Performance of unprotected steel and composite steel frames exposed to fire

摘要

Steel and composite construction are both common and popular forms of construction used around the world. Structural steel commonly has thermal protection for fire resistance, which may not always be necessary according to recent full-scale experimental fire tests and analytical studies. The fire behaviour of an unprotected structural steel beam or an unprotected steel and concrete composite beam in a fire compartment depends on many factors including the nature of the supports at the ends of the beam. In addition, it is not well understood how the fire resistance mechanisms of unprotected steel work, nor how the changes in material properties influence the behaviour of the composite or steel beam. This paper describes a detailed analytical investigation of the fire performance of steel and composite steel-concrete beams supported on steel columns and exposed to three-sided heating, simulating the effects of a compartment fire. The support conditions are provided by a range of columns having varying levels of axial and flexural restraint. In each case, a comparison of the deflected shape with the evolving internal forces shows unusual but predictable behaviour, strongly dependent on the stress-strain relationship of the steel. These cases show that the column stiffness has a very considerable influence on the behaviour of the beam and the time to failure under an increasing fire temperature. Thus this paper gives new insight into the fire resistance of unprotected steel and composite beams and frames, showing how the changes in column stiffness and material properties can influence the behaviour. The displacements, axial force and bending moments along the beam were found to be very sensitive to the stress state in the beam compared with the temperature-reduced proportional limit stress and the yield limit stress in the steel.