The structural steel skeleton for the tall building evolved to include diagonal members to increase stability, eventually giving way to a dominance of diagonal members. The “bundled tube” type provides added stability by allowing the base of the structure to be substantially larger than the decreased number of “tubes” toward the top of the building. The “belt truss” provides both stability and a place for mechanical floors. The “braced rigid frame” (also known as a “framed shear truss”) concentrates wind bracing to a vertical band that runs up multiple faces of the tower. The “diagonalized core system” extends the diagonal members over the entire façade on each face, using the diagonals to supplement the vertical load path provided by the columns. The “diagrid” eliminates vertical columns and uses the diagonal members to support the floors while simultaneously resisting lateral forces.
Bundled Tubes
Belt Truss
Braced Rigid Frame
Diagonalized Core System
DIAGONALIZED CORE BUILDINGS Skyscrapers brought with them particular structural problems related to their height and the necessity to resist wind loads. A tall building is essentially acting as a very long cantilever. Early buildings used strong moment-resisting connections within a simpler framed system to resist bending in the structure. These major moment-resisting connections were hidden within the frame and so did not impact the design of the façade. Additional steel was added to the hinge-type framed connections to stiffen the joints. As the design of tall buildings evolved architecturally, new structural systems were developed that chose to express wind resistance by exposing the diagonal braces in the façade. These diagonal braces reinforced a framing system that remained fairly consistent with the standard portal framing that had been developed in the earlier part of the 20th century.
Left: The Millennium Tower in Dubai, UAE by Atkins Architects uses a modernized variation of the exterior diagonal bracing system on its exterior. The exterior extensions of the floor plate use a vertical K-truss to add rigidity. This is an example of a “braced rigid frame” or “framed shear truss”. Right: The 100-storey John Hancock Building in Chicago, IL, USA, designed by Skidmore, Owings & Merrill, expressed the diagonal reinforcing of its frame as an overlay to the rectilinear pattern made by its strip windows, column covers and spandrel panels. The tower also tapers toward the top in response to wind loads. This system is known as a “braced tube” or “diagonalized core system”.
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Diagrid