Structural behaviour of precast concrete frames subject to column removal scenarios
Date of Issue2015
School of Civil and Environmental Engineering
NTU-MINDEF Protective Technology Research Centre
From time to time, structural collapse incidents throughout the world prompt research works on the robustness of building structures to mitigate progressive collapse. Among the design approaches, alternate path method tends to prevent the spread of local damage through mobilisation of compressive arch action (CAA) and catenary action in the bridging beam and the floor system. However, development of alternate load path is contingent on structural ductility and integrity at large deformations. This study aims to investigate the ability of precast concrete joints to develop CAA and catenary action. An experimental programme was conducted on beam-column sub-assemblages and frames under column removal scenarios. The middle beam-column joint and double-span beam over the removed column were extracted from a typical precast concrete structure and scaled down to one-half models. Two enlarged end column stubs were designed in the sub-assemblages, to which horizontal restraints were connected. In addition, engineered cementitious composites (ECC), with strain-hardening behaviour and superior strain capacity in tension, were utilised in the cast-in-situ structural topping and beam-column joint. In the precast frames, side columns were curtailed between the column inflection points below and above the bridging beam to represent realistic boundary conditions to the bridging beam. In comparison with flexural resistance, development of CAA and catenary action substantially enhanced the progressive collapse resistance of beam-column sub-assemblages. Besides, the effects of reinforcement detailing in the joint, longitudinal reinforcement ratios in the beam, and horizontal interface preparation between the precast beam unit and cast-in-situ concrete topping, on the resistance and deformation capacity of sub-assemblages were studied experimentally under relatively rigid boundary condition. Furthermore, a comparison was made between sub-assemblages with conventional concrete and ECC to highlight the effect of ECC on structural behaviour of sub-assemblages under column removal scenarios. In the experimental tests on precast concrete frames, the influence of joint detailing and boundary conditions on progressive collapse resistance of the frames was investigated. Special attention was placed on the behaviour of side columns subjected to CAA and catenary action. Design recommendations were made in accordance with experimental results. Based on previous studies, an analytical model was proposed to predict the CAA of beam-column sub-assemblages. The tensile strength of ECC and stress-strain model of concrete were considered in the model. A series of parametric studies was conducted through the analytical model to identify dominant parameters on the resistance of sub-assemblages subjected to CAA. In addition, the pseudo-static resistance was calculated through the energy balance method. In the catenary regime, the component-based joint was developed for precast concrete joints to provide an efficient and explicit representation of joint behaviour. Interactions between the structural members and beam-column joint were modelled by zero-length springs with specific constitutive relationships. The average bond stress for calculating the force-slip relationship of a spring was evaluated. Furthermore, a tension spring representing pull-out failure of embedded reinforcement was derived for precast concrete joints. Finally, the model was calibrated by experimental results of precast and reinforced concrete beam-column sub-assemblages.
DRNTU::Engineering::Civil engineering::Structures and design