How Does Sheltering Affect Wind Pressures?
The magnitude of the pressures generated by wind on a building are not only correlated to the speed of the wind passing by, but they are also affected by the degree of the exposure of the building to the direction of the wind.
“Exposure” is commonly defined as the lack of protection, covering, or sheltering from a harmful action or condition. In the evaluation of wind pressures on a particular building, the degree of the exposure needs to be considered.
Official recorded wind speeds are commonly measured in an open field at a height of about 33 feet. In urban areas these measurements are usually taken in the open fields of an airport.
An open field provides little resistance to the differential atmospheric pressures that generate the wind. This limited resistance allows the wind to freely accelerate. When winds encounter a change in typography, fences, trees, buildings, etc. they encounter resistance and they are deflected or slowed.
A building surrounded by other buildings or trees will encounter a lesser wind speed and a lesser wind pressure than a building approached by the same wind in an open field.
The effect of the exposure factor in wind pressures is accounted for with coefficients (multipliers) applied to the wind pressure computed from the wind speed. The coefficients may reduce or increase the pressure.
The base line exposure is called “Exposure C” and is the open field condition that has been previously discussed. Other exposures are:
Exposure A – a setting with numerous obstructions (trees, houses, other building, etc.) providing shelter. Usually found in an urban area with closely spaced housing or a heavily wooded setting. This exposure has been discarded. Buildings in this setting are now included in Exposure B.
Exposure B – a setting with relatively numerous obstructions providing shelter, usually found in an urban or suburban areas with obstructions not as closely spaced.
Exposure C – the baseline setting with relatively open terrain and scattered obstructions having a height of less than 30 feet that provide little to no sheltering, usually found in flat open country, grasslands or adjacent to agricultural lands.
Exposure D – a setting with little to no obstructions that provide sheltering, usually found within 5,000 feet of the shore of a large body of water.
Since the measurements of most wind speeds reported in the local media are taken in an Exposure C setting at an airport, it is probable that the actual wind speeds at a particular building in an urban or suburban setting were actually less than that recorded at the airport. Additionally, since wind speeds are also affected by height, the winds around buildings with a height of much less than 33 feet would probably see an additional reduction from those measured at a height of 33 feet at the airport. However, the wind speeds at a building in an Exposure D setting may actually be higher when the wind is approaching from across the body of water.
From this it can be seen that an evaluation of wind speed at a building usually requires more than a knowledge of the wind speed measured at the airport.