Accidental events, such as impact loading or explosions, are rare events with a very low probability of occurrence. However, their effects often lead to very high human losses and economic consequences. An adequate design against these events should reduce the risk for the life of the occupancy, minimize the damage extension and enable a quick rebuilding and reuse. A structure fulfilling these requirements is 'robust'. Different strategies can be pursued for accidental events, and among them, methods based on the residual strength or the alternate load path are frequently adopted because applicable to a vast range of structures. Adequate design strategies based on them require an in-deep knowledge of load transfer mechanisms from the damaged to the undamaged part of the structure. As to the frames, the important role of joint ductility was pointed out in recent studies. Besides, the flooring systems substantially affect the spread of the damage, but the research on this subject is still very limited. The present study focuses on steel-concrete composite frames under the column loss scenario. It aims to better understand the influence of both frame continuity and floor systems in the development of 3D membrane action. Two geometrically different 3D steel-concrete composite full-scale substructures were extracted from reference buildings and tested simulating the column collapse scenario. This paper illustrates the preparatory studies, the main features of the specimens and the outcomes of the first test. The test provided an insight in the need for an enhanced design of joints and pointed out the key features of the response of the floor system.

Slim floor systems represent an economical and competitive solution for building applications that combine the advantages of concrete floors, prefabricated steel sections and a shallow depth. The distinctive feature of this form of construction relies on the fact that the lower steel flange is wider than the top flange to provide a continuous support along the beam length for the slab formwork. The latter is usually specified in the form of either profiled sheeting or cellular slabs. In this structural typology, the steel section is embedded within the thickness of the slab.This paper presents an experimental study aimed at evaluating the service behaviour of slim floor beams induced by the time-dependent behaviour of the concrete and at establishing the possible influence of creep effects on their ultimate response. For this purpose, two slim floor samples were prepared in a simply-supported configuration. Their long-term deflections and deformations were monitored over time for about ten months, after which the specimens were tested to failure. The specimens possessed identical concrete and steel geometries. The shear connection was provided by transverse steel reinforcing bars installed within regularly spaced holes incorporated in the steel web. The samples were cast unpropped so that the self-weight of the wet concrete was carried by the steel member. The only difference between the two specimens consisted in the loading history specified over time. In particular, one slim floor sample was kept unloaded for the entire duration of the test to monitor the influence of shrinkage effects while the second specimen was subjected to a sustained load to evaluate the effects of both creep and shrinkage. The experimental data reported in this study provides insight into their long-term and ultimate response, and valuable benchmarking data for the calibration of numerical models and design procedures related to the serviceability and ultimate limit states of slim floor systems.

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Accidental events, such as impact loading, are rare events with a very low probability of occurrence but their effects often leads to very high human losses and economical consequences. An adequate design should not only reduce the risk for the life of the occupancy, but should also minimize the disastrous results and enable a quick rebuilding and reuse. A robust design prevents the complete collapse of the structure when only a limited part is damaged or destroyed. Design against disproportionate collapse is usually based on the residual strength or the alternate load path methods. Identification of an alternate path may lead to an effective and cost efficient design for progressive collapse mitigation by redistributing the loads within the structure. The continuity of the frame and of the floor represent essential factors contributing to a robust structural response. They in fact enable development of 3D membrane action. A European project focusing on robustness of steel and steel and concrete composite structures subjected to accidental loads is still ongoing. In the framework of the project the authors concentrated their studies on the redundancy of the structure through slab-beam floor systems as well as through ductile joint design. At this aim, two 3D full scale substructures were extracted from a reference building and experimentally investigated with the purpose to get an insight into the mechanisms allowing the activation of the alternate load paths resources, when a column collapse. The paper illustrates the main features of both the specimens tested and the experimental campaign. The preliminary results of the tests are presented and discussed. KEYWORDSRobustness, alternate path strategy, collapse of a column, steel and concrete composite structures, membrane action, full-scale tests.