## 1 Introduction  
Engineers have traditionally employed structural components made of steel forms combined with either plain or reinforced concrete. Earlier constructions only focused on the fire and corrosion resistance that the concrete provided for the steel shapes. However, studies on the strength of these members were carried out in the early 1900s, and by 1924, design guidelines had been developed. With the introduction of contemporary composite frame construction in tall structures in recent years, engineers have proposed innovative and practical ways to increase the resistance and ductility of concrete and steel reinforcement bars by using steel sections encased in concrete, also known as concrete-encased steel (CES) sections [1]. The use of concrete-encased columns in structures can reduce the size of the columns and reduce construction costs. In addition, its corrosion and fire resistance are both highly improved. Compared to ordinary columns, steel columns encased in concrete have substantially fewer long-term failures. It has been demonstrated that the ultimate axial load capacity of the CES columns is higher than a reinforced concrete column. This means that the structural steel improved the ultimate load resistance of the CES column [2]. Furthermore, an analytical and experimental study reveals that the axially loaded CES columns can sustain higher axial loads and undergo a smaller axial deformation comparedto a conventional reinforced concrete column [3]. The steel and concrete composite building technology combines the flexibility and stiffness of reinforced concrete with the assistance of reinforcing bars to create a cost-effective structure. Generally speaking, the idea that two or more components are combined to make an efficient structure is what gives composite buildings their strength [4]. Nevertheless, the CES columns have certain limitations too. In some cases, it is difficult to cast the concrete since the steel ratio is high and the space between the steel section and the rebars makes it difficult to pour the concrete. During the design phase, the engineers must be extremely mindful of any potential issues with the positioning of the reinforcing steel and congestion as they impact the pier's constructability, just like with any other concrete column with reinforcing steel [1]. Furthermore, the initial cost of construction of a CES column could be higher than a conventional reinforced concrete column. However, the increase in durability of the CES columns can justify a higher initial cost. Major infrastructure, including railroads and bridges, ought to be built to withstand significant seismic ground motion. In the CES columns, integrated concrete and steel profile sections help to obtain a high load-carrying capacity and high ductility. Figure 1 presents an example of a recent bridge construction using concrete-encased steel columns in China in 2024 [5]. The application of the CES column in buildings is more frequent than in bridges. Most applications on bridges until now have been on the steel-concrete composite high bridge pairs. More specifically, in the mountainous regions, where large quantities of rebars are required as well as concentrated stirrups are required for a conventional reinforced concrete column. In addition, a large number of stirrups and reinforcing bars lowers the quality of construction and reduces efficiency while also making construction more challenging. As illustrated in Figure 2, hybrid hollow high piers, which are a form of CES columns, offer a new approach to reducing the excessive use of steel bars in traditional bridge piers. A portion of the axial direct reinforcing bars installed using the conventional method are replaced by structural steel and stirrups or spiral reinforcement. When compared to typical reinforced concrete piers, the hybrid hollow high pier has a number of advantages, including increased earthquake resistance, increased economy and efficiency during construction, improved quality and aesthetics, and the preservation of the environment [6].