Development of a General-Purpose Component-based Connection Element for Structural Fire Analysis

In fire, elevated temperatures undermine the resistance of structural materials, which leads to steel-framed buildings being subject to very large deformations. Elevated temperatures also cause the affected members to expand, and subsequent cooling induces contraction and recovery of strength. Because of the irreversible nature of plastic straining this causes extremely complex force combinations in connections. Connections which are traditionally idealized as “pinned” or “rigid” in design actually display considerable semi-rigid behaviour, which may contribute to the structure’s survival during and after an internal fire.
It will be necessary in future for structural engineers to understand how joints perform in fire, which has been emphasized by a series of case studies, including the official forensic reports on buildings of the New York World Trade complex which collapsed during the “9/11” events in 2001. Eventually, advances in analysis, testing and design codes must allow engineers to design structures which will survive fires without experiencing disproportionate collapse.
This PhD study describes the development of a general component-based connection element, which has been implemented in the Vulcan software in order to enable modelling of the robustness and ductility of the connections in fire scenarios. The component-based method which has been adopted is generally accepted as an efficient intermediate way of treating the behaviour of connections in small-deflection ambient-temperature design of semi-rigid frameworks, which is included in Eurocode 3 Part 1.8. This has been developed in the course of several projects at the University of Sheffield towards high-temperature large-deflection representation of connections in a series of stages, including the characterization of individual components, joint testing and component assembly for some conventional connection types. The RFCS-funded project COMPFIRE, of which this work forms a part, extended the data-set to an innovative connection type, the reverse channel, which offers the prospect of greatly enhanced ductility as a way of improving structural robustness in fire. The new data derives from both structural furnace testing and detailed Finite Element analyses. Used in combination with the “static/dynamic” solver in Vulcan, the use of the general-purpose component-based connection element has been demonstrated in studies of the performance, including progressive collapse, of planar steel frames in fire scenarios.
The development should allow engineers to identify local failure of joints, and to predict the subsequent failure of the remaining structure, in analytical design. This will enable vulnerable areas to be identified in the structure and their design details to be amended in order to produce a building which is more robust in fire.