| With the acceleration of urbanization in China,profound changes have taken place in urban construction.Remarkable achievements have been made in the construction of Bridges.But,with the rapid development of our country’s urban transportation,the increase of transportation of flammable and explosive vehicles and new energy vehicles,the probability of fire caused by bridge is also rising.Vehicles carrying hazardous chemicals can burn or explode in summer when high temperatures occur or traffic accidents occur.Once a bridge fire occurs,it will have a significant impact on trusses,cables and other key components,posing a direct threat to the operation safety of the bridge,and even cause partial component damage or even overall collapse,resulting in huge economic losses and casualties.At present,the research on bridge fire is not perfect.On the one hand,there is a lack of relevant bridge fire codes in China,and bridge fire risk assessment lacks test data and theoretical support,so it is urgent to carry out bridge fire research.On the other hand,the researches on bridge fire mainly focus on the changes of the mechanical properties of bridge structures in fire,and the influencing factors are relatively simple,while the researches on the temperature distribution of key components in fire are few,and the temperature is a key parameter affecting the mechanical properties of bridge structures.In addition,due to the high economic cost of carrying out bridge fire experiment,most researches on bridge fire are carried out by numerical simulation.Therefore,it is of great theoretical and practical significance to study the variation law of temperature field of double-layer suspension bridge under multi-factor conditions through experiment and numerical simulation.In this thesis,partial restricted space formed by double-layer suspension bridge is taken as the research object.By combining experiment and numerical simulation,the fire temperature field of double-layer suspension bridge is studied by changing environmental wind speed,environmental wind direction,net height between floors and other factors,which mainly includes the following aspects:(1)The dimensionless temperature rise on the upper deck decreases exponentially with the increase of the distance from the fire source,which is consistent with the experimental results obtained by predecessors in the windless environment.Moreover,the temperature attenuation rate under the inward wind is significantly higher than that under the outward wind.By fitting the maximum dimensionless temperature rise of the truss with the average flame height,the relationship between the maximum dimensionless temperature rise of the truss and the average flame height under different ambient winds is obtained.Based on the dimensionless analysis,the prediction model of the temperature at the top of the truss is established.It is found that the relationship between the dimensionless temperature rise and Froude number is a power function.(2)The prediction formula of longitudinal temperature rise under the upper bridge deck under the impact of continuous flame under the effect of ambient wind is obtained by modifying the characteristic length of tunnel roof proposed by predecessors.Based on the dimensionless analysis,the prediction model of the maximum temperature rise of truss with dimensionless fire power is established.By fitting the experimental data,the prediction formulas of infinite temperature rise at the top of the truss under three conditions: buoyant plume impact on the upper deck,intermittent flame impact on the upper deck and continuous flame impact on the upper deck are obtained.(3)When the net height between layers was the same,with the increase of wind speed,the high temperature area of the fire shifted to the downwind side,and the influence range increased gradually.When the wind speed was the same,the fire affected area gradually decreased with the increase of the net height between floors,and this phenomenon was more obvious in the full-scale numerical simulation compared with the scale numerical simulation.The temperature distribution of upper deck and trusses measured by experiment is close to the numerical simulation results,and the temperature distribution presents a similar law with the change of wind speed and trusses. |
