Optimization of Concrete-Filled Steel Tubular (CFST) Columns Using Meta-Heuristic Algorithms

Introduction

Increased ductility, ease of construction, and high strength are some of the advantages associated with concrete-filled steel tubular (CFST) columns. Various methods have been developed to predict the load-carrying capacity (N_u) of CFST members and these methods have been included in design codes such as the ACI and AISC codes. However, most of these methods are only applicable when the design variables such as the yield strength of the steel (f_y) and the compressive strength of the concrete (f_c^’) are in a certain range. As an example of these limitations, the equations given in the AISC code can predict the ultimate load-carrying capacity (N_u) of CFST columns only for f_y ≤ 525 MPa and for 21 MPa ≤f_c^’≤ 70 MPa. The proper definition of the ultimate-load carrying capacity is another crucial factor that has to be agreed upon to prevent the wrong interpretation of the values predicted by the code equations. According to (Tao, Wang & Yu, 2013), (Uy, Tao & Han, 2011) and (Wang, Tao, Han, Uy, Lam & Kang, 2017), N_u is defined as the maximum load if this load level is reached at an axial strain less than 0.01. Otherwise, N_u is defined as the load level that corresponds to a compressive strain of 0.01. (Sakino, Nakahara, Morino & Nishiyama, 2004) carried out a comprehensive research program for the experimental study of CFST short columns. A total of 114 specimens consisting of both hollow and concrete-filled steel tubes were tested under central axial load. The steel tube tensile strength in these experiments varied between 400MPa and 800MPa whereas the concrete compressive strength varied between 20 MPa and 80 MPa. The outside diameter of circular tubes (D) was varied between 122mm and 450mm whereas the wall thickness of the tubes was in a range from 2.96mm to 9.45 mm.

The equations for the prediction of N_u developed by (Wang, Tao, Han, Uy, Lam & Kang, 2017) consider the contributions of the steel and concrete parts of the CFST member to the axial load carrying capacity separately as shown in Eq. (1).

In Eq. (1a) N_s and N_c are the contributions of the steel tube and the concrete core to the ultimate strength of the member respectively. In Eq. (1b) A_s and A_c are the areas of the steel tube cross-section and concrete core cross-section respectively. Under an axial load, the concrete core of the CFST column is confined by the steel tube and this confinement effect leads to the development of the hoop stresses in the steel tube which is accompanied by a decrease in the wall thickness. Therefore, the development of hoop stresses in the steel tube lead to a decrease in the axial load-carrying capacity of the steel tube. Furthermore, local buckling before reaching N_u can also decrease the load-carrying capacity of the steel tube. The reduction factor η_s in Eq. (1b) takes into account the effect of these conditions on the axial load-carrying capacity of the steel tube. In contrast to the steel tube part, the confinement by the steel tube has a favorable effect on the concrete core and the amplification factor η_c takes into account the increase in the load-carrying capacity of the concrete core due to this confinement effect. The equations for η_s and η_c obtained through nonlinear regression are given in Eqs. (1c,1d))). These equations are valid for CFST stub columns with circular cross-sections for certain ranges of the variables D/t, f_c^’, and f_y as listed in Table 1. In Eq. (1c), D and t are the diameter and wall thickness of the steel tube respectively as depicted in Figure 1.