Brian Uy

Strength of short concrete filled high strength steel box columns

Abstract The use of thin walled steel sections coupled with concrete infill has been used on various building projects with great advantage. The currently available international standards for composite structures are limited to the design of concrete filled steel columns with compact sections. However, there is limited research work in the literature available which is concerned with slender concrete filled thin-walled steel columns. This paper presents a comprehensive experimental study of thin walled steel sections utilising high strength steel of a thin walled nature and filled with normal strength concrete. A numerical model is developed herein in order to study the behaviour of slender concrete filled high strength steel columns incorporating material and geometric non-linearities. For this analysis, the equilibrium of the member is investigated in the deformed state, using the idealised stress–strain relationships for both the steel and concrete materials, considering the elastic and plastic ranges. This paper presents both an experimental and theoretical treatment of coupled local and global buckling of concrete filled high strength steel columns sometimes termed interaction buckling. The experimental results of columns with high strength steel casings conducted herein by the authors are used for comparison. The effect of the confined concrete core is also addressed and the method shows good agreement with the experimental results of concrete filled steel columns with compact sections. The behaviour of concrete filled steel slender columns affected by elastic or inelastic local buckling is also investigated and compared with relevant experimental results. The paper then concludes with a design recommendation for the strength evaluation of slender composite columns using high strength steel plates with thin-walled steel sections, paying particular attention to existing codes of practice so as not to deviate from current design methodologies.

In-plane stability of arches

Classical buckling theory is mostly used to investigate the in-plane stability of arches, which assumes that the pre-buckling behaviour is linear and that the effects of pre-buckling deformations on buckling can be ignored. However, the behaviour of shallow arches becomes non-linear and the deformations are substantial prior to buckling, so that their effects on the buckling of shallow arches need to be considered. Classical buckling theory which does not consider these effects cannot correctly predict the in-plane buckling load of shallow arches. This paper investigates the in-plane buckling of circular arches with an arbitrary cross-section and subjected to a radial load uniformly distributed around the arch axis. An energy method is used to establish both non-linear equilibrium equations and buckling equilibrium equations for shallow arches. Analytical solutions for the in-plane buckling loads of shallow arches subjected to this loading regime are obtained. Approximations to the symmetric buckling of shallow arches and formulae for the in-plane anti-symmetric bifurcation buckling load of non-shallow arches are proposed, and criteria that define shallow and non-shallow arches are also stated. Comparisons with finite element results demonstrate that the solutions and indeed approximations are accurate, and that classical buckling theory can correctly predict the in-plane anti-symmetric bifurcation buckling load of non-shallow arches, but overestimates the in-plane anti-symmetric bifurcation buckling load of shallow arches significantly.

Local and post-local buckling of concrete filled steel welded box columns

Abstract This paper considers the local and post-local buckling of steel plates in thin-walled concrete filled steel box columns. Concrete filled steel fabricated box columns and the many advantages that accrue from their use are reviewed. The concept of local buckling is described and the effects it has on the strength of concrete filled steel columns is discussed. An extensive set of experiments for the local and post local buckling behaviour of concrete filled steel box columns is presented. A semi-analytical finite strip method developed elsewhere which includes the beneficial effect of concrete is augmented to incorporate the true stress-strain behaviour and the residual compressive and tensile stresses produced by welding. The model is then calibrated with the experiments of this paper. Slenderness limits derived from this analysis are compared with existing Australian and British standards and these illustrate the advantages obtained from a rational local buckling analysis. A post-local buckling model based on the effective width principle is then established which can be used to determine the strength of a concrete filled box section.

Elastic local buckling of steel plates in composite steel-concrete members

A finite strip model for elastic local buckling is used to study the behaviour of steel plates in composite steel-concrete members. The accuracy of the numerical method is demonstrated by a comparison with solutions in the literature. Buckling of the steel skin is then considered, and width-to-thickness ratios suitable for design are suggested. The method is shown to be applicable to profiled composite slabs, profiled composite beams, profiled composite walls, composite steel-concrete columns and composite steel-concrete girders. A design example is given to show the applicability of the method.

Slenderness limits for filled circular steel tubes

Abstract This paper presents a theoretical study of the local and post-local buckling of thin-walled circular steel tubes that contain a rigid infill, with the motivation being an investigation of the cross-sectional behaviour of concrete-filled steel tubes used as columns in composite construction. An analysis is undertaken to determine a closed form solution for the elastic buckling of a circular tube with a rigid infill, and a model of the post-local buckling response is proposed. This generic approach is calibrated against test data, and a cross-section slenderness limit is proposed that delineates between a fully effective cross-section and a slender cross-section. A simple prescriptive equation is proposed for the buckling strength of the steel cross-section that is consistent with many design codes, and illustrates that the presence of an infill may enhance the cross-sectional strength, not only by the added strength of the infill itself, but by delaying the buckling of the steel tube.

Stress-Strain Curves of Structural and Reinforcing Steels after Exposure to Elevated Temperatures

AbstractFor the evaluation of damage to a structure after exposure to fire, the residual mechanical properties of structural materials need to be evaluated first. Many factors can affect the post-fire behavior of structural and reinforcing steels. In this paper, existing test data are collected from an extensive survey of the literature. A statistical analysis is conducted to analyze the effects of heat exposure on key parameters, such as the residual modulus of elasticity, residual yield strength, and residual ultimate strength, which control the full-range stress-strain curves of steel. A simplified stress-strain model is developed, which can be used for both structural and reinforcing steels after heating and cooling down to room temperature.

Theoretical study on the post-local buckling of steel plates in concrete-filled box columns

This paper presents a theoretical study on the post-local buckling behaviour of steel plates in welded steel box columns filled with concrete, by using the finite element method. The effects of various geometric imperfections, residual stresses and width-to-thickness ratios on the post-local buckling characteristics of steel plates in composite columns are investigated. A novel method is developed for evaluating the initial local buckling loads and post-local buckling reserve strength of steel plates with imperfections associated with a theoretical analysis. Two effective width formulas are proposed for the design of clamped steel plates restrained by concrete and are employed in the ultimate strength calculation of short concrete-filled steel box columns. The proposed design models are compared with existing experimental results with a good agreement.

An analytical model for thin-walled steel box columns with concrete in-fill

Concrete-filled steel box columns are widely used as a major structural element in tall buildings as they provide efficient structural performance in resisting axial compression. Local buckling of steel plates is a common failure criteria when plate slenderness ratio is large in box sections. A method using an effective width principle is proposed in this paper to predict the behaviour and load carrying capacity of thin-walled steel tubes with concrete in-fill. The present investigation addresses columns pinned at their ends and subjected to biaxial loading. Columns tested by other researchers have been analysed using the proposed method and the predicted results are compared with the corresponding test results. The proposed model is also verified against Eurocode 4. The comparison shows that the method could predict the ultimate load with sufficient accuracy.

Response of foam- and concrete-filled square steel tubes under low-velocity impact loading

This paper presents the results of experimental and numerical studies of the comparative behavior of square hollow section (SHS) tubes filled with rigid polyurethane foam (RPF) and concrete undergoing transverse impact loading. A series of instrumented drop hammer tests were performed on mild steel and stainless steel SHSs for both filled and unfilled constructions. The concrete-filled tubes had the highest impact resistance and energy absorption capacity, followed by the steel tubes filled with RPF, and then the hollow tubes. The results also show that RPFs can be used as an effective infill material in structural steel hollow columns when expedient enhancement of the energy absorption capacity is required, e.g., to increase blast and impact resistance of hollow structural elements. Nonlinear dynamic finite-element analyses were carried out to simulate drop hammer test conditions. The predicted impact forces, deformation histories, and failure modes were found to be in good agreement with the experimenta...

Large-Scale Experimental Validation of Steel Posttensioned Connections with Web Hourglass Pins

A new self-centering beam-to-column connection is proposed. The connection uses posttensioned high-strength steel bars to provide self-centering capability and carefully designed energy-dissipation (ED) elements that consist of steel cylindrical pins with an hourglass shape. The proposed ED elements have superior ED and fracture capacity, and are placed between the upper and the bottom flanges of the beam such that they do not interfere with the composite slab. A simplified performance-based procedure was used to design the proposed connection. The connection performance was experimentally validated under quasi-static cyclic loading. The specimens were imposed to drift levels beyond the expected design ones to identify all possible failure modes. The experimental results show that the proposed connection eliminates residual drifts and beam damage for drifts lower than or equal to 6%. A simplified analytical procedure using plastic analysis and simple mechanics was found to accurately predict the connection behavior. Repeated tests on a connection specimen were conducted, along with replacing damaged ED elements. These tests showed that the proposed ED elements can be easily replaced without welding or bolting, and hence the proposed connection can be repaired with minimal disturbance to building use or occupation in the aftermath of a major earthquake.