Skyscrapers: how do they resist wind?
Junior Khusuwan
Skyscrapers have become a big part of the skylines of modern cities, as well as symbols of power and wealth. Skyscrapers can be seen in almost everymajor city in theworld, especially in cities such as Hong Kong, New York and Dubai. As of 2020, there are currently 5071 completed skyscrapers worldwide. 1 However, for a building to qualify as a skyscraper, it must meet 3 specific criteria set by the Council on Tall Buildings and Urban Habitat (CTBUH). Firstly, at least 50 percent of the structure’s height must be safely and legally occupied by residents, workers, or other building users on a consistent basis. Secondly, the structure must be self-supporting and not require tension cables or supports in order to remain upright. Finally, the structure must be 150 metres in height or taller. 2 One of the most dominant factors in the skyscraper structural design is wind forces, since it is the primary source of lateral load. This is because when the wind swirls around a building, small whirlwinds, called vortices, are created in a process known as vortex shedding. In high winds, they create alternating pockets of low air pressure which create suction forces on the building that can cause lateral displacements and acceleration. 3 The vortex-shedding frequency around a building can be calculated as 𝑓 = St × v d where f is the vortex-shedding frequency in hertz (Hz), d is the width of the building which the flow passes in metres (m), v is the wind speed in metres per second (m/s), and St is the Strouhal number which represents the ratio of the inertial forces due to the unsteadiness of the flow or local acceleration to the inertial forces due to changes in velocity from one point to another in the flow field. 4 The Strouhal number is a dimensionless constant with a value typically ranging from 0.1 to 0.3. All buildings have a natural frequency, the rate at which they are likely to vibrate. Thus, if the natural frequency of a building matches the low frequency of vortex shedding, it would amplify the intensity of swaying which can cause structural damage and lead to unpleasantness to building occupants. 5 The traditional approach to this problem is by adding stiffness to the building. This is typically done by using a reinforced concrete or clustering steel columns and beams in the skyscraper's core which often doubles-up as an elevator shaft. 6 However, to achieve significant reduction in building motion, structural approaches can be relatively costly. The remainder of this essay will focus on the more popular and less expensive approaches to reduce the effects of vortex shedding, which is to develop a building with aerodynamic shape or install supplementary damping systems. The five main shaping strategies are softened corners, tapering and setbacks, varying cross-section shape, spoilers and openings. 7
1 The Council on Tall Buildings and Urban Habitat 2020b. 2 The Council on Tall Buildings and Urban Habitat 2020c. 3 Mills 2018. 4 Engineering ToolBox 2004. 5 Irwin et al. 2008: 916. 6 WGBH 2000. 7 Irwin et al. 2008: 917.
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