Non-linear structural modelling of a fire test subject to high restraint

The computer code VULCAN has been developed for the three-dimensional struc tural analysis of composite and steel-framed buildings in fire. In this pap er, the main features of the program are outlined, with particular emphasis on the most recent development to the layered procedure for modelling of c oncrete floor stabs. This development has introduced geometric non-linearit y into the modelling of slabs, whose layer structure already allowed temper ature distributions and change of material properties through the thickness , as well as modelling the effect of the ribs at the bottom of composite de cking slabs. The capabilities of the model are firstly tested at ambient te mperature for a uniformly loaded ribbed reinforced concrete slab with simpl y supported edges, and this is followed by a very detailed modelling of the Cardington restrained beam fire test. In both cases the development of mem brane action is demonstrated and the structural behaviour is compared with the geometrically linear case. A number of studies are carried out to demon strate the influence of the major floor slab details on the behaviour of th e structure in fire conditions. These studies provide evidence that when ex posed steel temperatures are less than 400 degreesC the concrete slab has l ittle influence, other than to play a part in generating thermal curvature to composite beams. For temperatures higher than about 500 degreesC the eff ect of the slab progressively becomes much greater, and it is very importan t to model concrete slabs correctly. The influence of membrane action canno t be ignored, particularly when the fire compartment is subject to high res traint because it is surrounded by cool, stiff structure. At very high temp eratures the floor slab becomes the main load-bearing element and the floor loads above the fire compartment are carried by the membrane forces develo ped in the slab, with tension being carried mainly by the steel anti-cracki ng mesh or reinforcing bars. However, the effect of the very high in-plane restraint to thermal expansion in the particular Cardington test considered is to enhance the peripheral zone of compressive membrane force and to red uce the extent of the central area of tensile force compared with more usua l cases. (C) 2001 Elsevier Science Ltd. All rights reserved.