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.

Basic material properties and cross-sectional data (stress-strain curves and load-end shortening curves) are presented for square, rectangular and circular hollow section specimens in Grade 1.4301 stainless steel. The material tests cover flat material in tension and in compression as well as corner material in tension. Modifications to the Ramberg-Osgood representation are suggested to ensure a close fit to both tensile and compressive behaviour over the full range of strains of interest. Results, including full load-end shortening curves, for a total of 37 stub column tests have been presented.The results have been used to develop an explicit relationship between cross-sectional slenderness and cross-sectional deformation capacity, which forms the basis for a proposed new design approach for stainless steel structures.

This paper describes numerical modeling of the structural response of stainless steel hollow sections. The aim of the investigation was to develop a consistent approach to the modeling of stainless steel structures. The developed FE models are more sophisticated than any other reported attempts to model stainless steel structural behavior, with general expressions defined for material stress-strain behavior, enhanced strength corner properties, initial geometric imperfection modes and amplitudes (local and global), and residual stresses. The general expressions define a consistent means of describing the key input parameters. A compound (two-stage) Ramberg-Osgood model is developed to describe stainless steel material stress-strain behavior in tension and compression. For the prediction of enhanced strength corner properties, a simple, though accurate model is proposed. Characterization of local plate imperfection amplitudes is described whereby a model originally devised for hot-rolled carbon steel cross-sections was re-calibrated and applied to stainless steel cross-sections. Numerical prediction of the key performance measures from tests is achieved with a high degree of accuracy: On average, ultimate load was predicted to within 3% and with a low standard deviation; deformation at ultimate load was within 6%, but exhibited a higher standard deviation; and the general form of the load-deformation response and the failure modes were similar.

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...

An experimental study has been conducted into the degree of composite action that can arise between cold-formed steel joists and wood-based flooring panels. A series of material, pushout and four-point bending tests have been carried out, and alternative means of shear connection, featuring fasteners and adhesives, have been investigated. It has been found that the spacing of the fasteners as well as the application of structural adhesive at the beam-board interface has a significant influence on the attained degree of shear connection and hence the moment capacity and flexural stiffness of the system. The highest degree of shear connection (up to about 60%) was obtained using the structural adhesive, bringing corresponding increases in capacity and stiffness of around 100% and 40% respectively over the bare steel.Smaller, but still very significant, increases in capacity and stiffness were achieved through the use of screws alone. Based on the results of the push-out tests, a load-slip relationship for screw fasteners in wood-based floorboards has been proposed; this is designed for use in future analytical and numerical models. The findings of this research demonstrate, for the first time, the benefits, in terms of enhanced structural performance and efficiency of material use, that can be derived through the practical exploitation of composite action in cold-formed steel flooring systems.

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