Skyscrapers
Softened corners can be achieved by rounding off the sharp edges, chamfering or small cut-outs on the edges. A good example of this is Taipei 101 in Taiwan. The buildingwas originally designedwith square corners, but when a scale model was tested in a wind tunnel, the building accelerated beyond the applicable comfort criteria. After sawtooth corners were added, they reduced based movement by approximately 25%. 8 Tapering and setbacks reduce the wind forces on the building by having a different cross-section as the building rises to ‘confuse the wind’. This means that the vortices never become organized as wind encounters different building shape at different height. The most famous example is the Burj Khalifa in Dubai. As you go up the building, there are 27 different cross-sections of the tower and each of them will generate vortices slightly different than the ones above or below them, so the wind never becomes organized and establishes harmonic resonance. Combining this with the y-shaped buttresses which provide excellent lateral bending resistance and curved edges, this design significantly reduces the forces of the wind on the building. 9 A similar result can be achieved by using varying cross-section shapes. The building can use this technique by changing the building shape from square to round as the height increases. 10 Adding spoilers to the outside of the building, such as vertical fins at intervals up the building height or helical strake along the length of cylindrical structures (often used by industrial chimneys), disrupts the wind flow along the length of the building, preventing the vortices from forming coherently. The effect could also be achieved by twisting the building as spirals redirect the wind and guide it upward and off the building. 11 During the wind tunnel testing of the Shanghai tower, the designer incorporated an elegant twist throughout its 632 metre height, which combined with tapering shape and consistently rounded corners to reduce the lateral loads to the building by 24%. This saved the developers approximately US $ 58million in construction costs as less structural material is used to the stiffness to the building. 12
Finally, openings increase the building ’s porosity by having large gaps or cut outs in the structure which allows air to flow through andweaken the formation of vortices. 432 ParkAvenue inNewYork achieves this effect by opening up two adjacent floors at five locations evenly spaced throughout its height. 13
Although making the skyscraper more aerodynamic greatly reduces the effect of wind on the building, supplementary damping systems are still needed. 14 Supplementary damping systems are divided into several different categories, after which they are then further divided into different subcategories. The two main categories are distributed damping systems and mass damping systems. The remainder of this essay will be focused onmass damping systems as distributed damping systems are not widely used due to a number of practical issues to in integrating them into the structural system. 15 The two main subcategories of mass damping systems are tuned mass dampers and tuned liquid dampers.
8 Irwin 2007: 703. 9 Baker, Korista and Novak 2008:7. 10 Irwin et al. 2008: 918. 11 Ibid. 12 Xia, Poon and Mass 2010: 13. 13 RWDI 2015.
14 Poon et al. 2004: 274. 15 Irwin et al.2008: 922.
121
Made with FlippingBook Digital Publishing Software