Heat Modeling Method And Performance Research For Double Suspension Bridge Based On Thermal-Structure Coupling

At present,the domestic and foreign research mainly centre on a large number of building fire and tunnel fire,while the research on long-span bridge fire,especially the large flexibility suspension bridge fire is rarely involved.Most of the research on bridge fire adopts the method of heating curves in building fire to impose heating load on the bridge,and this may lead to large difference compared to the actual situation.Some parts of the bridge may be damaged before the whole bridge is destruction for suspension bridge with complex loading is the statically indeterminate structure.Therefore,it is necessary to analyse mechanical propertie of the suspension bridge under tanker fire and do the fire resistance design of bridge to ensure the safe operation of bridge structure.Wuhan Yang Si Gang Yangtze River bridge is a double-layer highway steel truss girder suspension bridge with the span of 1700 m,and it is an important river crossing passageway with large traffic flow and high fire risk level.This paper takes the Wuhan Yang Si Gang Yangtze River bridge as the analysis project,and uses the method of least squares to fit polynomial of material thermal parameters and mechanical parameters which changing with temperature based on the summary of thermal parameters and mechanical parameters of materials(steel and concrete).The most unfavorable condition of tanker fire heat release rate function growth model is determined according to the heat release rate theory.Three-dimensional thermal model of Wuhan Yang Si Gang Yangtze River bridge is established by the fire dynamics software FDS.The 3D temperature field distribution characteristics of the bridge in different parts is identified,and the results of thermal analysis is applied to the three-dimensional finite element model of ANSYS.By the thermal-structure coupling analysis,the mechanical properties of main cables,slings and stiffening girders under high temperature is calculated.The shortest failure time of main cable and sling is obtained and the shortest buckling instability time is calculated in order to obtain the bridge fire rescue time.Fire resistance design method of Wuhan Yang Si Gang Yangtze River bridge is put forward,and the fire resistant measures is made based on the calculation results.The main work and innovation are as follows:(1)Expressions of steel and concrete thermal parameters and mechanical parameters which changing with time are fitting out.The thermal parameters and mechanical parameters of steel and concrete are significantly changed with temperature.During the thermal analysis process,the thermal parameters and mechanical parameters of the materials are discrete and not easy to be selected.Using a single thermal parameter and mechanical parameter changing with time will get a large calculation error.Based on the domestic and foreign expression of material thermal parameters(coefficient of thermal conductivity,specific heat,thermal expansion coefficient)and mechanical parameters(elastic modulus,strength)changing with time,the polynomial of steel and concrete thermal parameters and mechanical parameters changing with temperature is fitting out by the method of least squares,This can minimize the value of material thermal parameters and mechanical parameters to make the results more accurate.(2)Thermal analysis model and three-dimensional mechanical model of Wuhan Yang Si Gang Yangtze River bridge under tanker fire are established.By the large eddy simulation method from fire dynamics software FDS,the thermal analysis model of the bridge under tanker fire is establish,and the combustion characteristics of fire is simulated by the method of fire heat release rate.Temperature in the bridge corresponding parts is calculatedt.The mechanical finite element model of whole bridge is established using the finite element software ANSYS.In the model,the middle section in the main span is made up of precise space shell solid elements and other sections use beam and link elements.The results show that the FDS thermal analysis model can accurately simulate the dynamic convection radiation effect of fire on the suspension bridge and determine the characteristics of 3D transient temperature field distribution of the key parts;The ANSYS 3D finite element mechanic model can greatly reduces the number of model units.The thermal analysis results can be defined by the thermal-structure coupling principle to the mechanical analysis model,and the fine segments of the main cables,slings,steel plate,the truss rod is connected with the suspension bridge to calculate mechanical properties of key parts.(3)Transient temperature field distribution and mechanical properties of the key parts of Wuhan Yang Si Gang Yangtze River bridge are obtained.Fire heating load was defined by the fire heat release rate curve based on the calculation of maximum temperature and flame scale of different vehicle types.The most dangerous fire vehicle model was 200 MW tanker fire The applicability of the three models of heating release rate of fire is analyzed,and the model of exponential growth model is established for the most suitable heat release rate model.200 MW tanker heat release rate curve is used to simulate the fire burning process.Three-dimensional transient temperature field distribution characteristics of main cables,slings,bridge deck,stiffening girder steel plate are obtained when the tanker fire happens in the different position of the bridge.The elastic modulus,strength,stress and deflection of main cables,slings,bridge deck,steel plate,truss rod under high temperature are obtained by thermal-structure coupling theory analysis.(4)Failure characteristics of main cable and sling and buckling instability characteristics of steel plate under high temperature of tanker fire are obtained;the shortest failure time of the main cable and sling and the shortest buckling instability time of stiffened steel plate are obtained.Tensile strength and elastic modulus of the main cable and sling will decrease rapidly with the increase of temperature.The highest temperature range of the main cable and sling under the burning tanker is obtained based on high temperature transient temperature field of the main cable and sling and the mechanical parameters of material changing with temperature.The characteristic of stress varing with time for main cable and sling is calcelated by thermal-structure coupling calculation.So the failure time of the main cable and sling under tanker fire is obtained.The buckling shape and buckling time of steel plate under high temperature are calculated by the steel plate stability theory.Results show that under dead load and live load case,when a tanker fire happenes in the upper outer lane of the centre span,the shortest failure time of the sling is 36 min,and the shortest failure time of the main cable is 58min;when a tanker fire happenes in the lower outer lane of the centre span,the shortest buckling time of steel plate is 43 min,and failure characteristics of the bridge is buckling instability rather than strength failure.(5)Fire resistance design method of Wuhan Yang Si Gang Yangtze River bridge is proposed.Protective thickness,height and protection range of the Outsourcing fire protection layer of the main cable and sling is determined,and the steel plate fireproof coating type and the corresponding filling thickness is obtained.Heat transfer law of the main cable and sling under the action of the fire protection layer is simulated by giving the outsourcing fireproof layer and the steel wire materials with different thermal properties.The failure time of the main cable and sling changing with the thickness of outsourcing fire protection is got.Results show that the outsourcing of fireproof of the main cables and slings is 0.5cm aluminum silicate fireproof layer,and the protection range of the main cable is 28 m around the centre span,and the protection height of the sling is 10 m above the main girder.By setting the main cable and sling outsourcing fireproof layer and steel fireproof coating,the bridge will not be destroyed during the tanker fire.Through the analysis of different types of fire retardant coating,the ultra-thin fire retardant coating fireproof coating is determined to be the fire retardant coating of the bridge.Thickness of fireproof coating is determined to be 3mm With comparison of the high temperature mechanical properties of stiffer girder under different thicknesses of fire insulation.Under the protection of fire retardant coating,the bridge will not happen buckling instability and strength failure during the tanker fire.(6)The best fire rescue time of the Wuhan Yang Si Gang Yangtze River bridge under tanker fire is obtained,and relevant fire rescue measures are formulated.When a tanker fire happenes in the upper outer lane of the centre span,the best fire rescue time should be not less than 16min;when a tanker fire happenes in the lower outer lane of the centre span,the best fire rescue time should be not less than 20 min.According to the calculation results of the tanker burning in the upper layer of the bridge,the middle lane in the upper layer of the bridge is determined to be a safe lane.The tanker should be guided to go in this lane of the bridge.The fire rescue procedure and related fire rescue measures are established,which provide reference for the safety of the bridge.