Wind Pressure Exerted on Vertical Wall

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Wind Pressure Exerted on Vertical Wall

Civil Engineering and Design Resources

Wind Pressure and Force Exerted on Vertical Wall Equations

The wind can vary both in terms of its speed and its direction. As a result different pieces of equipment are needed to measure these different characteristics. Wind direction is commonly determined by weather vanes or wind socks which will swing around and show which direction the wind is blowing from.

The main instrument used to measure the speed of the wind is an anemometer. The little cups on a typical anemometer catch the wind and spin around at different speeds according to the strength of the wind. A recording device is used to count how many times they spin around in a given amount of time. Positioning the anemometer properly to accurately record the wind speeds is important and there are detailed standards that must be followed.

Wind conditions have been measured for years and the information provided in local design codes is based on decades of recorded data. By analyzing this data the average wind speed with a given return period can be obtained for a region. The "return period" refers to the most probable average wind speed that will be equaled or exceeded once during a period of time compared to the life of a vertical wall. Thus, a shorter return period would provide lower wind speed. Longer return periods would increase the probability for higher wind speeds. For example, a 10-year average velocity will be much less than a 50-year average velocity.

The wind speed can be used to calculate a wind pressure using the Bernoulli Equation relating velocities to pressures. Since wind is air in motion the resulting wind pressures are related to its kinetic energy and can be determined by the following expression:

q = γ ( V² / 2 g )

where:

q = Wind pressure in lb/ft² or N/m² (pascal)
γ = Specific weight of air in lb/ft³ or kg/m³
g = Gravitation force in ft/sec² or m/sec²
V = Average wind speed in ft/sec or m/sec

q is more commonly seen in the following form:

q = (1/2) ρ V²

where:

q = Wind pressure in lb/ft² or N/m² (pascal)
ρ = Average air density, 0.0809 lb/ft² or 1.29 kg/m²
V = Average wind speed in ft/sec or m/sec

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Further simplification:

q = x · V²

where:

x = Unit conversion, based on which velocity units the designer is using, see Table 1

Table 1

Unit Conversions
V	x	Yields
ft/sec	0.00119	PSF
m/sec	0.00064645	kPa
mph	0.00256	PSF
km/hr	0.00004807	kPa

This is called the "stagnation pressure" or total pressure because it refers to the maximum positive increase over ambient pressure that can be exerted on the vertical wall by any given wind speed. Stagnation pressure is the basic, non factored pressure to which all other pressures are referred to and are usually referenced in regional building codes.

Table 2 provides a few stagnation pressures computed from the given wind speeds. If the 50-year return period velocity is equal to the wind speed in Table 1, the tabulated values for wind pressure can be used in your design.

Table 2

Wind Speed Verses Wind Pressure
Wind Speed (V)	mph	60	70	80	90	100	110	120
Wind Speed (V)	km/hr	96	113	129	145	161	177	193
Wind Pressure (p)	psf	9.3	12.6	16.4	20.8	25.6	31.0	36.9
Wind Pressure (p)	fPa	0.443	0.603	0.785	0.995	1.23	1.48	1.77

Wind pressures exerted on the vertical wall depend not only on the speed of the wind, but on the interaction of exposure effects as well. Any structure, including but not limited to, buildings, landscape features, general topography, and open areas such as fields, parks, parking lots, street corridors, and bodies of water all significantly affect the wind patterns and need to be considered. An exposure category that adequately reflects the characteristics of ground surface irregularities is determined for the site. Open terrain allows for the maximum exposure, while vertical walls found in developed or urban areas have minimum exposure. Described below are the three exposure categories used in the designs of vertical walls:

Exposure B: Surface roughness consisting of urban and suburban areas, wooded areas, or other terrain with numerous closely spaced obstructions having the size of a single family dwelling or larger.
Exposure C: Surface roughness consisting of open terrain with scattered obstructions having heights generally less than 30 ft (9.1 m) extending ½-mile (805 m) or more from the site. This category includes flat open country, grasslands, and bodies of water under 1-mile (1,609 m) in width.
Exposure D: Describes the most severe exposure with surface roughness consisting of flat, unobstructed areas and bodies of water over 1-mile (1,609 m) in width. Exposure D extends inland from the shoreline ¼-mile (402 m).

Additionally, calculations should also include site topography and structural importance when factoring wind pressures on a structure. These Pressure Coefficients are listed in Table 3 by the exposure category, but also account for general site topography, structure height and structural importance as well.

Again, for vertical wall designs the wind pressure is based on the 50-year average wind speed. The pressure should be multiplied by the appropriate Pressure Coefficient to determine a factored pressure. This factored pressure is then used in the design calculations.

Pressure Coefficient
Exposure	H<12ft (3.7m) Pressure Coefficient	H>12ft (3.7m) Pressure Coefficient
B	0.68	0.85
C	0.9	1.2
D	1.25	1.5