Behaviour of steel and composite beam-column connections in fire

Abstract

Recent fire tests on the Cardington full-scale test frame and observations from real fires have demonstrated the significance of connections in fire, when they can have beneficial effects on the survival time of the structure. The lack of experimental data on the behaviour of steel and composite connections in fire means that this is insufficiently addressed in current design codes and also limits the effective use of numerical models. However, recent experimental tests on small-scale specimens have shown that it is possible to derive accurately the moment-rotation relationships at elevated temperature and have established the principles by which this could be achieved. In order to extend the scope to include further parameters, five series of tests have been carried out in a portable connection furnace at the Building Research Establishment. The test series includes flush and flexible end-plate bare-steel connections, and flexible end-plate composite connections. The testing procedure and the resulting behaviour are described. The fire test temperature profiles across the connections are detailed and the connection failure mechanisms are discussed. From the test results, moment-rotation- temperature curves for different connection types are derived. The degradation of connection characteristics is compared with that of structural steel. The experimental behaviour is also compared with the results obtained from an existing finite element analysis developed to model connection response in fire conditions. The experimentally derived connection characteristics have been incorporated within a parametric study of a typical sub-frame, to study the effect of connection type, end-plate thickness, concrete strength, load ratio, and connection temperature. Analysis is extended to a three-dimensional sub-frame. The patterns of behaviour observed in the connection tests is compared with that of the connections in the large-scale fire tests on the composite building at BRE's Cardington laboratory. Based on knowledge about the behaviour of connections at elevated temperature, a component-based model is developed for the elevated temperature response for flexible end-plate connections, both as bare-steel and composite. This is based on the response of constitutive parts of connection. The model is easy to use, and capable of modelling the entire non-linear range of connection behaviour. The predicted response is compared with that recorded experimentally.