Structural Optimization of 2000-Foot Tall 7 South Dearborn Building

The 2000 foot (609 meter) tall 7 South Dearborn tower proposed for Chicago, will be, at time of completion, the tallest building in the world and also one of the most slender with an overall aspect ratio of approximately 8.5 to 1. The 110-story building is a multi-use project with retail comprising the ground floor, offices in the lower section, residential comprising the upper sections, and telecommunications (including digital television broadcasting) occupying the highest floors. The innovative lateral load resisting system developed for the tower consists of a reinforced concrete core wall, with structural steel outriggers at the mechanical floors, connecting the core to structural steel perimeter columns in the lower half of the building. An overall structural economy of the lateral system for the tower is a requirement for the feasibility of the project. Based on structural analysis and wind tunnel testing, the building responses to wind loadings (deflections and accelerations) were determined. The sizing of the members of the lateral system for high-rise buildings is often controlled primarily by serviceability requirements rather than strength. In order to achieve an economical building, it is prudent that these elements are appropriately sized and the structural materials are efficiently distributed among the various components to result in a least cost solution. This paper summarizes the structural system for the 7 South Dearborn tower and presents the optimization techniques utilized to determine a least cost solution for a given deflection or period incorporating member constraints.