Behaviour of beam-column substructures with typical steel joints in progressive collapse scenarios
Item statusRestricted Access
Embargo end date03/07/2020
In a progressive collapse scenario, the immediate effect due to a local triggering failure, i.e. the loss of a column as considered in the present study, is exerted on the beams bridging over the lost column. The behaviour of such double beam assembly plays a critical role in determining whether a progressive collapse would occur or not. This study focuses on the realistic behaviour of such beam assemblies, particularly the plastic deformation concentration, the associated degradations in the connection regions, and how these in turn affect the overall resistance behaviour and the ultimate strength of the beam assemblies. To start with, a generic axially-constrained beam is analytically examined in order to better understand the full range response including the large deflection regime, the development of the plastic deformation, and how this in turn affects the overall resistance and the deformability. The load-deflection relationship and the associated plastic deformation are first analytically derived for a bending scenario, and then the effect of the catenary action is introduced. The analytical solution provides some quantitative insight, although under idealised conditions, into the limiting local criterion (material strain in this case) and the global deflection and deformability. A laboratory experimental study is then carried out on six reduced scale double beam assemblies. The purpose-designed connection details allow different local plastic mechanisms and local failure modes to develop, with an intention to represent relative weak connections so as to supplement the existing experimental literature in the coverage of relationship between connection failure limits and the overall resistance capacities. The test results have revealed that the specimens went through elastic and plastic bending stages; however, the catenary actions could not be developed sufficiently in most of the specimens whenever a premature local failure, such as material/weld fracture, bolt shear and concrete cracking, became dominant. The outcome highlighted the crucial importance of enabling the plastic deformation to “spread” in order to ensure a total plastic deformation capacity in the plastic zones as well as the importance of the axial capacities of the joint connections. In conjunction with the existing studies, the experimental exploration provided further evidences for the future experimental and numerical studies to focus on the design details in typical connection types and how these could transpire into the actual development of the plastic regions and effective catenary action. With representation of the realistic connection behaviour in mind and for practical applications, a new analytical framework is proposed in this study on the basis of component-based connection modelling, for the solution of the resistance function and the ultimate deformation limits. An analytical framework is realized by an in-house program written in MATLAB. In order to apply the analytical framework to double beam assemblies, the existing quantitative formulations for typical joint components of common steel joints are scrutinized. Examples of double beam assemblies with web-cleat and TSWA connection are carried out to verify the analytical solutions and demonstrate the key characteristics of the resistance functions involving these types of joints. Analytical results show good agreement with the corresponding test data. A dedicated study is then devoted to the development of a complete component model for a widely used connection in steel gravity frames, namely the fin-plate connection, which is however relatively less investigated in terms of the component modelling into the large deformation regime. Subsequently the overall behaviour of double beam assemblies involving such joints is examined using the analytical framework. In the development of the component model for the fin-plate joints, existing models and formulations to describe general plate bearing and bolt shearing behaviour are incorporated to establish the basic constitutive properties of the bolted lap-plate component set. Upon the initial verification, improvement of the constitutive descriptions is found to be necessary. To this end, high-fidelity finite element analysis is conducted, and on this basis, modifications to the plate bearing and bolt shearing behaviour are proposed. The analytical solutions with the modified lap-plate component properties are found to be in good agreement with the experimental results. The outcome of this thesis has provided new insight and evidence on the realistic behaviour of beam assemblies, and pointed out the directions for more robust design for improved progressive collapse resistance of the steel framed structures. The analytical framework and the associated solution provide a useful tool for the analysis of the resistance functions for the critical beam assemblies for practical applications. With this approach, the key to the reliability and soundness in the analysis of the resistance function lies upon the adequacy and accuracy of the description of the joint components. In this respect, the future work should focus on more comprehensive quantification of the properties and deformation limits of joint components for a variety of joint types and design details.