A story of a building may collapse due to excessive column inclination caused by thermal elongation of connecting steel beams during fire. However, thermal elongation of steel beams is not investigated fully. It can be thought that the thermal elongation is restrained by the rigidity of adjacent members. Their restrain effects are modeled by a spring at the end of the heated beam. From the consideration of full scale fire furnace tests of steel beams which were constrained by the RC slab, it can be assumed that the constraining effect of the RC slab is presented by its bending stiffness. Furthermore, the stiffness reduction can be indicated by an exponential equation. This study indicates that the thermal elongation of steel beams can be predicted accurately by the simple theoretical formula for the performance based fire engineering design.
