Flexural behaviour of concrete filled tubular flange girders (CFTFGs)

Abstract

There is an ever-increasing need to develop innovative structural solutions which offer greater efficiency and resilience compared with existing technologies. In construction, both engineers and also society as a whole are becoming more intolerant of practices which are considered unsustainable such as excessive material usage, short life-spans, high maintenance costs, etc. In this context, the research presented in this thesis is focused on a novel and efficient structural shape called a concrete filled tubular flange girder (CFTFG) and the principle aim is to gain a deep understanding of their behaviour through both numerical and analytical modelling. CFTFGs are steel beams in which the top flange plate is replaced with a hollow steel section which is then filled with concrete. The concrete in the tube strengthens the compression flange of the girder, providing greater torsional stiffness and thereby increasing the lateral-torsional buckling (LTB) resistance of the girder, relative to a regular steel beam of similar proportions. In heavily loaded applications such as bridges and car parks, CFTFGs can result in time and cost savings relative to more traditional sections as much of the fabrication is conducted offsite with fewer splices and complex connections required on-site. The concrete filled tubular flange can theoretically be any shape and the focus in the current research is on simply supported members with either a rectangular or circular top flange, as well as stiffened webs. CFTFGs are complex members and their behaviour is governed by several inter-related parameters. In order to investigate these, and develop a deeper understanding of the behaviour, a nonlinear three-dimensional finite element (FE) model is developed using the ABAQUS software and validated using available experimental data from the literature. The validated models are then employed to conduct a series of parametric studies to investigate the influence of the most salient parameters on the performance. The finite element models consider the effects of initial geometric imperfections, as well as other geometrical and material nonlinearities, on the response. For comparison purposes, and to observe the effect of the concrete infill, the same girders with a bare steel tubular flange section are also studied. In addition to the FE model, and with a view to providing designers with convenient guidance, simplified analytical expressions for the flexural capacity of CFTFGs are also proposed, and the results are compared to those from the FE analyses. It is found that concrete filled members exhibit similar buckling shapes to similar sections without the concrete infill but significantly greater buckling resistance. This highlights the influence of the concrete infill which increases the stiffness of the upper flange, and hence allows the member to carry additional bending moments compared to bare steel sections. The analytical expressions, which are suitable for design, are also shown to be capable of providing an accurate depiction of the bending moment capacity. In a further development of the work, the ultimate strength of circular CFTFGs under combined axial tension and positive (sagging) bending moment is also investigated, as this is a common scenario in bridge applications. Current design codes do not explicitly include guidance for the design of CFTFGs, which are asymmetric in nature under the combined effects of tension and bending. A finite element model has been developed using ABAQUS to study this behaviour. Based on the finite element analysis, the moment–axial force interaction relationship is presented and a simplified equation is proposed for the design of circular CFTFGs subjected to combined bending and tensile axial force. The data and analysis presented in this thesis supports the use of concrete filled tubular flange girders in appropriate, heavily-loaded, design scenarios. They are shown to provide an efficient load-carrying solutions, and can cover large spans without the need for intermediary supports. The design expressions proposed are based on a fundamental review of the behaviour and are shown to provide an accurate depiction of the capacity of these sections either in bending or under combined loading.