火灾下箱形截面梁柱的截面承载力分析

Analysis of Plastic Axial Load and Moment Capacity Curves for Fire-Exposed Steel Sections with Thermal Gradients

摘要: 火灾下由于梁和柱间相互作用，构件一般会在弯矩和轴力的联合作用下破坏，此时，构件的承载力由轴力–弯矩相关曲线(P-M相关曲线)决定。规范中给出的计算P-M相关曲线是基于截面全截面均匀升温的假定。此假定针对梁柱四面受火的情况是可行的，但当梁柱1、2、3面受火时，截面上各点的温度不同，计算结果可能会偏于不安全。因此，本文针对箱形截面分析了不同的高宽比、宽厚比对P-M相关曲线形状的影响，结果表明截面的高宽比和宽厚比对P-M相关曲线形状的影响不大。并且，本文提出一种简化P-M相关曲线，用来考虑不均匀升温对P-M相关曲线形状的影响。最后，通过一个数值计算实例来验证所提出的简化P-M相关曲线的准确性，结果表明，简化P-M相关曲线能很好的预测三面受火的箱形梁柱构件的破坏时间，并且偏于保守。

Abstract: The load carrying capacity of steel beam-columns subject to fire is determined through axial and moment capacity curves (P-M curves). A uniform temperature distribution through the depth of the section is assumed in Codes and standards. This assumption may be acceptable for some members, such as a column exposed to fire from four sides, but it may not be valid for columns or beams exposed to fire from 1, 2, or 3 sides since significant thermal gradients develop across the section. This study evaluates the effects of section depth, and plate thickness on the plastic P-M interaction diagram of box section. And the P-M interaction diagram is developed by adjusting the P-M interaction diagrams based on a uniform temperature. The applicability of the proposed method to a design situation is illustrated through a numerical example. It is indicated from the results that the simplified method can be applied to predict the load-bending capacity of beam- columns with temperature gradient in section under fire exactly.