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Behavior of Concrete-Filled Steel Tube Columns Subjected to Axial Compression 时间:2018-09-13 13:14 来源:未知作者:admin 点击: 次 Abstract:The behavior of concrete-filled steel tube (CFST) columns subjected to axial compression was experimentally investigated in this paper. Two kinds of columns, including CFST columns with foundation and columns without foundation, were tested. Columns of pure concrete and concrete with reinforcing bars as well as two steel tube thicknesses were considered. The experimental results showed that the CFST column with reinforcing bars has a higher bearing capacity, more effective plastic behavior, and greater toughness, and the elastoplastic boundary point occurs when the load is approximately 0.4–0.5 times of the ultimate bearing capacity. The change of rock-socketed depth and the presence of steel tube will affect the ultimate bearing capacity of rock-socketed pile. The bearing capacities of the rock-socketed CFST columns are lower than those of rock-socketed columns without a steel tube under a vertical load; besides, the greater the rock-socketed depth, the greater the bearing capacity of the rock-socketed piles. In addition, a numerical comparison between the ultimate load and the theoretical value calculated from the relevant specifications shows that the ultimate load is generally considerably greater than the theoretical calculation results.
1. Introduction
In the last few decades, high-rise, large-span, and large-scale building structures have become more common. Concrete-filled steel tubular (CFST) members are well recognized for their excellent performance owing to the combined merits of steel and concrete materials [1]. Therefore, concrete-filled steel tubes are being increasingly used in high-rise buildings and in large-span structures. CFST columns have been used in earthquake-resistant structures and bridge piers subject to impact from traffic and used to support storage tanks, decks of railways, and high-rise buildings as well as being used as piles. Concrete-filled steel tubes require additional fire-resistant insulation if the fire protection of the structure is necessary [2].
Studies of CFST have also been frequently performed, and tests of CFST with rectangular, square, and circular cross sections filled with high-strength concrete have been reported [3–6], including compression [7–12], bending [13–17], or torsion [18–20] tests. The influence of the behavior and strength capacity of CFST on the bonding and local buckling of the steel profile, confinement of the concrete, and strength of the materials were discussed. Pull-out tests on CFST columns have shown the contribution of shear connectors to the force transference mechanism that occurs in beam-column connections. Local buckling has been widely studied, and it is possible to consider its influence on the strength capacity of the CFST columns. The confinement effect is influenced by the shape of the cross section, the thickness of the steel profile, the type of loading, the slenderness of the composite column, and the strength of the materials used. The studies on CFST rock-socketed columns and predicting the ultimate capacity of CFST columns subjected to axial compression are limited, and there is no standard method for calculating the strength of concrete-filled steel tube with reinforcing bars.
There are several design codes for concrete-filled steel columns, such as the America Institute of Steel Construction (AISC) Load and Resistance Factor Design (LRFD) [21], Eurocode 4 (EC4) [22], Brazilian Code NBR 8800 [23, 24], and GB50396-2014 [25]. Since the AISC is focused on steel columns, use of the AISC [21] specifications is limited to composite columns with steel yield stress and concrete cylinder strength no greater than 415 and 55 MPa, respectively. Modified values for both the yielding strength (fmy) and elasticity modulus (Em) are assumed. These modified terms take into account the presence of the concrete filling the steel profile. EC4 [22] and NBR 8800 [23, 24] determine the resistance capacity of a section by adding the contribution of the steel tube and the concrete core. For columns with circular cross sections, the confinement effect is considered by using coefficients that increase the uniaxial compressive strength of the concrete (fck) and reduce the yielding strength of the steel (fy). The instability in these standard codes is considered by using a coefficient, which depends on the slenderness of the composite column. Limits for the steel tube slenderness are also considered to avoid local buckling.