Intrinsic and supplementary damping in tall buildings

Abstract

The dynamic response of a tall building plays a significant role in determining both design wind loading and seismic behaviour. The degree of energy dissipation, or damping, that a building can provide directly affects the resonant response and thus the effective design loading. Since there is very little codified guidance on what level of damping to assume for a tall building, it is usually left to a specialist consultant to determine a reasonable value, based upon measurements, rules of thumb and common practice. This paper reviews measurements of damping on tall buildings and compares them with values in common usage. It is found that, particularly for buildings above 250 m high, damping ratios commonly used in design are inappropriate and unconservative. This issue relates to serviceability, ultimate design wind loading and seismic response. The paper further discusses reasons for the variation in damping with building height, the consequences of overestimation of damping and possible methods of mitigation. The paper concludes that the addition of supplementary robust damping is often the most economic method of controlling the dynamic response of a tall building, as well as reducing the consequence of the uncertainty of intrinsic damping. This work aims to highlight the perceived gap between common practice and the latest data. The reasons for this gap – commercial pressures, lack of certainty and other mitigating factors – are also discussed.