Behaviour of axially loaded circular concrete-filled bimetallic stainless-carbon steel tubular short columns

doi:10.1016/j.engstruct.2017.05.064

Engineering Structures

Volume 147, 15 September 2017, Pages 583-597

https://doi.org/10.1016/j.engstruct.2017.05.064 Get rights and content

Highlights

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Bimetallic concrete-filled steel tubular (CFST) short columns are investigated.
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Using ABAQUS, 3D FE bimetallic CFSTs under axial compression are developed.
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High-strength concrete is considered.
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The best design formula for the strengths of bimetallic CFSTs is found.

Abstract

Circular concrete-filled bimetallic steel tubular (CFBST) columns are considered as a new type of structural composite members. An experimental investigation has recently been conducted on the performance of these concentrically-loaded circular CFBST stub columns. However, the fundamental response of these columns under axial compression has not been investigated numerically. Therefore, finite element (FE) analysis of axially loaded circular CFBST stub columns is discussed in this paper. An existing concrete constitutive model with the confinement mechanism is modified for the current CFBST columns. The nonlinear stress-strain relationship of stainless steel is utilised in the FE analysis. The current FE model accounts for the influences of initial imperfections, geometric and material nonlinearities. The ultimate strengths and load-strain responses predicted from the analysis are validated against the available test results and observations in literature. The comparisons indicate that the FE model accurately estimates both the ultimate strengths and load-strain characteristics of the concentrically-loaded circular CFBST stub columns. The developed model is then utilised to investigate the effects of the geometric configurations and material properties on the load-strain characteristics, ultimate capacity, ductility and steel contribution ratio of circular CFBST stub columns. The existing design recommendations for conventional circular concrete-filled steel tubular columns are then checked for computing the peak load of the circular CFBST stub columns, and the best strength predictor with the least deviation compared with the experimental values is recommended at the end for design.

Introduction

Conventional concrete-filled steel tubular (CFST) columns are utilised for the efficient construction of offshore structures [1]. The marine environments induce corrosion in the external carbon steel tubes in the conventional CFST columns. This corrosion reduces the strength and ductility of conventional composite columns. The barrier to the corrosion agents can be created by replacing the conventional carbon steel tubes with stainless steel ones. However, the initial high costs of the stainless steel greatly reduce its applications in the constructional industry. Therefore, the economical application of the stainless steel can be achieved by using bimetallic tubes filled with concrete. A typical circular concrete-filled bimetallic steel tubular (CFBST) column section is illustrated in Fig. 1. As can be seen, the bimetallic steel tube consists of external stainless steel tube with an inner layer made of carbon steel component. The overall costs of CFBST columns are expected to be lower than those of the conventional CFST columns in the long term, given that maintenance is not required for CFBST columns with their high corrosion resistance of the stainless steel envelopes [2]. The external layer of stainless steel, besides its corrosion and chemical resistances, offers many benefits including high strength, axial stiffness, strain ductility and extended hardening in compression.

The nonlinear characteristics of conventional CFST columns have been experimentally studied by many researchers [3], [4], [5], [6], [7], [8], [9]. These studies indicate that the confinement mechanism increases the strain ductility and the compressive strength of the conventional composite section. On the other hand, the stainless steel was recently investigated as an alternative to carbon steel in conventional CFST columns. The experimental studies for predicting the structural performance were conducted for concrete-filled stainless steel tubular (CFSST) columns [10], [11], [12], [13], [14], [15]. The peak loads and strain ductility of the CFSST columns were found to be higher than those of conventional CFST columns. Unexpectedly, although the CFSST columns offer the above mentioned structural benefits, current international standards in Australia (AS 5100.6-2004 [16]), America (AISC 360-05 [17]), Europe (Eurocode 4 [18]) and China (DBJ/T [19]) do not include any recommendations for the design of such columns.

Liew et al. [20] reported the design of conventional CFST columns with high strength materials. These materials offer various structural benefits in high-rise composite construction such as decreasing the cross-sectional size and subsequently increasing the floor area, but they reduce the ductility of these columns. However, experimental studies on circular CFBST columns with high strength materials have not been investigated yet. Only Ye et al. [2] tested ten axially loaded circular CFBST stub columns with normal strength materials. The test results reported by Ye et al. [2] indicated that the CFBST columns fail by the shear failure of the concrete component and the local buckling of the bimetallic tubes. In addition, CFBST short columns had higher peak strengths compared with the total strength of their individual components. Furthermore, it was found that the strain at the column’s ultimate strength is much higher than the peak strain of the individual components.

Extensive numerical investigations were performed in the past to study the structural response of conventional CFST columns [21], [22], [23], [24], [25], [26]. However, no numerical model was developed for analysing the fundamental performance of concentrically loaded circular CFBST columns. Ellobody and Young [27], Tao et al. [28], Hassanein et al. [29] and Patel et al. [30] conducted numerical studies on the structural behaviour of CFSST stub columns subjected to axial loading. Their numerical models considered the influences of the confinement mechanism and the extended strain hardening of the stainless steel. The numerical models proposed by these researchers were found to be accurate and computationally efficient.

Literature review indicates that no numerical analysis has been presented for simulating the compressive performance of the CFBST stub columns, as presented in this paper. The finite element (FE) model presented in this paper was developed by using the general-purpose FE code ABAQUS 6.13 [31]. The proposed model considers the influences of the confinement mechanism, high strength materials and stainless steel strain hardening. Its accuracy is verified by comparing the obtained predictions with the existing test results of the circular CFBST stub columns [2]. A parametric study is subsequently conducted to cover various geometric configurations and material properties of circular CFBST stub columns. Finally, the existing design methods given by the international standards in one hand and by Liang and Fragomeni [21] in the other hand were checked for the possibility of computing the peak load of the concentrically-loaded circular CFBST columns.

Section snippets

Modelling assumptions

The assumptions considered in the current FE simulation of the circular CFBST stub columns are:

1.
The perfect bond was assumed between the carbon and stainless steel components. Following the manufacture process used by Ye et al. [2], the bimetallic steel tube is assumed herein to be manufactured by rolling a flat stainless steel plate round over the carbon steel tube. Full penetrated groove welding is then used to connect the two ends in the longitudinal direction. The outer surface of the carbon

Stainless steel

The nonlinear material model given by Rasmussen [35] was used to model the material behaviour of the stainless steel which is the external layer of the bimetallic tubes. The austenitic stainless steel was used in the external layer of the bimetallic tubes [2]. It should be noted that the material model proposed by Rasmussen [35] represents the accurate behaviour of austenitic stainless steel [36]. Fig. 4 gives the adopted stress versus strain response for the stainless steel. The nonlinear

Ultimate strengths

The experimental data proposed from the independent study conducted by Ye et al. [2] was employed to validate the proposed FE model. The experimental investigation by Ye et al. [2] consisted of 10 circular CFBST stub columns, from which the $D / t$ ratio varied between 45 and 57. The length of the tested specimens was equal to 615 mm. The test specimens were made from bimetallic tubes and the measured concrete strengths were found as 18 MPa, 26 MPa and 36 MPa as given in Table 1. The elastic modulus of

Parametric study

In this section, a parametric study was performed to examine the inelastic response of the circular CFBST stub columns. A total of 200 models were developed with varying geometric and material properties. Three groups with various geometric configurations and material parameters were considered in this analysis as given in Table 2. The steel contribution ratio of the circular CFBST columns was determined as the ratio of load carried by the bimetallic tube $(P_{su})$ to the peak load $(P_{u})$ of the

Eurocode 4 [18]

Eurocode 4 [18] considers the confinement mechanism of the concrete component offered by the outer tube. The equation is modified for the circular CFBST column, which is expressed as: $P_{u . EC 4} = η_{a} (A_{sc} f_{y} + A_{ss} σ_{0.2}) + A_{c} f_{c}^{'} [1 + η_{c} \frac{(t_{sc} + t_{ss})}{D} \frac{(A_{sc} f_{y} + A_{ss} σ_{0.2})}{(A_{sc} + A_{ss}) f_{c}^{'}}]$ $η_{a} = 0.25 (3 + 2 \overline{λ}) ⩽ 1.0$ $η_{c} = 4.9 - 18.5 \overline{λ} + 17 {\overline{λ}}^{2} ⩾ 1$ $\overline{λ} = \sqrt{\frac{N_{pl.Rk}}{N_{cr}}}$ $N_{pl.Rk} = A_{sc} f_{y} + A_{ss} σ_{0.2} + A_{c} f_{c}^{'}$ $N_{cr} = \frac{π^{2} {(EI)}_{eff}}{L^{2}}$ ${(EI)}_{eff} = E_{s} I_{sc} + E_{0} I_{ss} + 0.6 E_{cm} I_{c}$

in which ${(EI)}_{eff}$ represents the effective flexural stiffness, $η_{a}$ and $η_{c}$ denotes as the factors related to the confinement of

Conclusions

The inelastic response of concentrically-loaded CFBST stub columns was investigated by employing the finite element (FE) method. An existing concrete confinement model was modified for accurately simulating the lateral pressure applied by the bimetallic tubes. The present model was verified with existing experimental results. The obtained predictions for ultimate loads and load-strain behaviour compared reasonably well with the experimental results with the mean and standard deviation (SD) of

Acknowledgements

This work is supported by the School of Engineering and Mathematical Sciences at La Trobe University with the startup research fund. This financial support is gratefully acknowledged.

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