火灾下张弦桁架结构防火涂料等效热阻取值研究

Study on Equivalent Thermal Resistance of Fire Protection Coating of Tension-String Truss Structure Under Fire

为研究张弦桁架结构在火灾中火源与其正上方钢构件之间的安全距离及防火保护涂料等效热阻Ri的最小取值,以140,188,220 m跨度的张弦桁架结构为研究对象,采用火灾模拟软件FDS模拟不同火灾场景下的空气温度场,考虑了空气对钢构件的热传导作用及高温对钢材特性的影响,基于有限元软件ANSYS进行高温下结构整体抗火性能验算,根据验算结果分析位移及内力,揭示张弦桁架结构达到极限耐火时间的内在原因。通过对不同尺寸张弦桁架结构抗火性能的分析,归纳火灾下张弦桁架结构需喷涂防火保护涂料的具体范围。最后从经济角度出发,以3组不同跨度的张弦桁架结构为背景,在防火保护涂料等效热阻系数Ri不同的情况下,对火灾下张弦桁架结构代表性节点的位移进行分析,探究防火保护涂料等效热阻系数Ri的最小取值。研究结果表明:张弦桁架结构在火灾中达到耐火极限的原因在于火灾危险区域的杆件内力达到屈服状态;张弦桁架结构在火灾中火源与其正上方钢构件安全距离为15.0 m;对安全距离以内的杆件采用喷涂防火保护涂料的方法可增强结构抗火性能,其中防火保护涂料的等效热阻Ri最小取值为0.02 m^(2)·℃/W。

In order to study the safety distance between the fire source and the steel members directly above the tension-string truss structure in a fire and the minimum value of equivalent thermal resistance Ri of fire protection coating,the temperature field under the action of different kinds of fire was simulated based on three truss string structures with the span of 140,188 and 220 meters by the fire dynamic simulation software FDS.The nonlinear analysis on the overall fire resistance of truss string structure with the influence of high temperature on the steel characteristics was conducted through using the finite element software ANSYS,according to the results of calculation,the internal force and displacement of the tensioned truss are analyzed,and the internal reason for the tensioned truss structure to reach the ultimate fire resistance time was revealed.Based on the analysis of fire resistance of different tensioned truss structures,the specific scope of fire protection coating should be sprayed on tensioned truss structures under fire is concluded.Finally,from the economic point of view,three groups of tension-string truss structures with different spans were taken as the background,and the displacement of representative joints of tension-string truss structures under fire was analyzed under the condition of different equivalent thermal resistance coefficient Ri,and the minimum value of equivalent thermal resistance coefficient Ri was explored.The research results showed that the reason for the tension-string truss structure reached the fire resistance limit was that the internal force of the rod in the fire danger area reaches the yielding state.The safety distance between the fire source and the steel members directly above the fire source was 15.0 m.It was feasible to improve the fire resistance performance of tension-string truss structures by spraying fire protection coating,and the minimum value of equivalent thermal resistance Ri of fire protection coating was 0.02 m^(2)·℃/W.