Investigation of the influence of relative wall area in plan and the role of their boundary elements in the seismic behavior of tunnel-form concrete system

The provision of a minimum relative wall area equivalent to 3 % of the total plan area is one of the prescriptive requirements in design process of tunnel-form concrete systems, which is independent of seismic demands and often governs determination of the wall thickness. The high lateral stiffness and resistance of the tunnel-form concrete system generally encourage designers to disregard boundary elements in the walls. Although wall thickness and boundary elements are both essential factors in ensuring load-carrying capacity and ductility of walls, their specific effects within the tunnel-form concrete system have not been thoroughly examined to date. The motivation behind and the necessity of adhering to the prescriptive requirement for minimum relative wall area in tunnel-form plans remain ambiguous. In this study, aiming to address these concerns, 5- and 10-story tunnel-form buildings have been analyzed. Within the considered model range and adopted assumptions, the results indicate that the requirement for providing a minimum wall area in each plan direction is overly conservative and unnecessary. Without adhering to this requirement and solely following the minimum requirements of the reinforced concrete design code, a reduction of 12.9 of concrete per story was achieved, with no observed change in the performance level of the system under design basis and maximum considered earthquakes. Furthermore, the investigations reveal that boundary elements of the walls do not significantly affect the seismic responses and behavior of the tunnel-form concrete system. Based on the results, even without boundary elements, the seismic reliability of the system for the immediate occupancy performance level in walls as the main lateral load-resisting elements was estimated to be 100 % under both design basis and maximum considered earthquakes.