title: Progressive Collapse Resistance of Reinforced Concrete Flat Plate Structures
Creators: name: Xue, Huizhong
Publisher: name: Griffith University
Year: 2019
Subjects: subject: Progressive collapse
subject: Reinforced concrete
subject: Flat plate structures
subject: Construction systems

description: Progressive collapse of structures caused by extreme or accidental loads may lead to significant loss of life and property. Considerable research efforts have been made to date to mitigate the probability of progressive collapse and its consequences. However, a vast amount of the existing knowledge is pertinent to the reinforced concrete (RC) frame structures whilst the understanding of progressive collapse mechanisms of RC flat plate structures is still limited. RC flat plate structures represent one of the most common construction systems used in residential buildings and car parks in Australia and internationally. The absence of beams, column capitals and drop panels which can help to redistribute the concentrated loads, makes flat plate structures susceptible to punching shear failure at their slab-column connections and subsequent damage propagation, potentially leading to a catastrophic progressive collapse. The post-punching shear behaviour of the slab-column connections and the load redistribution in the slabs are of particular interests, which essentially dominate the load-carrying capacities after punching shear failure. To investigate the progressive collapse mechanisms and resistance of RC flat plates, experimental tests were performed on two 1/3-scaled 2  2-bay RC flat plate substructures specimens under an interior column removal scenario. In addition to the uniformly distributed load (UDL) imposed on the slab, an incremental downward displacement was applied to the interior slab-column connection to simulate the column loss and subsequent collapse. Custom-built column load cells were designed and carefully calibrated to ensure an accurate measurement of the reaction forces and moments at column bases. The overall load-displacement responses, crack propagations, failure modes and strain developments, were recorded and analysed. The complete collapse-resistant behaviour and load redistribution pattern of the specimens were examined, from which three load-carrying mechanism phases, in the form of flexural, tensile membrane and a combination of oneway catenary and dowel actions were distinguished in resisting the applied concentrated load. To study the punching and post-punching shear behaviours of RC slab-column connections being isolated from their parent flat plate structures, a set of numerical modelling techniques was established and verified against the experimental results of eight slab-column connections. In this modelling strategy, the concrete was simulated using solid elements with calibrated Continuous Surface Cap material model (CSCM) and failure criterion for punching shear. The reinforcing bars were explicitly created using beam elements with material properties obtained from the uniaxial tensile tests as well as calibrated failure criterion for rebar rupture. As a result, a competent 3D nonlinear numerical model of RC slab-column connections without shear reinforcement was created, with which, the punching shear failure featuring a critical punching shear surface and an abrupt drop of the applied force in the load-displacement response was able to be accurately reproduced. The post-punching shear behaviour, taking the form of an increased load-carrying capacity which was ceased by rebar fracture in the suspension stage, was also well captured. Using the proposed numerical model, typical punching and post-punching shear failure mechanisms were studied in some detail, finally leading to a simple yet effective analytical solution to accurately and reliably predict the postpunching shear response of RC slab-column connections. To investigate the influences of critical slab design parameters on the progressive collapse resistance of RC flat plate structures subjected to an interior column loss, the already established numerical modelling strategy for RC slab-column connections was employed to simulate our own experiment of the 2  2-bay flat plate substructure under a concentrated load and a similar test under a UDL in the literature for model validation. In addition to the modelling techniques developed for RC slab-column connections, the modelling of bond-slip behaviour at the interface between concrete and reinforcing bars was also highlighted, which was found to have a significant impact on the structural performance of flat plate substructures in progressive collapse. The key structural behaviours of the substructures under large deformations including the tensile membrane and suspension actions were able to be replicated. Further, the validated numerical model being subjected to a concentrated load was used to conduct a series of parametric studies in which the influences of concrete strength, slab thickness and reinforcement ratio on the progressive collapse performance were examined. The outcomes of the parametric studies indicated that the concrete strength and the slab thickness only affected the flexural capacity to different degrees with no impact on the post-failure capacity, whereas the load-carrying capacity due to the tensile membrane action was primarily governed by the amount of slab reinforcement. This research, covering experimental, numerical and analytical studies of RC slab-column connections and flat plate substructures with a missing interior column, offers a further understanding of their punching and post-punching shear behaviours as well as collapseresistant mechanisms. The numerical modelling techniques developed for the substructures with an interior column loss can be readily used to simulate other column loss scenarios (edge column, corner column and multiple columns). Further detailed analyses of the 3D numerical models will help to establish a simplified numerical model to facilitate collapse simulations of entire flat plate structures induced by any potential column removal scenarios.