Keywords: progressive collapse, robustness, steel frames, column removal, full and partial strength connections.

In this paper the robustness and resistance to progressive collapse of steel framed structures under exceptional forces has been investigated. Such a topic is of wide interest when exceptional forces, that is either loads not considered in the design phase or loads greater than the design ones, are applied to structures. In particular, the robustness reserve of the structure has to be exploited in order to preserve its structural integrity. As a consequence, the direct damage deriving from the application of the loads should be prevented and the indirect one should be really limited in order to avoid global structural collapse.

After some calculation methods proposed by authors have been presented [1,2], a new general robustness assessment technique has been proposed and applied to some case studies, represented by steel framed structures (two-storey one-bay and three-storey three-bay) designed with both the old and the new Italian seismic codes. The robustness of the structures studied has been assessed under different column-removed conditions, related to different catastrophic events (blast, impact, fire, etc.), by means of a new non linear static analysis approach based on the alternative load path method in order to estimate their resistance against progressive collapse.

In particular, the computational model starts with the whole structural model where the loads are applied. Afterwards, the structural stiffness is decreased to take into account column loss and the applied loads are increased by means of a dynamic amplification factor to consider the dynamic nature of the phenomenon. This enables the assessment in a more precise way by permitting the stress redistribution into the structural elements, so leading to a load history dependent procedure.

The structure robustness index is determined as ratio between the direct damage caused by the exceptional event and the total damage, equal to the sum of the direct damage and the indirect one.

The analyses performed have enabled the evaluation of the robustness performance of the structures studied, by considering the variability of the joint types (full strength and partial strength), as well as to make a comparison between structures designed using the new seismic Italian code and the old one.

The results achieved have shown the best behaviour of structures designed using the old normative code as a result of the presence of more robust beams able to offer a better catenary effect. Analyses has also clearly shown the influence of the connection type. In fact, full strength and rigid connections enable high robustness levels to be achieved, whereas semi-rigid ones exhibit less performance, showing a better behaviour when they are of the full strength type. In addition, indications about the worst scenarios of column removal have been given. Bad situations are those connected with the loss of internal columns, immediately followed by the loss of the corner column. Finally, analyses have also shown that the scenario hazard increases as the structure level number increases.

1

A. Formisano, A. Marzo, F.M. Mazzolani, "Robustness based design of new and existing steel structures", in F.M. Mazzolani, J.M. Ricles, R. Sause, (Editors), Proc. of the Int. Conference STESSA 2009, "Behaviour of Steel Structures in Seismic Areas", Philadelphia, 16-20 August 2009, CRC Press-Taylor & Francis Group, Leiden, The Netherlands, 815-822, 2009. doi:10.1201/9780203861592.ch118

2

A. Formisano, F.M. Mazzolani, "Robustness assessment approaches for steel framed structures under catastrophic events", Proc. of the Int. Conference STESSA 2012, "Behaviour of Steel Structures in Seismic Areas", Santiago, 9-12 January 2012.

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