Optimal Geometric Configuration and Stiffness Distribution for Tall Building Structures
Tall buildings are constructed in major cities throughout the world today due to their economic benefits in dense urban land use and significant international symbolic power. Because of their enormous scale, tall buildings are built with an abundant amount of resources. This paper investigates optimal geometric configurations and lateral stiffness distributions of today’s prevalent structural systems for tall buildings, which will lead to design and construction of tall buildings with less amount of structural materials. Among various structural systems developed for tall buildings, the systems with diagonals are generally more efficient because these systems carry lateral loads by their primary structural members’ axial actions.
When the primary lateral load resisting system is located over the building perimeter, the system’s efficiency can be maximized. Tall building structural systems with perimeter diagonals include braced tubes and more recently developed diagrid structures. Braced tubes of various column spacings and bracing configurations are comparatively studied. Diagrid structures of various uniform and varying angle diagrids are studied to determine more efficient configurations. Another structural system prevalently used for today’s tall buildings is outriggers structures. Optimal stiffness distribution between the building core and perimeter mega-columns is investigated for outrigger structures of various heights.
INTRODUCTION
Structural design of a tall building is generally governed by lateral stiffness, and the required amount of structural material to resist lateral loads increas es drastically as the building height increases. Therefore, structural systems for tall buildings have evolved to produce higher lateral stiffness more efficiently. The efficiency of a structural system is significantly influenced by its geometric configuration. Once a particular structural system is selected for a tall building, its configuration should be determined very carefully to maximize its structural efficiency and, at the same time, satisfy other non-structural design requirements integratively. Lateral shear forces and overturning moments due to wind loads significantly influence the structural design of tall buildings.
These forces can be carried very efficiently by primary structural members configured to work in axial actions. When the primary structural members are located over the building perimeter, the system’s efficiency can be maximized. Braced tubes and more recently emerging diagrids are two typical examples developed based on these concepts. Braced tubes have continuously been used since its initial employment for the John Hancock Center of 1969 in Chicago. The system is configured with perimeter columns, typically spaced evenly, and large perimeter bracings. This paper studies optimal geometric configuration of braced tubes. Various perimeter column spacings are studied to determine more efficient alternatives. Comparative structural efficiency between different geometric configurations of perimeter diagonals, such as X, chevron and single diagonal bracings, is investigated. Further, structural impact of bracings placed at different angles is studied.
The optimal bending-to-shear stiffness distribution is also discussed in relation to the height-to-width aspect ratio of the braced tube system. Since its application to the Swiss Re Building of 2003 in London and Hearst Headquarters Tower of 2006 in New York, diagrid structures have widely been used for major tall buildings throughout the world. Diagrid structures of various uniform and varying angle configurations are studied to determine more efficient geometric configurations. Varying angle diagrid studies are carried out with diagrid structures with not only vertically but also horizontally varying angle configurations and their combinations. The optimal bending-to-shear stiffness distribution is also discussed in relation to the height-to-width aspect ratio of the diagrid system.
Outrigger structures are another prevalently used structural system for today’s tall buildings. An early example of the outrigger system can be found in the Place Victoria Office Tower of 1965 in Montreal. However, major application of this structural system can be seen on contemporary skyscrapers such as the Jin Mao Building and the Shanghai Tower in Shanghai. In the construction photo of the Shanghai Tower, the building core, belt trusses at the outrigger truss level and perimeter mega-columns can be clearly observed. Unlike braced tubes and diagrids, which carry lateral loads primarily by perimeter structural members, outrigger structures use stiff core structures and perimeter mega-columns connected to the core structures with outrigger trusses. Optimal stiffness distribution between the core and mega-columns is studied for outrigger structures of various height-to-width aspect ratios.
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Kyoung Sun Moon, Yale University School of Architecture