Mitigation of Motions of Tall Buildings with Specific Examples of ...

at lower levels of the building. The inclusion of openings and other such modifications may
adversely affect habitability if they reduce the resonant vortex frequency (Tamura 1997).
Through-building openings have been used in Japan for several
buildings and are being applied to the proposed new world’s tallest
building, the Shanghai World Financial Center, featuring a
54 m square shaft and diagonal face that is shaved back with the
aperture cut off to relieve pressure at this location. The opening,
shown atop the tower in Figure 9, measures 51 meters in diameter.
The design exploits not only the benefits of through-building
openings but also those provided by shifting and decreasing the
cross section with increasing height, essentially tapering the 460
m tower.
However, care must always be taken in order to engineer modifications
that will produce the desired effect, constantly consulting
wind tunnel tests to verify the effects of altering the plan shape or
employing other forms of aerodynamic modifications. Armed
with modifications which avoid increasing the projected area or
effective breadth of a building, engineers may achieve significant
response reductions (Kwok 1995).
5.0 Damping Sources
An increase in the effective damping of a structure, accomplished
by any of the four major sources of damping: structural, aerodynamic,
soil, and auxiliary, will also lead to decreased structural
motion. Structural damping is limited to the damping already
available inherently in the materials: steel, concrete, or their composite.
At times, aerodynamic damping may also contribute in the
alongwind direction, depending on the wind velocity, structural
shape, and building dynamic characteristics. However, the contribution
in the acrosswind direction is negligible and may even
become adverse at higher wind speeds, though the presence of
adjacent structures may introduce different effects. Although not
marked for high rise buildings, damping contributions may also be
Figure 9: Shanghai World
Financial Center. (taken from
Mori Building Co., Ltd.)
obtained from the soil-foundation interaction, i.e. soil damping. Unfortunately, these three forms
of damping make only limited contributions. In addition, the damping in the structure cannot be
engineered like the mass and stiffness properties of the structure, nor can it be accurately estimated
until the structure is completed, resulting a certain level of uncertainty (Kareem & Gurley
1996). In cases where the inherent damping is not sufficient, auxiliary damping devices may be
introduced, offering a somewhat more predictable, adaptable, and reliable method of imparting
additional damping to a system.
Mitigation of Motions of Tall Buildings with Specific Examples of Recent Applications 10