The American Institute of Steel Construction provides design equations for the lateral torsional buckling (LTB) of beams under fire. However, these equations are limited only to prismatic beams. Stepped beam factors are introduced by researchers that accounts the change in cross-sections of beams to determine capacities of stepped beams under normal temperatures. This paper assesses the validity of the stepped beam factors for the LTB capacities of stepped I-beams located at its midspans integrated to the AISC equation for beams under high temperatures. A set of numerical studies using finite element analysis program, ABAQUS, was conducted to assess the buckling behavior of stepped beams. The analysis is composed of heat-transfer analysis that evaluates the change in material properties of steel as heat propagates the material from 20 °C to 800 °C; and Static Riks analysis where the beams are applied with uniform end moments. Correlation between the results from the stepped beam equations and the simulated data from ABAQUS has been done. The comparison between data showed that the proposed equation generated conservative estimates with an average percentage difference of 11.48% for inelastic LTB, whilst, 2.07% for the elastic LTB. In addition, the ratio of the increase in strength and increase in volume of stepping of beams shows that the flange width of the stepped beam controls its efficiency in lateral torsional buckling capacity. Overall, the results of this research proved that the existing stepped beam equations can be used in calculating the structural capacity of stepped beams at midspan under both normal and elevated temperatures.

美国钢结构学会提供了梁在火灾下的横向扭转屈曲（LTB）的设计方程式。但是，这些方程式仅限于棱镜光束。研究人员介绍了阶梯梁因素，该因素考虑了梁横截面的变化，以确定常温下阶梯梁的容量。本文评估了在高温下集成在AISC方程中的阶梯式I形梁中跨的阶梯式梁的LTB容量的有效性。使用有限元分析程序ABAQUS进行了一组数值研究，以评估阶梯梁的屈曲行为。该分析由传热分析组成，该分析评估了当热量将材料从20°C传播到800°C时钢材料性能的变化。和静态Riks分析，其中以均匀的端力矩施加梁。步进梁方程的结果与ABAQUS的模拟数据之间已经建立了相关性。数据之间的比较表明，所提出的方程生成了保守估计，非弹性LTB的平均百分比差异为11.48％，而弹性LTB的平均百分比差异为2.07％。另外，强度的增加与台阶的体积的增加的比率表明，台阶的梁的凸缘宽度控制了其在横向扭转屈曲能力中的效率。总体而言，这项研究的结果证明了现有的阶梯梁方程可用于计算中跨阶梯梁在常温和高温下的结构承载力。