Robust design of steel and concrete composite structures

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

Progressive Collapse Assessment of Steel and Concrete Composite Structures Subjected to Extreme Loading Conditions

Accidental events, such as impact loading and explosions, are rare events with a very low probability of occurrence, but their effects often lead to very high human losses and economical consequences. Vulnerability of structures to the effects of local damages and its mitigation are issues widely discussed inside the scientific community. The structural property associated with such a vulnerability is named robustness. Depending on the type of the structural system and on the importance of consequences, specific design strategies can be adopted in order to ensure a robust structural response. Among them, the system redundancy, the joints and members ductility and the alternate load paths are the ones commonly adopted in case of multi-storey framed buildings. The present work focuses on the study of the behaviour of steel-concrete composite structures subjected to a column loss, and proposes a global overview to quantify the robustness of such systems subjected to this hazard scenario. The description of validated finite element models and of a new analytical tool to predict the response of flat concrete slabs subjected to large displacement are reported in this dissertation. Furthermore, important design hints for composite buildings are proposed. The starting point of the research is an experimental campaign conducted at the University of Trento. Two tests on 3D full-scale one storey composite steel-concrete frames, extracted from five storeys frames designed in accordance to the Eurocodes, were performed simulating the central column removal. The role of the beam-to-column connections and of the concrete slab for the force redistribution was investigated. The experimental data have been then taken as reference for the calibration of finite element models that allowed to conduct further numerical analyses on different structural configurations and design scenarios. In particular, it was studied the influence of the location of the removed column on the structural behaviour. The collapse of central, lateral and corner columns were investigated in order to understand the load transfer mechanism, the requirement of joint ductility and the influence of the concrete slab on the development of alternate load paths. Both experimental and numerical results showed that the concrete slab plays a key role on the load transfer mechanism within the structure: it can hence contribute significantly to the robustness of the system preventing progressive collapse. The knowledge of the response of reinforced concrete slabs subjected to large displacements, as in the case of a column loss, allows quantifying the contribution to the resistance of the building to collapse associated with activation of membrane forces. Regarding this aspect, a new analytical simplified method, based on the principle of virtual works, was developed to predict the load-deflection response of simply supported reinforced concrete slabs with planar edge restraints subjected to large displacement. In conclusion, the present work provides a significant contribution to the knowledge of composite steel-concrete structures subjected to extreme loading conditions and open the way to extend results to different structural configurations and loading scenarious

Experimental assessment of an asymmetric steel–concrete frame under a column loss scenario

Several noteworthy accidents clearly pointed out the risk of disproportioned collapse of framed structures. Design codes recently recognized it by adding a new requirement: the structural robustness. Among the different approaches to check robustness, the most popular is associated with the column loss scenario: the analysis should verify that, in case of a column loss, an alternative load path does exist, limiting the portion of structure affected by collapse. Consequently, numerous experimental and numerical studies of 2D and 3D structures were carried out in recent years to identify the mechanism of load transfer from the damaged to the undamaged part of the structure. This knowledge becomes an essential and fundamental key for assuring adequate resistance against progressive collapse by the development of catenary action in the beams and membrane action in the floor slab. Studies of reinforced concrete systems and of bare steel sub-assemblies are numerous. More recent is the focus on the response of steel–concrete composite structures subjected to accidental events. Furthermore, most of these studies focused on the characterization of 2D sub-assemblies or 3D in-scale framed structures. This paper presents an experimental assessment of the structural response of a 3D full-scale steel and concrete composite frame under the column loss scenario. The results are finally compared with the response of a frame with the same overall geometry but different columns’ layout, tested by the Authors within the same research programme

Verification of analytical and numerical tools - Deliverable D.5b - Robustimpact

The numerical recalculation performed on the slab, beam and joint tests as well as the analytical recalculation of these tests have been employed as support for the experimental investigations represented within the previous chapter. In particular, numerical and analytical analysis have been performed to design the experimental test and the test setup for the symmetric and the asymmetric 3D test performed by UTRE. For the 2D-frame tests a numerical simulation was performed and the analytical model was validated. In addition to the recalculation of the experimental investigations on composite joints some parameters were varied in order to evaluate the influence on the behavior of the composite joints. Furthermore an analytical approach to predict the response of joints subjected to M-N-loading was investigated. In addition, analytical calculations have been performed in order to predict the results of the static and dynamic experimental tests

Detailed results of the experimental tests - Deliverable D.5a - Robustimpact

At the Institute for Steel Structures crash tests on columns were carried out to investigate the dynamic response of the member during the impact as well as the dynamic interaction of the member with the surrounding structure. The tests' campaign planned by the Università degli Studi di Trento comprises of: - two full-scale tests on 3D specimens representative of the reference buildings; From two reference structures selected as cases study two representative substructures characterised by the same overall dimensions but by a symmetric and an asymmetric configuration (i.e., column layout), respectively, have been identified. The substructures, built in laboratory, are tested reproducing the collapse of a central column. The aim of the tests is to investigate the influence of the biaxial membrane effect associated with the concrete slab in a framed structure when the collapse of a column happens. - 20 tensile tests on T-stubs related to the beam-to-column joint adopted in the reference structures and in the full-scale specimens. T-stubs associated to both the column (10 specimens) and the end-plate (10 specimens) are tested. The tests are performed at two different loading speeds aiming at investigating the influence of strain rate effects on the joint performance, with particular reference to the deformation capacity. As a reference, also quasi-static tests are planned. At the Institute of Structural Design experimental tests were carried out investigating the behaviour of 2D-frames under a column loss. In addition the behaviour of composite joints under high speed loading was examined. Within the framework of the project, two tests on composite frames were tested as well as four tests on composite joints. The 2D substructure tests planned at USTUTT have the objective to evaluate the benefit of the concrete slab compared to the behaviour of the composite frame structure concerning the development of a membrane effect. For this purpose, the results of the composite frame tests will be compared to the 3D substructure tests performed at UTRE. Therefore, the test specimens of the 2D substructure tests are adapted to the 3D substructure tests. In order to compare also the results of the composite joint tests to the behaviour of the frames and to observe the influence of the joints on each other the test specimens for the joint tests are also adapted to the frame tests. Experimental impact tests were planned at the University of Liege (ULg): 22 tests on beam-to-column joints with 5 different joint configurations and 22 tests on column base joints with 4 different joint configurations. One of the objectives of the performed study was to propose realistic joints and column bases which could fit with the laboratory facilities of ULg, what induced a downscaling of the dimension of the members in comparison to the dimension of the ones met in the reference building. With the designed specimens, it is possible to investigate the response of different joint components under impact loading, for different levels of energy (small, medium and high levels), and to compare this response to the one obtained through a static test to highlight the dynamic effects. These static tests are performed for each type of specimens and will be used to clearly determine the different level of energy to be used for the impact tests. The general test setup for all tests was derived in Work Package 3 and can be found in the deliverable D.4.1 [18] of the project