Using Offset External Bracing to Optimize Tall Building Performance

The 56-story, 232 m tall Jinao Tower in Nanjing, China, incorporates steel bracing placed at the outside of a reinforced concrete frame providing an unusual lateral load resisting system. The Tower form was derived from the juxtaposition of an innovative double-skin façade and an external lateral braced steel frame that wraps the tower from crown to base and defines the dimensions and folds of the building envelope. The bracing was conceived three dimensionally by considering the faceted form as a volume rather than a planar system. Introducing the diagonal steel brace system on each side of the structure (outside of the tube-in-tube structure and between the double-skin facade) resulted in additional stiffening of the structure and a 40% design reduction in concrete and rebar in the concrete lateral load resisting system and a 20% design reduction in concrete and rebar for the overall building structure. Several studies beyond typical code checks were conducted to explicitly ensure the redundancy, ductility, and overstrength of the proposed structural system including a 3D nonlinear pushover and time history analysis, enhanced design criteria of key members and components, reanalysis after brace removal, and testing.