Torsional capacity of concrete-filled steel tube columns circumferentially confined by CFRP

Highlights

• Comprehensive finite element models were established and verified by test data.

• Parametric study on torsional behaviour of CFRP-CFST columns was conducted.

• Torsional capacity was compared between CFRP-CFST columns and CFST columns.

• Formula for predicting the torsional capacity was proposed and verified.

Abstract

The concrete-filled steel tube (CFST) column circumferentially confined by carbon fibre reinforced polymer (CFRP), i.e. CFRP-CFST column, is a promising structural member in engineering practice. In this paper, the torsional capacity of CFRP-CFST columns was investigated by means of finite element (FE) simulation and theoretical derivation. Comprehensive FE models were established considering suitable material constitutive laws and interface relationships, which were then verified by experiment results. Besides, parametric analysis was conducted to reveal the geometrical and material parameters' effects on the torsional capacity of CFRP-CFST columns. The results show that the yield strength, the thickness, and the outer diameter of the steel tube are the most influential factors, and the number of CFRP layers also affects the torsional capacity considerably. Meanwhile, the comparison between the CFRP-CFST and CFST column indicates that the confinement effect provided by the CFRP can enhance the torsional capacity. Finally, equations for calculating the torsional capacity of the CFRP-CFST column were proposed and proved to be accurate through comparison with the test data.

Introduction

Due to its high strength, good durability and corrosion resistance, the carbon fibre reinforced polymer (CFRP) has been widely applied to repair and strengthen structural members in recent years [[1], [2], [3]]. By wrapping the CFRP around the surface of a concrete-filled steel tube (CFST), a concrete-filled steel tube circumferentially confined by CFRP (CFRP-CFST) is formed, as shown in Fig. 1. CFRP-CFST columns inherit the advantages from CFST columns, such as the high capacity, the excellent seismic performance, and the construction flexibility [4]. Meanwhile, the CFRP can effectively reduce the local buckling of the steel tube and improve the strength of the core concrete [5]. Moreover, the CFRP-CFST column can achieve a high corrosion resistance ability with the protection of the CFRP in the corrosive environment [6]. All the above features combined make the CFRP-CFST column a promising component in building structures, bridge structures and especially marine structures.

In the past, extensive research has been conducted on the mechanical behaviour of CFRP-CFST columns under typical static loads. Choi and Xiao et al. [7] carried out several axial compression tests on CFRP-CFST columns, and the experimental results proved that the axial compressive capacity of the column is significantly enhanced since the local buckling of the steel tube can be effectively confined by the CFRP. Tao and Han et al. [8] investigated influence of the section type, the diameter-thickness ratio and the CFRP layers on the axial compressive strength and ductility of CFRP-CFST columns through experimental results. Teng et al. [9] proposed a formula based on their active restraint concrete model to calculate the axial compressive capacity of CFRP-CFST columns. At the meantime, Lu et al. [10] derived a capacity formula of CFRP-CFST columns under axial compression by parametric analysis. In order to examine the flexural behaviour of CFRP-CFST columns, numerous tests were conducted by Wang et al. [11,12]. In the studies, they argued that the flexural capacity and stiffness of the columns will improve if the number of longitudinal fibre layers increases and a simplified flexural capacity formula was also proposed. Wei and Wu et al. [13] studied CFRP-CFST columns and CFST columns subjected to flexural loading, and they proved that the CFRP can improve the hardening effect of the steel tube after yielding. As a result, the flexural capacity of the CFRP-CFST column is higher than that of the CFST column. Sundarraja and Prabhu [14] investigated square CFRP-CFST columns to determine the maximum flexural strength and ductility, and to verify the influence of CFRP wrapping positions on the flexural behaviour of columns. To discover the mechanical behaviour of the CFRP-CFST column under combined compression and bending, several studies have been conducted. For instance, Park and Hong et al. [15] investigated the effect of the CFRP on the local buckling of the steel tube subjected to combined compression and bending load. Che [16] proved that the flexural performance of the CFRP-CFST column is related to the ratio of axial compression stress to strength.

As mentioned above, previous studies about CFRP-CFST members concentrated on the mechanical properties under compression, bending or their combination. While in engineering practice, structural components are commonly in the combined compression-bending-torsion state as the structure is subjected to various external forces [17]. For instance, marine structures are often subjected to the combined load made up of wind, waves and currents. Fig. 2 shows an offshore floating wind turbine support structure designed by researchers and engineers recently [18]. This structure system consists of wind turbine tower, edge auxiliary columns (CFST columns), the central main column (CFST column), mooring lines, and trusses. The wind turbine tower will be subjected to wind load, leading to a large bending moment at the top of the central main column; Edge auxiliary columns on both sides of this system will be subjected to different levels of wave loads, causing different levels of shear forces to act on them, and then the central main column will be subjected to torsion load. Under the combined action of the bending moment, the torsion and the self-weight, the central main column is in the combined compression-bending-torsion state. Hence, complicated stress distribution exists in the area from the point A (the joint of the wind turbine tower and the central main column, as shown in Fig. 2) to point B (the connection of the central main column and the mooring line, as shown in Fig. 2). Additionally, this area is prone to corrode since it is near the interface of seawater and air. In this scenario, the CFRP can be wrapped externally around the central long CFST column to enhance its strength as well as corrosion resistance. Similarly, the CFRP-CFST column can also be used in buildings, curved bridges and other structures in the complex combined load [[19], [20], [21]].

In order to understand the performance of the CFRP-CFST column in compression-bending-torsion combined state, the research on the mechanical properties of the CFRP-CFST column under torsion is indispensable [[22], [23], [24]]. However, limited relevant investigations have been reported at present. The authors recently presented an experimental study on the torsional behaviour of CFRP-CFST column [25]. The testing results demonstrated that the confinement provided by CFRP can significantly improve the column capacity and ductility under torsion, and the behaviour of such a composite column is affected by the number of CFRP layers. In the tests, the parameter scopes selected, such as the sectional size of the steel tube, the material strength and the number of CFRP layers, are limited. For purpose of carrying out necessary extension of the experimental study as well as rationally understanding the structural behaviour of CFRP-CFST columns and its key affecting parameters under torsion load, an analytical study was performed in this paper. The targets thus are threefold: (1) to present a comprehensive finite element (FE) modelling on CFRP-CFST columns, which is verified using the test data reported in reference [25]; (2) to extend the parameter scopes and to reveal their effects on the torsional capacity of CFRP-CFST columns by conducting parametric investigations; (3) to propose design formulae for calculating the torsional capacity of the CFRP-CFST column.