Flexural behavior of concrete-filled double-skin steel tubular beams after subject to high temperature

Highlights

• CFDST beams under pure bending subject to high temperature.

• The in-depth mechanism and systematic analysis of CFDST after subject to high temperature are carried out.

• The proportion of ultimate bearing moment of each component is studied by ABAQUS.

• A temperature effect coefficient k_r was proposed to predict the ultimate bending moment after subject to high temperature.

Abstract

Concrete-filled double-skin steel tubular (CFDST) specimens that exhibit lightweight, good ductility, and better bending resistance, can be used for beams. However, fire often causes great damage, and the flexural behavior of CFDST beams after subject to high-temperature has not been studied. To investigate the flexural behavior of CFDST beams after subject to high temperature, seven CFDST specimens after subject to high temperature were carried out four-point bending tests. The main parameters explored in the test include hollow ratio (0.31, 0.52, 0.72) and temperature (300 °C, 500 °C, 800 °C). The corresponding finite-element analysis (FEA) models were established, and the results showed good agreement with the test. The in-depth mechanism and systematic analysis including the influence of different hollow ratio, material property, and temperature on the whole process curve of CFDST after subject to high temperature are carried out using FEA models. It is found that the ultimate bending moment of CFDST after subject to high-temperature increases with hollow ratio and material property, but decreases with the temperature. The outer steel tube contributes the most to the ultimate bending moment, and the inner steel tube improves the ductility at a later stage. Specimens with a large hollow ratio (χ ≥ 0.52) have better bending capacity, and the ultimate bending moment of specimens with a small hollow ratio (χ < 0.52) decrease the least with the change of temperature due to the protection of concrete. Based on the findings from the test and numerical studies, a temperature effect coefficient k_r is proposed to represent the residual bearing capacity after subject to high temperature, and combined with three different codes to evaluate the ultimate bending moment. A comparison of test results showed the calculated results following the T/CCES 7–2020 (2020) had a good agreement.

Introduction

The concrete-filled double-skin steel tubular (CFDST), unlike the traditional concrete-filled steel tube (CFST), is a novel composite structure comprising two concentric steel tubes with concrete sandwiched between them. There are four common types of CFDST specimens fabricated using different combinations of a circular hollow section (CHS) and a square hollow section (SHS). Moreover, recent emerging technologies and engineering applications have led to the development of novel CFDST shapes. For instance, the CFDST with an octagonal outer steel tube and a circular inner steel tube [1] or with a dodecagonal outer steel tube and an inner steel tube [2]. The interaction between concrete and steel in a CFDST, whereby the sandwiched concrete laterally supports the outer and inner tubes, leads to high stiffness and strength. Moreover, placing a section of the inner steel tube away from the column centroid could significantly improve the bending stiffness [3]. Additionally, the fire resistance of the CFDST specimens is higher than that of the CFST specimens for the following two reasons: Firstly, the outer steel tube and the sandwiched concrete absorb most of the heat, thereby delaying the temperature increase in the inner steel tube. Secondly, the inner steel tube acts as a ventilation channel in case of a fire [[4], [5]]. Besides, the absence of concrete inside the inner steel tube of a CFDST makes the self-weight lower and the steel ratio higher, which improves ductility and seismic performance. Hence, owing to the abovementioned advantages and easy installation, the CFDST can be used in structures such as high-rise buildings, long-span bridges, offshore platforms, electric transmission towers, marine tunnels, and viaducts. Besides, the hollow sections of the CFDST have the benefit of transmitting liquid and electric lines.

Pure bending is the special pattern of CFDST beam-columns. While in practical engineering, the flexural behavior of CFDST always combines pure bending with axial load, eccentric load or cyclic load: CFDST has been used as high-rise bridge piers in Japan against earthquake loading and electric transmission poles in Zhejiang Province, China. Therefore, investigating on pure bending is fundamental to further analysis of the axial load-moment interaction of CFDST. Most of the former researchers focus on analyzing the CFST and CFDST under axial loading and only a few researchers study the flexural performance of them and mainly focus on room temperature. For instance, Zhao et al. [6] conducted five tests on CFDST beams (SHS outer and inner) and proposed a theoretical model to predict their ultimate bending moment, which was widely accepted. The results of the tests also showed that the CFDST beams exhibited good ductility. Similarly, Han et al. [7] conducted four tests on CFDST beams (SHS outer and CHS inner) and proposed a confinement factor, which described the interaction between the steel tubes and the sandwiched concrete. Tao et al. [8] conducted three tests on CFDST beams (SHS outer and CHS inner) and proposed a simplified formula for flexural stiffness. Furthermore, Tao et al. [9] conducted four tests on CFDST beams (rectangle hollow section outer and inner) and proposed theoretical and simplified models to predict and estimate the ultimate bearing moment. Huang et al. [10] conducted three tests on CFDST beams (CHS outer and inner) and proposed a simplified formula to calculate the ultimate bending moment. Besides, some CFST specimens with special sections and materials are widely being investigated. Patel [11] studied the uniaxial compression of Round-ended concrete-filled steel tubular (RCFST) beam-columns at room temperature by using a numerical fiber-based methodology and the results showed good agreement. Li et al. [12] designed an experiment on six high-strength concrete- filled high-strength square steel tube (HCFHST), which showed the ultimate bearing capacity increased with the rise of steel ratio.

As a fire is a major disaster for buildings, CFDST specimens have been investigated for fire and temperature resistance. For instance, Yang et al. [13] proposed a theoretical model to predict the temperature field of CFDST specimens and reported that the bearing capacity decreased rapidly as the fire duration increased. Lu et al. [14] conducted six tests on CFDST columns with self-consolidating concrete under fire exposure. The results showed that the limiting temperature of the CFDST specimens was higher than that in the CFST specimens: the temperature of the inner steel tube in the former was very low and thus, these specimens exhibited good fire resistance. Lu et al. [15] developed a finite element analysis (FEA) model to simulate the fire behavior of CFDST columns and reported good agreement with the experimental results. Imani et al. [16] proposed an analytical procedure for the calculation of the axial load capacity of the CFDST columns subjected to fire. Yao et al. [17] developed a 3D FEA model to simulate the fire performance of CFDST columns and proposed an engineering design to promote fire resistance, and then extended the conventional Rankine approach to predict the fire resistance of the CFDST columns subjected to axial load. Wan et al. [18] proposed a series of new unified design formulas for calculating the temperature field and fire resistance of short and slender CFDST columns under axial loading when exposed to fire. Yao et al. [19] conducted tests on CFDST columns subjected to non-uniform heating and the results indicated a positive correlation between the hollow ratio and fire resistance of the CFDST columns, and a negative correlation between the load ratio and slenderness ratio. Li et al. [[20], [21]] tested the seismic performance of 12 CFDST specimens under fire exposure and proposed simplified and numerical models with reasonable accuracy. To date, most research studies have focused on analyzing the post-fire behavior of CFDST stub columns under axial load. However, the behavior of CFDST beams under pure bending after exposure to high-temperature has not yet been studied and there is no test data found in such condition. Therefore, this paper presents the test results on seven CFDST beams under pure bending after exposure to high-temperature. The main test parameters considered are the high temperature and the hollow ratio of the cross-section. Besides, the corresponding FEA models were provided for comparison with the experimental results and for analyzing different design parameters. Finally, a temperature effect coefficient k_r has been proposed and used to predict the ultimate bearing moment of the beams.