Development of a component based model of steel beam-to-column joints at elevated temperatures.

The response of steel-framed structures to applied loading depends to a large degree on the behaviour of the joints between the columns and beams. Traditionally designers have assumed that these joints act either as 'pinned', with no ability to transmit moments from beam to column, or as 'rigid', providing perfect continuity between the connected members. Advances in analysis, and developments in modem codes of practice, permit designers to account for the real behaviour of steel joints where this is known or can be predicted. Even though experimental studies of joints conducted at many research centres around the world have provided a large bank of test data, the vast number of variables in joints (beam and column sizes, plate thicknesses, bolt sizes and spacing, etc.) often means that data for a specific joint arrangement does not exist. As a result, researchers have turned their attention to ways of predicting the behaviour of such joints. One approach which has gained acceptance is based on the "Component Method" in which overall joint behaviour is assumed to be produced by the responses of its various simpler components. To date, data on the response of joints at elevated temperatures has been gathered from full-scale furnace tests on cruciform arrangements, which have concentrated exclusively on moment-rotation behaviour in the absence of axial thrusts. However, when steel-framed structures are subjected to fire, the behaviour of the joints within the overall frame response is greatly affected by the high axial forces which are created by restraint to the thermal expansion of unprotected beams. If momentrotation- thrust surfaces were to be generated this process would require prohibitive numbers of complex and expensive furnace tests for each joint configuration. The alternative, and more practical, method is to extend the Component Method to the elevated-temperature situation. The basic theme of the Component Method is to consider any joint as an assembly of individual simple components. Each of these components is simply a non-linear spring, possessing its own level of strength and stiffness in tension, compression or shear, and these will degrade as its temperature rises. The main objective of this study was to investigate experimentally and analytically the behaviour of tension and compression zones of end-plate connections at elevated temperatures. A series of experiments has been carried out and a simplified analytical model has been developed, and this has been validated against the tests and against detailed finite element simulations. The simplified model is shown to be very reliable for this very common type of joint, although similar methods will need to be developed for other configurations. The principles of the Component Method can be used directly in either simplified or finite element modelling, without attempting to predict of the overall joint behaviour in fire, to enable semi-rigid behaviour to be taken into account in the analytical fire engineering design of steel-framed and composite buildings.