Before 1990, research on fire-resistance of steel structures was mainly focused on isolated members. In 1990, a fire occurred in a partly completed 14-storey office building at Broadgate in London
. The investigation after the fire showed that behavior of a beam was strongly influenced by the restraint provided by the surrounding structural components. Although the possible beneficial effects of the catenary action of the beam or the membrane action of the composite slab were not evident because relatively low steel temperatures less than 600 °C were reached during the fire, interactions between different structural members in a completed structure subjected to a fire drew the attention of researchers. In 1996, a program of full-scale fire tests was completed on an eight-storey steel-framed building in the UK at Cardington Laboratory, to investigate the behavior of a real steel framed structure under real fire conditions. The typical “runaway” failure of an isolated steel beam in the standard fire test did not occur to the steel frame beam, even though the temperature of the bottom flange of the beam had exceeded 800 °C, which indicted that a steel beam in a framed structure, with the aid of restraint from surrounding members, has better fire-resistant capability than an individual steel beam
. The local buckling of the bottom flange occurred near the beam-to-column connection during heating, because of tremendous compression stress at this place resulting from the restrained thermal expansion. Damage of beam-to-column connections was also observed due to thermal contraction of the beam during cooling
, as shown in Fig. 7.1.