Modelling of axial restraints for limiting temperature calculation of steel members in fire

At ambient temperature, design actions calculated for a beam under lateral loading may remain unchanged irrespective of whether axial restraints at the beam's ends are present or not. However, at elevated temperatures, axial restraints may induce significantly different design actions from those calculated at ambient temperature, mainly due to thermal restraining reactions. As a result, the limiting temperature may be reduced. In this paper, a method is presented for limiting temperature calculation of steel beams in fire in multi-storey buildings. The method takes into account the fact that the axial force generated in a steel beam in fire by axial restraints may increase to such a magnitude that the strength criterion, in terms of axial-bending interaction, may be breached. In addition, it has been recognised that the degree of axial restraints provided at the ends of a steel beam varies, depending on the axial restraint stiffness provided by neighbouring members. A simple technique to model the axial restraints provided to a steel beam by neighbouring members and the procedure for its limiting temperature calculation in a multi-bay situation in a building will be presented. The approach caters for different end conditions of the columns below and above the beam under consideration. Catenary action due to large deflection effect for beams in fire will also be discussed.