Lateral Torsional Buckling Strengths of Stepped Beams at Midspan Exposed to Elevated Temperature

Abstract

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.