Composite columns are generally installed in high-rise buildings and bridges. As well as being smaller and lighter than conventional reinforced concrete columns, composite columns offer high efficiency and ductility. In this research, experimental and analytical studies were carried out to investigate the optimal distribution of the total steel area of the Concrete-Filled Hollow Steel Tube (CFHST) columns; the experimental program included 20 CFHST columns. The main variables of the experimental study were the distribution of the total steel cross-sectional area over the internal concrete core (steel reinforcement) and the external steel tube, the cross-sectional shape, the width-to-breadth ratio t/b, and the presence or absence of shear connectors. In addition, using equations of different codes, comparison was conducted between the experimental and analytical results. For a constant steel cross-sectional area, the use of internal steel bars and external steel tubes, together with the use of shear connectors, was found to exhibit a better ultimate load capacity, stiffness, and ductility for the CFHST columns.
Studies have shown that steel fibers improve concrete performance specially shear and tensile strength. The degree of improvement gained depends mainly on the type, shape and volume of the steel fibers. The main objective of this study is to investigate the shear behavior of low volume content steel fiber concrete under static or repeated loading. Low volume steel fibers (V f < 1 %) presents an economic solution that guaranties suitable workability for concrete. Three groups with a total number of one hundred twenty-six push-off concrete specimens were casted and tested. The experimental variables were the concrete compressive strength, the shape of the steel fibers (corrugated or hooked end steel fiber), the volume content of the steel fiber (0.15, 0.30 and 0. 45%) and the type of loading (static or repeated). The experimental test results showed up to 30% increase for the shear capacity of the concrete as well as an increase for the fatigue life due to the presence of low volume content of steel fibers. An equation is proposed to predict the fatigue life of the steel fiber concrete.
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