THE ANALYSIS OF SEMIRIGID FRAMES IN FIRE - A SECANT APPROACH

A secant stiffness method was previously developed for efficient analysis of isolated steel beams under the influence of thermal distributions caused by fire. This method has been developed considerably further, to include the thermally degrading characteristics of the connections between adjacent members and also the effects of axial force, the latter including both material and geometric nonlinearities. This enables full planar skeletal frames' to be analysed up to failure under local or overall fire conditions. In particular terms it allows the investigation of the real behaviour in fire of frames designed as 'simple' for ambient-temperature, strength and serviceability criteria. Although the normal design assumption for such frames is that beams transfer no moments to columns, the rotational stiffnesses of the connections can considerably affect fire survival, as can other restraint forces from the surrounding structure. In this paper the theoretical background to the analysis is developed, and validation studies are reported. These include comparisons with previous analytical work and with fire test results. An original example is presented of a three-bay x three-storey frame with a local compartment fire in the bottom storey.