Square concrete-filled steel tubular columns with internal high-strength steel confinement

Abstract

Concrete-filled steel tubular (CFST) columns have been extensively studied and widely used in practice. Existing research has shown that CFST columns of non-circular section is much less ductile than their circular counterparts, particularly when high-strength concrete and thin/high strength steel (HSS) tubes are used, which is a major concern for structures designed to resist seismic loading. This thesis presents the results of a systematic research project on a new form of CFST columns recently proposed by Professor Jin-Guang Teng at The Hong Kong Polytechnic University. These columns consist of a steel tube filled with concrete that is confined with high-strength steel (HSS) spirals typically with a yield stress exceeding 1000 MPa. The new form of CFST columns, referred to as confined concrete-filled steel tubular (CCFST) columns, also maintains the ease for connection to CFST or steel beams. The combined experimental and theoretical study presented in the thesis is aimed at the demonstration of the expected advantages and a good understanding of the structural behavior of CCFST columns. Only CCFST columns with a square steel tube and an internal HSS spiral are examined in this Ph.D. thesis. Obviously, the general concept of internal HSS spiral confinement can be used with all steel tube cross-sectional forms and all sorts of multi-spiral arrangement over the cross-section. The concrete in a square CCFST column can be divided into two parts: the core concrete enclosed by the HSS spiral and the sandwiched concrete between the steel tube and the concrete core. In order to fully understand the confinement mechanism in a CCFST column, the behavior of core concrete confined with HSS spirals needs to be understood first. As a result, the first objective of the Ph.D. research was to conduct axial compression tests on HSS spiral (HSSS)-confined concrete columns and develop accurate stress-strain models for HSSS-confined concrete. The second objective of the Ph.D. research was to demonstrate the advantages of CCFST columns through axial compression tests on such columns. Axial compression tests on CFST columns and hollow steel tubular (HST) columns were carried out to better understand the behavior of CCFST columns. The test results showed that CCFST columns possess much greater ductility than the corresponding CFST columns as a result of the use of only a small amount of HSS as confining spirals. Apart from the experimental study, three-dimensional (3D) finite element (FE) analysis of CCFST columns was also carried out to further understand the interaction between different components in a CCFST column. Finally, on the basis of the understanding of the stress-strain behavior of HSSS-confined concrete and confined concrete in CCFST columns, a simple theoretical method based on section analysis was developed for the section capacity of CCFST columns. The theoretical method was verified using test results as well as results from FE analysis. A parametric study was then conducted using the theoretical method, based on which simplified design equations were proposed for the section capacity of CCFST columns.