The nature of wind storms and windinduced damage - Wind Loading of Structures - page 1

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The nature of wind storms and wind-
induced damage
1.1 Introduction
Wind loading competes with seismic loading as the dominant environmental loading for
structures. They have produced roughly equal amounts of damage over a long time
period, although large damaging earthquakes occur less often than severe wind storms.
On almost every day of the year a severe wind storm is happening somewhere on the
earth—although many storms are small and localized. In the tropical oceans, the most
severe of all wind events—tropical cyclones (including hurricanes and typhoons)—are
generated. When these storms make landfall on populated coastlines, their effects can be
devastating.
In this introductory chapter, the meteorology of severe wind storms— gales produced
by large extra-tropical depressions, tropical cyclones and downbursts, squall lines and
tornadoes associated with thunderstorms—is explained, including the expected
horizontal variation in wind speed during these events. The history of damaging wind
events, particularly those of the last 30 years, is discussed, focussing on the lessons learnt
from them by the structural engineering profession. The behaviour of flying debris, a
major source of damage in severe wind storms, is outlined. Insurance aspects are
discussed, including the recent development of loss models, based on historical data on
the occurrences of large severe storms, the spatial characteristics for the wind speeds
within them, and assumed relationships between building damage and wind speed.
1.2 Meteorological aspects
Wind is air movement relative to the earth, driven by several different forces, especially
pressure differences in the atmosphere, which are themselves produced by differential
solar heating of different parts of the earth's surface, and forces generated by the rotation
of the earth. The differences in solar radiation between the poles and the equator produce
temperature and pressure differences. These, together with the effects of the earth's
rotation, set up large-scale circulation systems in the atmosphere, with both horizontal
and vertical orientations. The result of these circulations is that the prevailing wind
directions in the tropics and near the poles tend to be easterly. Westerly winds dominate
in the temperate latitudes.
Local severe winds may also originate from local convective effects (thunderstorms)
or from the uplift of air masses produced by mountain ranges (downslope winds) . Severe
tropical cyclones, known in some parts of the world as hurricanes and typhoons, generate