Robust frames require relatively weak beams, but strong connections and columns. Limit state design may provide the reverse. The composite beams used in the majority of steel framed buildings are shown to possess a ‘hidden’ reserve of strength. When subjected to the large sagging deformations associated with terrorist attacks, these beams are capable of resisting typically twice their design load. This creates a situation whereby the weak point in a frame can be transferred to the connections, leading to non-ductile and potentially catastrophic failures in the event of severe overloading. Furthermore, certain industry standard nominally pinned connections are shown to lack the ductility required to accommodate large beam end rotations. This lack of ductility could result in the premature failure of connections due to the high tensile forces generated in the connection bolts. To address these problems an alternative approach is advocated, whereby ductile beams are designed to resist only working loads, albeit elastically. Thereafter the upper-bound flexural strength is established. It is the corresponding upper-bound reactions that are used for the subsequent design of components lower down in the load path. This approach would improve robustness and provide economies in the use of construction materials in structures considered at risk from terrorist attack.
M. P. Byfield, BEng, PhD, CEng, MICE, MIStructE
School of Civil & Environmental Engineering, University of Southampton, Highfield, Southampton, SO17 1BJ