A Simplified Steel Beam-to-Column Connection Modelling Approach and Influence of Connection Ductility on Frame Behaviour in Fire

Steel beam-to-column connections are vulnerable structural elements when a fire strikes a building, as observed in fire incidents and full-scale fire tests. Existing techniques allow researchers to model the behaviour of different types of connections in fire but are difficult to use when conducting simulations on full-scale frames with multiple connections due to time and computation requirements. Therefore a need for a simplified connection modelling approach that can significantly reduce the computational time required without compromising on the accuracy of the simulation results so that large-scale simulations of structures with multiple connections in fire can be performed. A simplified spring connection modelling approach for steel flush endplate beam-to-column connections in fire has been developed in this research project so that the realistic behaviour of connections can be incorporated into full-scale frame analyses at elevated temperature. The proposed modelling approach divides the connection into two or three (depending on the connection size) T-stubs and employs ABAQUS as a pre-processor to generate the force-displacement characteristics for each T-stub by detailed finite element modelling. These characteristics are then input into specialised software (VULCAN) to simulate the behaviour of structure in fire including realistic representation of the steel beam-to-column connections. As a result of its simplicity and reliability, the proposed approach permits full-scale frame analysis in fire to be conducted efficiently. The proposed simplified spring connection modelling approach has been used to investigate the influence of connection ductility (both axial and rotational) on frame behaviour in fire. 2D steel and 3D composite frames across a range of spans were modelled to aid the understanding of the differences in frame response in fire when the beam-to-column connections have different axial and rotational ductility assumptions. The research study highlights that adopting the conventional rigid or pinned connection assumptions does not permit the axial forces acting on the connections to be predicted accurately, since the axial ductility of the connection is completely neglected when the rotational ductility is either fully restrained or free. By including realistic axial and rotational ductility of the beam-to-column connections, the frame response in fire can be predicted more accurately, which is advantageous in performance-based structural fire engineering design.