Experimental study of rectangular CFST columns subjected to eccentric loading

This thesis presents an experimental and analytical investigation of the non-linear behaviour and design of high-strength rectangular concrete-filled steel hollow section (CFSHS) columns under axial loading, pure bending and eccentric loading. In the experimental programme, 117 specimens were tested to examine the effects of material strength, slenderness and aspect ratio on the behaviour of the columns. Ductility performance of the high-strength concrete was found to be significantly enhanced by the steel hollow section. Analytical models are developed for estimating the behaviour of high-strength rectangular CFSHS columns. The calibration of the models shows that they agree reasonably well with the present experimental results as well as those previously reported by other researchers. The experimental results are also employed to calibrate the specifications in the design codes, i.e. EC4, ACI and AISC. It indicates that modifications should be incorporated into the codes so that a safe and accurate design can be achieved for high-strength rectangular CFSHS columns. Based on the experimental and analytical investigation, design guidelines are developed and presented.

Experimental and numerical investigation of incident angle effects on the dynamic response of RC beams under near-field blast loading

Strain compatibility method for the design of short rectangular concrete-filled tube columns under eccentric axial loads

Behaviour of high strength rectangular concrete-filled steel hollow section columns under eccentric loading

Experimental investigation on the fire behaviour of rectangular and elliptical slender concrete-filled tubular columns

High strength steel square and rectangular tubular stub columns infilled with concrete

An experimental investigation on eccentrically loaded stainless steel tubular columns filled with steel-fibre-reinforced concrete (SFRC) is presented in this paper. The composite columns had different depth-to-plate thickness (D/t) ratios, stainless steel tube cross-sections, lengths, non-dimensional slenderness, concrete strengths, steel fibre percentages and concrete type. The pin-ended composite columns were subjected to eccentric compressive loads with a large eccentricity of 0·25D. The composite column strengths, load–axial shortening behaviour and failure modes were measured and observed in the tests. The experimental investigation showed that eccentrically loaded SFRC-filled stainless steel tubular columns offer a considerable enhancement in column strength compared with those filled with plain concrete (PC). In addition, it was found that an increase in the steel fibre percentage enhanced the strength behaviour of the composite columns. Furthermore, the composite column test strengths were compared with design strengths predicted using Eurocode 4 for composite columns. Generally, the investigation showed that the code accurately predicted the strengths of the eccentrically loaded PC-filled stainless steel tubular columns of high concrete strength and was quite conservative in estimating the column strengths of the eccentrically loaded SFRC-filled stainless steel tubular columns.

This paper presents the experimental results on the strength and axial behavior of rectangular steel tube columns filled with cellular lightweight concrete (CLC) under axial compression. A total of 24 specimens, including 6 reinforced cellular lightweight concrete (RCLC) columns and 18 cellular lightweight concrete-filled steel tube (CLCFT) columns are investigated. The nominal dimension of the rectangular columns are 150×75 mm in cross-section and 750 mm in height. The parameter used in all tests are the ultimate compressive strengths of the CLC, which are 15 MPa, 20 MPa and 25 MPa, and the wall thicknesses of steel tubes, which are 3.0 mm, 4.5 mm and 6.0 mm. All specimens are prepared and loaded concentrically in axial compression to failure. The results of these tests demonstrated that the CLCFT columns have a linear behavior up to the approximately 80-90% of their maximum compressive load. Then, the behavior of the columns is nonlinear. The nonlinear behaviors are due to the crushing of the concrete core and local wall buckling of the steel hollow tube. In addition, it is found that the CLCFT columns have high axial deformability at the failure when compared to the reference RCLC columns. Finally, by comparing the maximum compressive load of the test results with those obtained from the ACI composite design equation, the comparison results show that calculation formula in ACI code can be applied to compute the axial capacity of CLCFT columns under axial compression.

Element interaction of cold-formed stainless steel cross-sections subjected to major axis bending

In this study, high-strength steel (HSS) reinforcement with a yield stress of about 600 MPa was used in reinforced concrete columns to reduce both the reinforcing bar congestion and the construction costs. For this purpose, 16 square concrete columns reinforced with either the conventional normal-strength steel (NSS) or HSS rebars were subjected to axial and eccentric compression loads. The primary test variables included longitudinal reinforcement with two strength grades, axial load eccentricities, two different concrete compressive strengths, and different longitudinal reinforcement ratios. The structural response of the columns reinforced with reduced HSS rebars (Grade 600) was compared with that of the columns reinforced with grade 420 MPa rebars in terms of their load-carrying capacity, failure mechanism, axial force-bending moment (P-M) interaction, and ductility. Experimental results showed that although the amount of longitudinal steel reinforcement was reduced by about 34% in columns containing grade 600 MPa rebars, their load-carrying capacity and P–M interaction diagrams were comparable to those of the reference columns containing conventional NSS rebars. It was also concluded that simultaneous use of high-strength rebars and high-strength concrete below a balanced point would lead to slightly higher values of ductility index (by up to 4%) than when normal concrete strength and conventional reinforcement steel rebars of Grade 420 MPa are used.

The compressive strength of concrete according to certain codes can be based on the compressive strength of unconfined plain standard concrete cylinders tests at the age of 28 days. In this paper, the standard concrete cylinders were spirally confined with steel bars and with/without hooked‐end steel fibers. The influence of the use of hooked‐end steel fiber in spirally confined concrete with various pitches was investigated. It can be seen that the use of hooked‐end steel fiber contributes significantly in improving both compressive strength and ductility of concrete. The compressive strength and ductility of steel fibered concrete also increase with the reduction of the spiral’s pitch.

Due to high bearing capacity and excellent seismic performance, the section steel reinforced concrete-filled rectangular steel tube has been widely applied in engineering. The application of stainless steel for outer steel tube can improve the durability performance of members. In this study, ABAQUS software was used to perform nonlinear seismic analysis on section steel reinforced concrete-filled rectangular stainless steel tubular columns, and the numerical model was validated against experimental data. The mechanism and parametric study of the hysteresis performance were conducted. These parameters mainly include: concrete strength, strength of stainless steel, strength of section steel, steel ratio of stainless steel tube, steel ratio of section steel, axial compression ratio, and slenderness ratio. The analysis results showed that: increasing the strength of stainless steel tube and concrete can significantly improve the bearing capacity of members, but the strength of section steel has little effect; the bearing capacity decreases with the increasing slenderness ratio; when the axial compression ratio is smaller than 0.4, increasing axial compression can improve the bearing capacity. The strength of each material shows little influence on the shape of load–displacement skeleton; the axial compression ratio presents high effect on the shape of descending stage of curve, and the larger the axial compression ratio, the steeper the descending stage; the slenderness shows high influence on curve shape, and the larger the slenderness, the less the elastic stiffness, and the more gentle the descending stage of curve. Finally, a simplified load–displacement hysteretic model was proposed, which exhibits good agreement with finite element numerical analysis.

Most reinforced concrete (RC) structures are constructed with square/rectangular columns. The cross-section size of these types of columns is much larger than the thickness of their partitions. Therefore, parts of these columns are protruded out of the partitions. The emergence of columns edges out of the walls has some disadvantages. This limitation is difficult to be overcome with square or rectangular columns. To solve this problem, new types of RC columns called specially shaped reinforced concrete (SSRC) columns have been used as hidden columns. Besides, the use of SSRC columns provides many structural and architectural advantages as compared with rectangular columns. Therefore, this study was conducted to explain the structural performance of slender SSRC columns experimentally and numerically via nonlinear finite element analysis. The study is based on nine RC specimens tested up to failure, as well as eighteen finite element (FE) models analyzed by Abaqus soft wear program. The use of SSRC columns led to increase strength by about 12% and reduce deformations, especially with slenderness ratio more than 40 as compared with equivalent square-shaped columns. Two design formulas were proposed to determine the compressive strength of SSRC columns under concentric loading. The results obtained indicate a good structural performance of SSRC columns when compared with equivalent square-shaped columns.

This thesis presents both experimental and theoretical studies on the structural behaviour of externally confined concrete-filled-steel-tube (CFST) short columns under uni-axial compression. External confinement in the form of tie bars, steel rings, spirals and jackets is proposed. From the study, it can be concluded that the external confinement is highly effective in improving the uni-axial behaviour of CFST columns by improving confining stress and suppressing inelastic local buckling.
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\nThe first part of the thesis covers the experimental behaviour of concentrically loaded confined CFST columns. The experimental programme involved testing a total of 208 specimens that consisted of 46 hollow-steel-tube columns with or without external confinement and CFST columns: 28 unconfined, 25 tie-confined, 77 ring-confined, 24 spiral-confined and 8 jacket-confined columns. The test results are summarized and discussed. From analysing the obtained experimental results, a better understanding of the structural behaviours, such as steel-concrete interface bonding, stress state of steel tube, behaviour of core concrete of CFST columns can be obtained. Besides, the test results obtained are useful for deriving and verifying theoretical models in this study.
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\nIn the second part of the thesis, a theoretical stress-strain model is proposed based on the obtained test results. This model is obtained through a four-step procedure. As the first crucial step, a new and accurate hoop strain equation is proposed and verified based on experimental results. Having validated the assumption of stress path independence of the core concrete in CFST columns under monotonic uni-axial compression, an actively confined concrete constitutive model is adopted to predict the axial stress-strain response of the core concrete. Next, with the aid of Generalized Hooke’s Law and Prandtl-Reuss theory, the complicated stress-state of steel tube is accurately modelled. Lastly, by using the newly proposed hoop strain equation together with the interaction models among the core concrete, steel tube and external confinement, the uni-axial behaviours of CFST columns can be predicted. The validity of this model is subsequently verified with the author’s and published experimental results of CFST columns and FRP-confined concrete.
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\nFinally, a parametric study using the proposed model is conducted in this thesis, involving a wide range of parameters, i.e. steel tube yield strength, concrete compressive strength, steel ratio (steel tube outer diameter / thickness) and external confinement (types and arrangements). From the parametric study, two sets of critical steel ratios based on two levels of ductility at different steel yield and concrete compressive strengths are established. Moreover, the maximum column strengths of CFST columns obtained from the model and previous experimental studies are compared with the design strengths calculated using different design codes and the results indicate that the design codes cannot give reasonably accurate predictions. Therefore, new design equations are proposed in this thesis to predict the maximum strength of the CFST columns.

Seismic behaviour and modelling of rectangular FRP-concrete-steel tubular columns under axial compression and cyclic lateral loading