This paper proposes a novel simplified framework for progressive collapse assessment of multi-storey buildings, considering sudden column loss as a design scenario. The proposed framework offers a practical means for assessing structural robustness at various levels of structural idealisation, and importantly it takes the debate on the factors influencing robustness away from the generalities towards the quantifiable. A major feature of the new approach is its ability to accommodate simplified as well as detailed models of the nonlinear structural response, with the additional benefit of allowing incremental assessment over successive levels of structural idealisation. Three main stages are utilised in the proposed assessment framework, including the determination of the nonlinear static response, dynamic assessment using a novel simplified approach, and ductility assessment. The conceptual clarity of the proposed framework sheds considerable light on the adequacy of commonly advocated measures and indicators of structural robustness, culminating in the proposal of a single rational measure of robustness that is applicable to building structures subject to sudden column loss. The companion paper details the application of the new approach to progressive collapse assessment of real steel-framed composite multi-storey buildings, making in the process important conclusions on the inherent robustness of such structures and the adequacy of current design provisions.

The companion paper presents the principles of a new design-oriented methodology for progressive collapse assessment of multi-storey buildings. The proposed procedure, which can be implemented at various levels of structural idealisation, determines ductility demand and supply in assessing the potential for progressive collapse initiated by instantaneous loss of a vertical support member. This paper demonstrates the applicability of the proposed approach by means of a case study, which considers sudden removal of a ground floor column in a typical steel-framed composite building. In line with current progressive collapse guidelines for buildings with a relatively simple and repetitive layout, the two principal scenarios investigated include removal of a peripheral column and a corner column. The study shows that such structures can be prone to progressive collapse, especially due to failure of the internal secondary beam support joints to safely transfer the gravity loads to the surrounding undamaged members if a flexible fin plate joint detail is employed. The provision of additional reinforcement in the slab over the hogging moment regions can generally have a beneficial effect on both the dynamic load carrying and deformation capacities. The response can be further improved if axial restraint provided by the adjacent structure can be relied upon. The study also highlights the inability of bare-steel beams to survive column removal despite satisfaction of the code prescribed structural integrity provisions. This demonstrates that tying force requirements alone cannot always guarantee structural robustness without explicit consideration of ductility demand/supply in the support joints of the affected members, as determined by their nonlinear dynamic response. Keywords IntroductionA new relatively simple yet sufficiently accurate methodology is presented in the companion paper [1] , which aims at appraising the efficacy of multi-storey buildings to resist progressive collapse triggered by sudden local column failure, as a consequence of an extreme loading event. The potential for progressive collapse is assessed in three independent stages based on the ductility demand and supply in the critical regions of the affected structural members. A significant advantage of the developed procedure is that it can explicitly account for the dynamic effects associated with the instantaneous column removal through a simplified energy equivalence approach, thus avoiding the need for nonlinear dynamic analysis. With respect to its applicability, the proposed method accommodates both simplified and detailed models of the nonlinear static response. Moreover, it can be implemented at various levels of structural idealisation, depending on the required level of sophistication, the feasibility of model reduction and the availability of analytical tools [1] . These levels correspond to either the full structure, excluding the damaged column, or critical sub-structures in which ductility demands are concentrated.The componen...

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A simplified framework is proposed for progressive collapse assessment of multi-storey buildings, considering sudden column loss as a design scenario. This framework can be applied at various levels of structural idealisation, and enables the quantification of structural robustness taking into account the combined influences of redundancy, ductility and energy absorption. Three main stages are involved in the proposed approach: i) determination of the nonlinear static response, ii) dynamic assessment using a novel simplified approach based on energy conservation, and iii) ductility assessment at the maximum dynamic deformed configuration. The application of the proposed method is illustrated on a multi-storey steelframed composite building, where the relative importance of various joint details and levels of axial restraint is highlighted. Importantly, the study underlines the inadequacy of prescriptive tying force requirements that neglect ductility issues, and demonstrates that typical composite buildings must rely on bending or compressive arching rather than tensile catenary action for enhanced structural robustness.

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