Outrigger system analysis and design under time-dependent actions for super-tall steel buildings

The outrigger system has been widely adopted as an efficient structural lateral-load resisting system for super-tall buildings in recent years. Although the outrigger system has many structural advantages, it has a significant defect due to differential shortening, which cannot be neglected. Due to the shrinkage and creep of concrete, as well as the differential settlement of foundation, the shortening of the structural member is an important time-dependent issue, which leads to additional forces in the outriggers after the lock-in of the outriggers. As a result, it will increase the size of the structural member cross section in the design. In a real project, engineers can delay the lock-in time of the outrigger system to release the additional forces caused by the differential shortening during the construction phase. The time-dependent actions, such as the column shortening and the differential settlement of the foundation, were estimated. A mega frame steel structure was employed to illustrate the analysis and design of the outrigger under the time-dependent actions. Furthermore, a simple optimal method, considering the structural stability and overall stiffness, was proposed to optimize the construction sequence of the outrigger system.