Compressive behavior of partially prefabricated concrete-encased-and-filled steel tubular columns

Abstract

This paper develops a novel concrete-encased-and-filled steel tubular (CEFST) column that employs a concrete-filled steel tube encased into a high ductility concrete (HDC). The steel tube and the HDC are prefabricated parts of the column while concrete infill is cast on-site. To investigate the axial compressive behavior of the novel prefabricated column, an experimental campaign was carried out involving twelve prefabricated CEFST columns, five conventional concrete-filled steel tubular (CFST) columns, and three typical reinforced concrete (RC) columns. The test variables included the width-to-thickness ratio of the steel tube, the number of tie bars arranged in rows along the height of the column to restrain the evolution of tube local buckling, the tensile strength of the tie bars, and the shape of the tie bar nuts used to enhance the mechanical connection between the materials, i.e., the inner steel tube and the HDC. Overall, prefabricated CEFST columns of the same steel content ratio and material strengths as the corresponding RC columns exhibited higher ductility. Test results indicated that tie bars in CEFSTs can effectively restrain the outward buckling of the steel tube by limiting the formation of local buckling within their pitch distance. By increasing the number of tie bars or using ring-type nuts instead of conventional nuts, the onset of local buckling can be delayed, hence the initiation of column's strength degradation. It was also found that by arranging the tie bars in more than two rows within the critical height of the column, the tensile strength of the high-strength tie bars can be fully utilized resulting in an improved post-peak strength of CEFSTs. Based on test results, a fairly accurate design formula was proposed for calculating the compression strength of CEFST columns.