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

Abstract

A simplified spring connection modelling approach for steel flush endplate beam-to-column connections in fire has been developed to enable realistic behaviour of connections to be incorporated into full-scale frame analyses at elevated temperature. Due to its simplicity and reliability, the proposed approach permits full-scale high-temperature frame analysis to be conducted without high computational cost. 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 with a range of beam spans were modelled to aid the understanding of the differences in frame response in fire where the beam-to-column connections have different axial and rotational ductility assumptions. The modelling results highlight that adopting the conventional rigid or pinned connection assumptions does not permit the axial forces acting on the connections to be accurately predicted, since the axial ductility of the connection is completely neglected when the rotational ductility is either fully restrained or free. By accounting for realistic axial and rotational ductilities of beam-to-column connections, the frame response in fire can be predicted more accurately, which is advantageous in performance-based structural fire engineering design.

Figure 1. Development procedure of the proposed simplified spring connection modelling approach.

Figure 2. Flush endplate details.

Figure 3. Contact surface groups.

Figure 4. Connection rotational behaviour.

Figure 5. Connection deformation.

Figure 6. Tensile bolt rows under pure bending.

Figure 7. T-stub grouping configurations.

Figure 8. T-stub force-displacement curves.

Figure 9. A connection transformed into a simplified spring connection model.

Figure 10. Simplified spring model (SSM) validating against FEM.

Figure 11. Simplified spring model transformation.

Figure 12. T-stub force-displacement curves.

Figure 13. Validation results of FE1, FE5 and FE8.

Figure 14. Spring connection model proposed by Sun et al. (2015).

Figure 15. 2D Frame in VULCAN & ABAQUS.

Figure 16. Simplified spring connection.

Figure 18. Validation of the simplified spring model in VULCAN and ABAQUS at elevated temperature.

Figure 19. 2D steel frame model and connection details.

Figure 20. Flush endplate connection details and T-stub stiffnesses.

Figure 21. Fire protection scheme.

Figure 22. Axial forces at connections for various spans of the beam.

Figure 23. General floor layout.

Figure 24. Flush endplate connection details and T-stub stiffnesses.

Figure 25. Composite slab strips and downstand beam stubs in the frame models.

Figure 26. Loadings applied to the composite frame.

Figure 27. Steel frame protection scheme and locations of connections investigated in this study.

Figure 28. 3D composite frame simulation results.

Figure 29. Typical axial force on the edge secondary beam-to-column connections in 3D composite frame.

Figure 30. Typical changes in axial force, ΔF_ax, acting on connections.

Figure 17. Validation of the simplified spring model at ambient temperature.

Table 1. Beam section selection

Table 2. Maximum axial force at the connections comparisons

Table 3. Beam section sizes

Table 4. Structural member temperature profile with reduction factors

Table 5. Maximum axial force at the connections comparisons