Design approaches against wind excitation - Tall Buildings: Structural Systems and Aerodynamic Form - page 159

Figure 6.3 The Millennium Tower, Tokyo,
Japan, 1993, proposed

Figure 6.4 Chicago Spire, Chicago, USA,
never completed

Plan variation

Varying the building plan results from the variation in plan dimensions or shape
throughout the height of the building and can be achieved by:

a. reducing the plan area
b. changing the plan shape.

Plan variation by reducing the plan area toward the top of the building results in a
reduction in the surface area affected by the wind at the upper levels of the building,
which lessens the wind intensity and thus the excess pressure. The reduction in the
plan area of the building as it rises can be in the form of:

• tapering
• setbacks.

Creating an inward-tapered facade (resulting in the building narrowing upward)
or providing setbacks are effective methods for reducing the across-wind building
response (Ali and Armstrong, 1995; Davenport, 1988; Irwin, 2008; Irwin, 2009; Irwin
et al., 2008a; Irwin et al., 2008b; Kim et al., 2008; Schueller, 1977; Scott et al., 2005).
By designing tall and slender buildings in this way, lateral drift can be reduced by
10 to 50 per cent (Schueller, 1977). An analytical study by Khan (1972) has shown
that, by creating a slope of 8 per cent in the facade of a 40-storey building, a 50 per
cent reduction of the lateral drift in the upper stories can be obtained.