## 1 Introduction  
Composite elements are designed by combining different materials. The purpose of composite elements is to increase structural performance by combining the advantages of these different materials [1, 2]. These elements are preferred in many engineering applications, such as high-rise structures or long-span bridge beams, where high strength is required. The use of composite structural elements reduces the amount of steel by 30% to 50% compared to steel elements and provides a more economical design [3]. In order to increase structural performance, the strength and ductility of the columns used in the building system should be high. Composite columns exhibit less displacement than steel columns because they show more rigid behavior [4]. Although steel columns have high tensile strength and high ductility, they exhibit poor behavior under the influence of pressure. However, in composite columns, the concrete material increases the buckling strength of the steel element [5, 6]. In addition, the concrete layer increases the durability of the column by protecting the steel profile from external effects such as corrosion and fire [7, 8, 9]. Reinforced concrete-steel composite columns generally consist of concrete-encased steel profiles, steel tube sections filled with concrete, and partially encased steel columns. Optimum design aims to make the most economical design within safety limits. For this purpose, the material in the building system should be used in the most effective way. In castellated elements, empty opening shapes in the body of steel elements increase the body height and bending rigidity of the steel profile. Castellated elements are produced by cutting, shifting, and welding steel profiles (Figure 1). Studies in the literature indicate that by designing the body of steel beams with circular openings, the body height and bending rigidity increase by 40-60% [10]. Thanks to the increase in rigidity in the steel element, building loads are carried with smaller sections, and a more economical design is provided. Opening shapes such as hexagonal, circular, sinusoidal, or square can be used to increase steel profile body heights [11]. Within the scope of this study, topology optimization was used to determine the optimum opening shape in the steel profile used in the composite column. Then, the shape obtained from topology optimization was transformed into a form suitable for production. In literature studies, it is recommended to use high-strength concrete to increase the structural performance of composite columns [12]. By using high-strength concrete, damage is prevented from occurring in the concrete material, and ductile behavior is ensured. In the Turkiye Steel Regulations, the concrete strength to be used in composite columns is limited to 20-70 MPa [13]. For this reason, 70 MPa was chosen as the concrete strength in the nonlinear analysis of composite columns in this study. Using high-strength concrete provides advantages such as increasing load-bearing capacity, decreasing cross-section dimensions and the self-weight of columns. When the studies on composite columns are examined, it is observed that increasing the strength of the steel profile causes damage to occur in the concrete and therefore brittle behavior [14]. In order to prevent brittle damage caused by the high yield strength of the steel profile, the steel profile strength is limited in the regulations [13; 15]. In this study, the yield strength of the steel profile used in composite columns has been selected as 235 MPa. In this study, it is aimed at increasing the structural performance of concrete-encased steel composite columns. For this purpose, the steel profile used in the composite column was designed with an opening shape. The optimum opening shape was determined by using topology optimization in the optimum design of the steel profile. Then, these opening shapes were made suitable for production, and a new composite column design was made.