Experimental, numerical study and design of concrete-encased concrete-filled steel tube columns and beam-column joints

• Main Author: Ma, Youxin
• Other Authors: Tan Kang Hai
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Civil engineering::Structures and design
• Online Access: https://hdl.handle.net/10356/160731
• Institution: Nanyang Technological University

Steel-concrete composite construction has been widely used for many years in medium to high rise buildings in Singapore, China, America and Japan. The structural and economic advantages of both steel and concrete materials are effectively combined in composite construction. Several structural types have been developed in the steel-concrete composite system, for example, steel beam-to-reinforced concrete (RCS) column frame, steel and concrete beam-to-concrete-filled steel tube (CFST) column frame. Recently, a new type of composite columns formed by encasing the conventional CFST column with the reinforced concrete encasement, known as concrete-encased concrete-filled steel tube (CECFST) column, is developed. Steel and concrete are combined and used more efficiently in CECFST columns. The steel tube of CECFST columns provides confinement effect to the inner concrete core. In return, the concrete encasement can enhance the buckling, fire and corrosion resistance of the inner steel tube. After a comprehensive review of previous research works, the author found that there were limited studies on effects of load eccentricity and biaxial loading for CECFST columns and the seismic performance of concrete beam-to-CECFST column joints. Moreover, there has not been any codes of practice that includes the design of CECFST columns under combined loading (both uniaxial and biaxial) and concrete beam-to-CECFST column joints under seismic loading. In addition, the widely used European code EN 1993-1-8 for joint design mainly address the design of steel structure and cannot be applied to the design of RCS joints. To address these research gaps, comprehensive experimental studies, including two series of tests, were undertaken to study the structural behaviour of CECFST columns and seismic performance of concrete beam-to-CECFST column joints (both cast-in-place and precast), respectively. Design approaches were developed for the cross-section strength prediction of CECFST columns and concrete beam-to-CECFST column joints. Mechanical models were developed for capturing the behaviour of both the concrete beam-to-CECFST column joint and the RCS joint. To gain a deep insight into the performance of CECFST columns and concrete beam-to-CECFST column joints, two series of tests consisting of 15 column specimens and 6 joint specimens were conducted. The first test series including 15 CECFST columns was aimed to investigate the effects of uniaxial and biaxial load eccentricities on the behaviour of CECFST columns. The test programme for CECFST columns provided new experimental data on the structural behaviour of CECFST columns under uniaxial and biaxial eccentric compression including load versus mid-height deflection curves, moment versus curvature curves, failure modes and cross-section strengths. A finite element (FE) model was then established to capture the structural behaviour of CECFST columns under combined loading and generate mode data. Traditional cross-section design methods for composite columns were evaluated based on the obtained test and FE results. After comprehensive experimental and numerical investigations on the structural behaviour of CECFST columns, a unified design method was proposed to calculate the cross-section strength of CECFST columns under combined loading. A normalised interaction surface of CECFST columns under combined loading could be generated based on the proposed unified method, which considered the confinement effects from both inner steel tubes and stirrups. The proposed design method was applicable to a broad spectrum of material strength, steel tube diameter-to-thickness ratio, spacing of stirrups, aspect ratio between the steel tube and column cross-section for CECFST columns. The seismic performance of concrete beam-to-CECFST column joints was experimentally studied through the second test series. The effects from the construction method, axial load ratio and connection detail to the joint specimens were investigated in the second test series. Two specimens were designed as benchmarks, which were cast-in-place (CIP) concrete beam-to-CECFST column joints. The remaining four specimens were designed as precast concrete (PC) beam-to-CECFST column joints with two proposed connection details. Arising from this test programme, there were new experimental results on the structural behaviour of concrete beam-to-CECFST column joints under lateral cyclic loading including storey shear force versus horizontal displacement curves, energy dissipation, ductility, strength and stiffness degradations. The experimental results showed that PC beam-to-CECFST column joints with the proposed connection details could satisfy the ductility requirement specified in the codes of practice. In addition, the proposed connection details for the PC specimens could work effectively and reliably in comparison with the CIP specimens. According to the test results, a design formula was then proposed for concrete beam-to-CECFST column joints under seismic loading. After a comprehensive assessment of the seismic performance of concrete beam-to-CECFST column joints, a simplified model was then proposed to capture the shear behaviour of such joints. In addition, a new design approach was proposed for RCS joints which enable structural engineers to design composite joints based on a simplified and rational approach. A new calculation method was first developed for predicting the strength of RCS joints. The proposed design method for RCS joints was applicable for all the possible details and could capture two different failure modes of RCS joints. The proposed design was validated against a large set of experimental data including 54 RCS joint specimens from the literature and showed better accuracy and consistency compared to the previous design method. After that, a component-based model (CBM) was developed for capturing the nonlinear RCS joint moment–rotation response.