Study On Structure Fire Resistance Of Double-deck Suspension Bridge With Horizontal Wind

Building a strong transportation country is a major strategic decision in China.With the rapid development of transportation infrastructure construction and improvement of transportation efficiency,fire loads on bridges with large-span cablebearing systems have increased and accidents are frequent.Compared with the study of confined space fires in buildings and tunnels,the study of vehicle fire in the open space of bridge upper deck and the semi-open space of bridge lower deck,which are affected by ambient winds,is still relatively cursory.Therefore,it is especially important to clarify the fire plume behavior and structural collapse risk during vehicle fires in open space on the bridge upper deck and semi-open space on the bridge lower deck with the action of ambient wind to scientifically carry out research on fireresistant design of bridges with large-span cable-bearing systems.In this thesis,the spatial and temporal distribution characteristics of the fire nonuniform temperature field in the open space of the bridge upper deck and the semiopen space of the bridge lower deck with the action of horizontal wind are studied,and the spatial and temporal distribution model of the fire plume of the vehicle fire in the open space on the bridge upper deck with the action of horizontal wind and the mathematical model of the maximum smoke temperature of the roof of the vehicle fire in the semi-open space on the bridge lower deck are established by combining theoretical analysis and numerical simulation.The distribution law of the temperature field of non-uniformly fired steel members of the bridge with different influencing factors is analyzed.The non-uniformly distributed spatial and temporal temperature field of the fire-bearing steel members is applied to the full-bridge thermal-structural coupling model to reveal the change law of structure fire resistance performance index of a double-deck suspension bridge with the action of horizontal wind.The main research work of this thesis is as follows:(1)Using the FDS field simulation technology,the spatial and temporal distribution model of the non-uniform temperature field of the fire plume of vehicle fire in the open space on the bridge upper deck with different horizontal wind speed and different heat release rate was investigated,and a model of the spatial and temporal distribution of the fire plume of vehicle fire in the open space on the bridge upper deck with the action of horizontal wind was proposed.At the same time,more than dozens of fire with different heat release rate,different horizontal wind speed and different steel truss beam size were studied,and a mathematical model of the maximum smoke temperature of the roof of the vehicle fire in the semi-open space on the bridge lower deck was proposed.(2)Using the FDS field simulation technology,a thermal analysis model of large-span double-deck suspension bridge sections with the action of horizontal wind was established to analyze the effects of heat release rate,horizontal wind speed,fire source location and other factors on the temperature field of the mid-span sling and main cable surface.The most unfavorable fire scenarios were determined from the perspective of considering the maximum surface temperature of the steel members on fire,and the most unfavorable fire scenarios for the main cable on fire were the heat release rate of 300 MW,horizontal wind speed of 5 m/s,and the fire source located in the emergency lane in the span(fire scenario 10)or the second lane(fire scenario 8);the most unfavorable fire scenario for the sling on fire were the heat release rate of300 MW,horizontal wind speed of 5 m/s,and the fire source located in the emergency lane in the span(fire scenario 10)or the first lane(fire scenario 8).(3)Using the finite element software ANSYS,the temperature field variation law of the cross-section of the main cable subjected to fire with the action of horizontal wind was obtained.It is shown that the temperature field distribution on the outer surface of the main cable is different in different fire scenarios,resulting in different temperature field distribution in the inner cross section.When the high temperature area(>600℃)covers about 1/4 of the outer surface of the main cable,the internal temperature of the main cable cross-section is firstly distributed in layers from high to low,followed by a "few" pattern;when the high temperature area(>600℃)covers about 3/4 of the outer surface of the main cable,the internal temperature of the main cable cross-section is distributed in circles from high to low.(4)Using the finite element software ANSYS,the variation law of the structural fire resistance index of a large-span double-deck suspension bridge with the action of horizontal wind was obtained.The study shows that the span deflection only increased by 80 mm when the fire source was located in the emergency lane of the upper deck span(fire scenario 10)with the heat release rate of 300 MW and the horizontal wind speed of 5 m/s.However,irreversible wire damage occurred at four of the six measurement points of the main cable section,and the span sling broke at1170 s.When the fire source was located in the second lane of the upper deck span(fire scenario 8),the deflection of the span increased by 112 mm and the temperature of the main cable was higher than that in fire scenario 10,so the steel wires at the six measurement points of the section were irreversibly damaged and the span sling broke at 1800 s of the fire.When the fire source was located in the emergency lane of the lower deck span(fire scenario 12),the straight web of the steel joist was damaged at2160 s of the fire.The top plate of steel joist was damaged at 2340 s when the he fire source was located in the fourth lane of the lower deck span(fire scenario 13)with the heat release rate of 300 MW,horizontal wind speed of 2 m/s.This thesis investigates the structural fire resistance of double-deck suspension bridges with the action of the horizontal wind,and provides data support and theoretical guidance for the fire hazard analysis and fire resistance research of bridges in this type of environment.