COLLAPSE AND FLEXURAL-TORSIONAL BUCKLING TEMPERATURES AT FIRE FOR BEAMS IN STEEL FRAMES

Abstract

When a steel beam arranged in a steel frame is exposed to fire, compressive force, that is thermal stress, is generated in it with member temperature increase, because the liner thermal expansion is restricted by the adjacent members. It is, however, well known that the generated thermal stress is attenuated by plastic deformation occurred in the steel frame at elevated temperature, finally disappears when the heated beam exhibits collapse mode. The collapse temperature is independent of the thermal stress in case of stable plastic collapse mode. On the other hand, there is a possibility that the open cross-section beam unstiffened in the out-of-plane direction exhibits flexural-torsional buckling in the process of fire by acting the large compressive force. Since the flexural-torsional buckling resistance depends on the compressive force, it is considered that the member temperature at the flexural-torsional buckling depends on the thermal stress. The main purpose of this study is to clarify the flexural-torsional buckling behavior of the beam at the fire arranged in the steel frame and propose theoretical solutions on the flexural-torsional buckling temperature considering influence on the thermal stress. Three dimensional finite element analyses are conducted to verify the theoretical solutions. A line element being capable of analyzing the flexural-torsional behavior of the member is used. Both geometrical and material nonlinearity are considered in the numerical analysis. Three types of partial frame models including a fire compartment room are analyzed and their numerical flexural-torsional buckling and collapse temperatures are estimated, respectively. By comparison of the theoretical solution with the analytical results obtained by the parametric analyses, the following 1) and 2) were clarified. 1) The unstiffned beam at the fire exhibits the flexural-tosional buckling and the large buckling deformation in both in-plane and out-of-plane directions occurs with the temperature increase. The flexural-torsional buckling temperature becomes lower when the liner thermal expansion is strongly restricted by the adjacent members. However, the beam can sustain vertical loads after the buckling. The thermal stress is attenuated by the plastic deformation in both in-plane and out-of-plane directions generated in the buckled beam. The beam can resist up to the temperature when the thermal stress disappears. The collapse temperature is higher than the flexural-torsional buckling temperature, however, the plastic deformation in the out-of-plane direction is very large. 2) The flexural-torsional buckling temperature of the heated beam at the fire can be safely estimated by the theoretical solution proposed in this study. The collapse temperature of the beam after the buckling can be estimated by the theoretical collapse temperature based on the simple plastic theory. It is considered that fire resistance of the beam in the case when the plastic deformation in the out-of-plane direction cannot be allowed, for instance, in the case when the heated beam adjoins fire compartment walls, should be estimated by the theoretical flexural-torsional buckling temperature.