桥梁钢绞线拉索防火隔热的理论分析及设计

Theoretical Analysis and Design of Fire Protection and Thermal Insulation of Steel Strand Cables for Bridges

桥梁拉索易在火灾情况下因高温而发生破坏,因此有必要对桥梁拉索的防火隔热开展研究设计。采用集中热质法建立了钢绞线拉索瞬态热流模型,求得拉索各层钢绞线表面温度随时间变化的数值解,并与有限元模拟结果进行了对比,取得了较好的一致性。参考美国后张法协会规范,根据限制拉索最外层钢绞线表面温度的防火要求,对计算进行分析,结果表明:受火时间为30 min时,拉索满足防火要求的最小防火层厚度与拉索等效模型的截面形状系数基本呈线性相关,且函数斜率约等于防火层的导热系数。以此为根据提出了计算桥梁钢绞线拉索防火隔热系统的最小防火层厚度的简便方法,并用37孔12.7 mm和单孔15.2 mm的钢绞线拉索进行了验算,数值计算结果表明受火时间为30 min时,前者最外层钢绞线的表面温度为261.3℃,后者为272.1℃,均满足防火要求。该计算方法简单可靠,为桥梁拉索防火隔热系统的设计提供了理论依据。

The cables can be easily destroyed in case of a fire due to high temperatures,thus,it is important to carry out the study and design of a fire protection for the bridge cables.A transient heat flow model was established for steel strand cables by the lumped thermal mass approach,and the numerical solution of the surface temperature changing with time in each layer of steel strand was calculated.Then,the numerical calculation results are compared with the finite element simulation results and they have achieved good consistency.With reference to the specifications provided by the Post-Tensioning Institute(PTI),the calculations were analyzed in accordance with the fire protection requirements limiting the surface temperature of the outermost layer of the steel strands.It is observed that the minimum thickness of the fire protection layer required to meet the fire protection regulations for 30 min is basically linearly related to the section factor of the cable equivalent model.And the slope of the function is approximately equal to the conduction coefficient of the fire protection layer.Based on this,a simple method is proposed to calculate the minimum thickness of the fire protection layer for the fire protection and thermal insulation system for the steel strand bridge cables,and this method is checked using 37-holeφ12.7 mm and single-holeφ15.2 mm steel strand cables.The calculation results show that the surface temperature of the outermost layer was 261.3℃for 37-holeφ12.7 mm cables and 272.1℃for single-holeφ15.2 mm cables when the fire duration is 30 min,all meeting the fire protection requirements.The calculation method herein is simple and reliable,providing a theoretical basis for the design of the fire protection and thermal insulation system of bridge cables.