Behaviour and design of composite steel-concrete beams subjected to flexure and axial load

Abstract

Composite steel-concrete beams are common structural elements formed by connecting a concrete slab to the supporting steel beam with shear connectors. This type of construction is considered favourable for bridge design and is extensively used in high rise buildings. These beams are also used frequently in situations where axial forces are introduced. Such situations may occur through the installation of post-tensioning cables or through the inclination of the beam such as in stadia or ramps. In these situations, the beam may be subjected to any combination of flexure and axial load. However, modern steel and composite construction codes currently do not address the effects of these combined actions. The primary objective of this thesis is to study the behaviour and strength of composite steel-concrete beams subjected to combined flexure and axial load. The research comprises two main parts: a thorough experimental program and an analytical model. The experimental program consists of 24 full-scale composite beams. Each specimen is tested to failure to determine the behaviour, ultimate strength and failure mode. The analytical model is based on cross-sectional analysis (CSA) using a strategy of successive iterations. Results derived from the model show an excellent agreement with the experimental results. Using the analytical model, a parametric study is conducted to investigate the effect of axial load on the flexural strength of composite beams with full and partial shear connection (PSC). The study shows the hogging bending capacity of a member is unaffected by a relatively low axial tensile force. For beams subjected to sagging bending, the tensile capacity of the section is unchanged by moments up to approximately 40% of the sagging moment capacity. The application of axial compression to a beam is immediately detrimental to the hogging moment capacity of the section. This is due to initiating an earlier onset of buckling. The strength of a specimen subjected to combined sagging bending and compression is shown to be crucially dependent on the degree of shear connection. With full shear connection, the CSA and rigid plastic analysis (RPA) both show that the ultimate sagging moment capacity remains unchanged for axial compression loads less than approximately 60% of the squash load. However, significant additional stiffening is required in order to achieve axial compression forces greater than the squash load of the bare steel section. The RPA is shown to be a feasible design tool for axially loaded composite beams. In most cases, the RPA produced results similar to the CSA and the experiments and is generally more conservative. Some overestimation of the flexural strength occurs particularly for the sagging bending and axial compression case with low degrees of shear connection. Design models are proposed for estimating the flexural strength of an axially loaded member with full or PSC. These design models are based on results from the CSA, RPA and experiments as well as a finite element analysis performed in a concurrent study. It is hoped that the findings herein provide an improved understanding of the complex behaviour of composite steel-concrete beams and contribute to the continuing evolution of composite steel-concrete building structures.

Date of Award	2014
Original language	English