The advancement in the building and construction industry, coupled with high-rise buildings and heavy haul transportation networks, has increased the demand for concrete-filled steel tubular (CFT) beams and columns. CFTs exhibit excellent load-bearing capacity, high seismic performance, and economic benefit attributed to the combined advantages of supporting effects of the concrete and steel tubular. Their combinational response is majorly dependent on the cross-sections of its members that exhibit different structural behaviors. With the increasing demand for novel, attractive and efficient constructions suitable for modern society, CFT beams and columns offering various cross-sections have attracted significant research attention. Among them, the oval-ended elliptical concrete filled with steel tubular (OECFT) has been identified as a potential candidate for numerous structural applications owing to its good aesthetics, high structural efficiency, and low fluid resistance properties.
Over the past few years, significant amount of research has been conducted to understand the behavior of OECFT columns and beams. Nonetheless, these works have mainly focused on the properties of OECFT columns under single load conditions, with limited research on the effects of combined forces. In engineering practice, subjecting structural members to different loads at the same time is a common occurrence. As such, the behavior of various forms of CFT columns has also been investigated under compression and torsion loading conditions. The torsional resistance of CFT columns with normal cross-sections is reportedly influenced by the axial forces. Unfortunately, the design behavior and properties of OECFT columns with different sections subjected to axial compression and torsion loading are sparsely explored despite their crucial applications.
In this paper, researchers at Hefei University of Technology: Wenbing Xing (Ph.D. student), Dr. Qihan Shen, Professor Jingfeng Wang, and Mingyu Sheng (Ph.D. student) studied the torsional behavior of axially loaded OECFT columns through numerical analysis. In particular, they investigated the structural performance and design provisions of the OECFT columns under combined axial compression and torsion loadings. The aim was to provide a basis for the engineering application of OECFT columns. Their research work is currently published in the journal, Journal of Construction Steel Research.
Briefly, the research team utilized ABAQUS solver for the numerical analysis. First, a modified equivalent of oval-ended elliptical confined concrete was developed as a simplified version of the finite element model and verified against the existing experimental. Next, a comprehensive parametric analysis of the OECFT column was conducted to examine the effects of material and geometric characteristics on the properties of the columns under combined axial comprehension-torsion. Finally, a simplified model for predicting the load-bearing capacity of OECFT columns under combined axial comprehension and torsion was proposed taking into account the aspect ratio and confinement factor.
The authors observed that the torsional resistance of the column increased with an increase in the concrete strength, steel ratio and steel strength and decreased with an increase in the aspect ratio. Moreover, the failure patterns of the OECFT column were accompanied by diagonal concrete cracks inclined at 45 degrees and an inclined tensile strain on the steel tube. After the damage of the core concrete, the fracture strain of the steel tube exceeded its critical value and was also damaged completely. Based on the numerical analysis, the curve was divided into three phases: elastic, elastic-plastic and plastic, which were depended on the changes in typical torque ratio, axial compression ratio, material strength, and steel ratio.
The study presented a detailed numerical analysis of the structural design and performance of OECFT columns under combined axial compression-torsion. Based on the results, a design provision for assessing and accurately predicting the torsional strength of OECFT columns under combined axial compression-torsion was proposed and validated against the existing experimental data. In a statement to Advances in Engineering, the authors stated that their study expand the knowledge of OECFT columns and their applications in the construction industry.
Xing, W., Shen, Q., Wang, J., & Sheng, M. (2020). Performance and design of oval-ended elliptical CFT columns under combined axial compression-torsion. Journal of Constructional Steel Research, 172, 106148.