Ultra-high strength circular short CFST columns: Axisymmetric analysis, behaviour and design
Introduction
CFST columns are considered as an economical option for the composite structural members since most of the concentrically compressive force is carried out by the concrete which is cheaper than the steel. Accordingly, further economies could be obtained by utilizing UHSC in CFST columns. The use of UHSC in CFST columns considerably decreases the cross-sectional size, and thus saves the workspace area. However, the use of UHSC in CFST columns significantly reduces their ductility [1]. To overcome this problem, steel tubes with high yield stress and small diameter-to-thickness ratio should be employed to ensure the ductility of CFST columns with UHSC. The workability of the UHSC is lower than that of the NSC [1]. The prefabricated columns should be used to avoid the workability problems at the construction sites. The current structural codes restrict the use of UHSC in CFST columns [1]. This is due to the limited research on ultra-high strength CFST columns. Techno station in Tokyo Japan [2] is one of the well-known example in adopting the application of UHSC in CFST columns. Although extensive research on CFST circular columns with high strength concrete and steel has been undertaken, the numerical analysis is very limited for the simulation of circular short CFST columns with UHSC. Therefore, the numerical investigation on ultra-high strength short circular CFST columns is needed [3].
Many researchers have experimentally assessed the characteristics of CFST columns with NSC and High Strength Concrete (HSC) [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. The constructional applications of CFST columns were recently reviewed by Han et al. [15]. A comprehensive experimental study was first performed by Liew and Xiong [1] and Xiong et al. [3] to investigate the axial performance of concentrically compressed ultra-high strength CFST short columns. Their specimens were made from UHSC with compressive strength up to 193 MPa and hollow steel tube having yield stress up to 428 MPa. The results of their experiments demonstrated that the ductility is decreased because of the brittleness of UHSC, but it increases by utilizing the small ratios. They concluded that Eurocode 4 [16] method safely estimates the maximum capacities of axially loaded ultra-high strength short circular CFST columns. Moreover, the required ductility of short circular CFST columns with UHSC occurred at a steel contribution ratio of 0.3 or more. The steel contribution ratio is defined as the ratio of ultimate strength of steel section to the ultimate strength of CFST column section. They also concluded that there is a limited research performed on CFST columns with UHSC.
Nowadays, the concrete core may be reinforced by fibres or steel bars to enhance the ductility of the column. Australian Standard, AS3600-2009 [17], allows the use of fibre reinforced concrete in the structural design of bridges. The influences of high performance constructional materials on the characteristics of axially and eccentrically compressed slender CFST beam-columns were examined by Portolés et al. [18]. They showed that the filled UHSC generally decreases the ductility but it increases by using the steel fibres. They carried out experiments on 24 beam-column specimens with NSC, HSC and UHSC. The results indicate that the contribution of UHSC on concentrically loaded CFST columns is more significant than that of eccentrically loaded ones. Xiong et al. [19] experimentally assessed the characteristics of ultra-high strength slender CFST beam-columns with circular sections. The test results showed that pin-ended slender CFST beam-columns are mainly failed by the overall buckling.
Considerable numerical research on the structural characteristics of CFST columns has been devoted over the past two decades [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. However, very limited numerical study on CFST columns with UHSC can be found in the literature. Wang et al. [30] employed the finite element (FE) simulation code Abaqus to assess the structural characteristics of concentrically the compressed ultra-high strength short circular CFST columns. A new stress-strain relationship for the confinement mechanism of UHSC is proposed. Xiong et al. [19] numerically investigated the performance of eccentrically compressed slender circular CFST beam-columns with UHSC. The material law suggested by Eurocode 2 [31], [32] for the filled concrete was implemented in the FE model based on Abaqus. Le Hoang and Fehling [33], [34] recently developed a FE model using Atena-3D for the simulation of concentrically compressed circular ultra-high strength CFST columns. An equation was reported to estimate the peak capacities of ultra-high strength circular CFST short columns. The results indicated that the proposed model safely estimates the maximum load of the ultra-high strength circular short CFST columns.
As mentioned, the research study on UHSC is very limited in comparison with the investigation on NSC and HSC. An axisymmetric simulation is undertaken in this paper to model the characteristics of axially loaded circular CFST short columns with UHSC as illustrated in Fig. 1. The proposed axisymmetric model is verified with the specimens tested by Xiong et al. [3]. The verification study demonstrates that the proposed axisymmetric model is not only much simpler than detailed 3D model, but also predicts well the behaviour of axially loaded circular CFST short columns with UHSC including elastic stiffness, ultimate load-carrying capacity and post-peak response. A parametric study is also performed on ultra-high strength short circular CFST columns for a wide range of parameters including ratio, concrete compressive strength and steel yield stress. Finally, the design of ultra-high strength circular short CFST columns using the existing code provisions and proposed model is discussed in details.
Section snippets
Confined concrete
As the lateral strain of the concrete component in CFST columns becomes more than that of the outer tube, the radial pressure develops at the contacting surfaces between the concrete and steel. A steel tube in circular CFST column is bi-axially stressed in the hoop and axial directions. The hoop stress component induces the confinement to the inner concrete while the axial stress accelerates the steel tube buckling. The confinement mechanism offered by the steel tube to the concrete component
General description
Abaqus [42] is used for the computational simulation of concentrically loaded short circular CFST columns with UHSC. The axisymmetric model and corresponding three dimension model are shown in Fig. 6. The initial geometric imperfection was employed in the hollow steel tube and considered in the axisymmetric simulation to allow for the outwards local buckling of steel tubes. The shape of the imperfection of steel tubes is based on the first buckling mode obtained from the eigenvalue analysis of
Peak load of concentrically loaded CFST short columns
The dimension of tested columns and concrete and steel material parameters are listed in Table 1. The experimental investigation carried out by Xiong et al. [3] consists of 14 ultra-high strength circular short CFST columns with ratio varied from 18 to 44. The column length was equal to either 250 mm or 600 mm. The experimental specimens were made from the hot-rolled steel with yield stress from 300 MPa to 428 MPa. The compressive strength of UHSC is ranged between 149 MPa and 193 MPa. The
Parametric study
The parametric study based on the verified axisymmetric model is undertaken to examine the effects of material and geometric parameters on the residual strength ratio, ductility, column diameter and axial load-strain behaviour of ultra-high strength circular short CFST columns. Table 2 presents the geometric and material properties of 99 columns used in this parametric study with a yield stress varying from 250 MPa to 420 MPa, compressive strength varying from 60 MPa to 190 MPa, and
Design codes
The current design methods limit the material strengths for the design of CFST columns. The international standards of AS/NZS 5100.6 [44], Eurocode 4 [16], AISC [45] and GB 50956 [46] are used to check their feasibility for ultra-high strength short CFST columns. These design models are compared to the FE predictions and the test data given by Xiong et al. [3].
Conclusions
An efficient axisymmetric simulation for circular short CFST columns with UHSC has been presented in this paper. The axisymmetric simulation considers the influences of initial geometric imperfection, material and geometric nonlinearities. The developed axisymmetric models were verified by the independent test data. The axisymmetric results agree with the experimental results including elastic stiffness, peak load and load-strain response. The ratio of the peak loads predicted by the
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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