Design of steel structures under fire

Abstract

The Australian Steel Design Code, AS4100 provides guidance for design of steel elements. The fire section of the code details the procedure necessary to determine the time and temperature of a steel member to fail under fire conditions. Researchers all over the world have conducted studies and have reported their findings and possible design criteria’s to use in the prediction of the temperature rise of steel when subjected to elevated temperatures. The material properties that affect the behaviour of structural steel members exposed to fire have been reviewed and they are the thermal, mechanical and deformation properties of steel which change at elevated temperatures. This report describes the design methods used for design of steel building elements required to have a fire resistance level in accordance with the Australian Steel Code AS4100.1998. This report also contains sample design calculations to predict failure of simply supported unprotected beams subjected to elevated temperatures. The review of AS4100 indicates that a simple method is used for estimating temperatures for unprotected steel elements, whilst standard fire test curves are to be used for estimating temperatures for protected steel elements. Structural behaviour of steel structures in fire depends upon a number of variables such as material degradation at elevated temperature and restraint stiffness of members surrounded by fire. High temperatures and gradients across a structural element are the driving force behind large deflections and axial forces. In buildings exposed to fire they all interact, thus influencing the structural behaviour.