Damping Devices For Reducing Motion Perception

Engineers have learned from building occupants and owners, and from wind tunnel studies, that designing a tall building to meet a given drift limit under code-specified equivalent static loads is not enough to make occupants comfortable during windstorms. However, they have only limited control over three intrinsic factors, namely, the height, the shape, and the mass, that influence the dynamic response of buildings. Additionally, the behavior of a tall building subjected to dynamic loads such as wind or seismic activity is difficult to predict

Figure 8.29. Structural steel unit quantities.

20 30 40

Unit quantity of steel, psf

Figure 8.29. Structural steel unit quantities.

with any accuracy because of the uncertainty associated with the evaluation of a building's damping and stiffness, as well as the complicated nature of loading.

The present state of the art is such that an estimate of structural damping can be made with a plus or minus accuracy of only 30% until the building is constructed and the nonstructural elements are fully installed. It is well-known that wind-induced building response is inversely proportional to the square root of total damping, consisting of aerodynamic plus structural damping. So, if damping is quadrupled (increased by four times), a 50% response reduction is achieved, and if damping is doubled, the dynamic response is reduced by 29%. Because of the inherent damping of a building responding elastically to wind loads in the range of 0.5 to 1.5% of the critical response, it is impractical to increase the damping to, say, four times as much by use of modified structural materials.

Suppression of excessive vibrations can be dealt with limited success in a variety of ways. Additional stiffness can be provided to reduce the vibration period of a building to a less sensitive range. Changes in mass of a building can be effective in reducing excessive wind-induced excitation. Aerodynamic modifications to the building's shape, if agreeable to the building's owner and architect, can result in reduced vibrations caused by wind. However, these traditional methods can be implemented only up to a point beyond which the solutions may become unworkable because of other design constraints such as cost, space, or aesthetics. Therefore, to achieve reduction in response, a practical solution is to supplement the damping of the structure with a mechanical damping system external to the building's structure.

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