Keywords: robustness, progressive collapse, multi-storey buildings, structural design.

This chapter presents recent developments in the design-oriented assessment of structural robustness with particular focus on multi-storey buildings. The benefits of sudden column loss as a standard test of structural robustness are identified in terms of its event-independence, its potential correlation with actual extreme dynamic events, and its computational benefits. In this context, it is proposed that the most realistic definition for the robustness limit state should be based on the avoidance of collapse in the floors above the damaged column. In turn, this could be related to the ductility demand being less than the corresponding ductility supply in the various affected parts of the structure for all states up to the maximum dynamic deformation state.

In dealing with the dynamic effects of sudden column loss, several load factor approaches based on amplified static gravity loading have been previously proposed for use with nonlinear static analysis. While the most recent variants of these approaches [1] attempt to account for the influence of ductility supply on dynamic amplification, such attempts lack generality for common forms of nonlinear static response. This could lead to significant conservatism in some cases but, most seriously, could also lead in other cases to considerable underestimation of dynamic amplification, thus resulting in unsafe designs.

It is advocated that the shortcomings of load factor approaches can be addressed in a rational manner through the explicit consideration of ductility demand and supply, taking into consideration the maximum dynamic deformations sustained by the affected floor system. Towards this end, a design-oriented ductility-centred approach has been developed recently at Imperial College London for sudden column loss scenarios [2,3]. This approach offers a practical multi-level assessment framework, which combines the commonly advocated indicators of structural robustness, including ductility, redundancy and energy absorption capacity, in a single rational measure termed the pseudo-static capacity. The application of the ductility-centred approach to steel-concrete composite multi-storey buildings clearly demonstrates the inadequacy of prescriptive and load factor approaches in robustness design, the importance of enhanced connection ductility, and the relative merits of such factors as additional connection reinforcement, axial restraint and infill panels.

[1]

D. Stevens, B. Crowder, B. Hall, K. Marchand, "Unified Progressive Collapse Design Requirements for DoD and GSA", Structures Congress'08 - Crossing Borders, Vancouver, Canada, 2008.

[2]

B.A. Izzuddin, A.G. Vlassis, A.Y. Elghazouli, D.A. Nethercot, "Progressive Collapse of Multi-Storey Buildings due to Sudden Column Loss - Part I: Simplified Assessment Framework", Engineering Structures, 30(5), 1308-1318, 2008. doi:10.1016/j.engstruct.2007.07.011

[3]

A.G. Vlassis, B.A. Izzuddin, A.Y. Elghazouli, D.A. Nethercot, "Progressive Collapse of Multi-Storey Buildings due to Sudden Column Loss - Part II: Application", Engineering Structures, 30(5), 1424-1438, 2008. doi:10.1016/j.engstruct.2007.08.011

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