局部火灾下钢框架中上翼缘无侧移工字梁的极限状态计算

本文根据钢框架中工字梁在梁下局部火灾下将产生较大温度轴压力，其受力状态发生改变的事实，建立了钢框架中上器无侧移工字形梁的下器侧移失稳挠曲微分方程，应用能量法近似求解获得了验算梁下翼缘侧向稳定的计算长度，并结合我国现行钢结构设计规范建立了梁下翼侧移失稳的极限状态方程。此外，本文还建立了局部火灾下梁形成三铰机构而破坏的极限状态方程。这两个极限状态方程可直接用于钢框架工字梁的抗火设计与计算。

Limit State Calculation of Steel Frame Girders with Laterally Restrained Upper Flanges Subjected to Local Fire

When the I section girders of steel frame buildings suffer local fire beneath the girders, large axial compression will be formed due to thermal expansion. It is possib1e that the lower flanges of the compressed girders viith laterally restrained upper flanges will buckle out Of the web planes.The differential equation governing the lateral buckling of the lower flanges of the girders is estab-lished. The effective lengths of the lower flanges of the fire-affected girders are obtained approxi-mately by the energy method- Based on the approach for checking the stability of axially com-pressed members adopted by the Chinese Code for Design of Steel Structures, the limit state equa-tion for lateral buckling of the lower flanges of the girders with laterally restrained upper flanges is established. In addition, the limit state equation for the destruction of three - hinge mechanism of the girders caused by local fire is also developed. The two limit state equations presented in this paper can be directly used for fire-resistant design of the girders in steel frame building.