,the deflection curve equation and torsion angle equation of the curved girder under action of temperature are derived.The deformation function of the curved girder under the condition of high temperature and large deformation is given according to the deformation function of its elastic state and assumptions.According to determinate temperature distribution of the section,stress distribution of the section and the solution of internal force of the section are given through analysis.The theoretical analysis method of curved steel box girders under the condition of high temperature and large deformation is proposed,and bending-torsion coupling mechanical behavior at high temperature is revealed.(4)The high-temperature response laws of curved steel box girders under complex fire scenarios are revealed.Side span fire,middle span fire and middle support fire are selected as the fire scenarios,and thermal-mechanical coupling analysis method is used to analyze thermal and structural response by the finite element model.The temperature rising and heat transfer process as well as section temperature distribution of single-box twin-chamber steel box girders are studied.The change laws of vertical displacement,horizontal displacement,support reaction,internal force distribution,bearing capacity and other indicators of main girders are investigated.The mechanical behavior and failure mechanism of continuous curved steel box girders under different fire scenarios are revealed.(5)The factors affecting the fire resistance of curved steel box girders are investigated.The influence factors are considered including fire parameters(fire intensity,fire length),load parameters(load ratio,load evacuation,eccentric load),structural parameters(longitudinal stiffeners,radius of curvature,reverse bending)and support conditions(different support arrangements).The high-temperature response and fire resistance of continuous curved steel box girders exposed to fire are studied under different parameters.(6)The improvement and analysis methods of fire resistance of curved steel box girders are proposed.Based on the results of previous research,from the perspective of active and passive fire protection,fire protection strategy and fire performance improvement methods under active and passive combined effect are proposed.Aiming at structural parameters in passive protection,the multi-parameter coupled fire resistance analysis method is proposed.The functional relationship between design parameters and structural fire resistance limit is constructed.The optimal configuration of design parameters is obtained through solving a nonlinear programming problem.The methods provide ideas and references for the fire resistance design of curved steel box girders.The innovation points of this article are as follows:(1)Considering the support characteristics of actual bridges,a fire resistance test method is proposed to achieve the bending and torsion coupling effect of curved steel box girders.The temperature rising and failure test of twin-chamber thin-walled hollow steel box girders under bending and torsion coupling loading during fire exposure are conducted.(2)The temperature rise analysis theory of closed steel box structures is established.The large-deformation theory analysis method for curved steel box girders exposed to fire is proposed.The evolution laws of high-temperature deformation and failure of curved steel box girders under complex environments are revealed.(3)The method for improving fire performance of curved steel box girders under the combined action of active and passive protection to resist complex environments is proposed.The fire performance analysis method of multi-parameter coupling under complex environments is proposed.