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 10year average velocity will be much less than a 50year 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 ( V^{2} / 2 g )
where:
q = Wind pressure in lb/ft^{2} or N/m^{2} (pascal)
γ = Specific weight of air in lb/ft^{3} or kg/m^{3}
g = Gravitation force in ft/sec^{2} or m/sec^{2}
V = Average wind speed in ft/sec or m/sec
q is more commonly seen in the following form:
q = (1/2) ρ V^{2}
where:
q = Wind pressure in lb/ft^{2} or N/m^{2} (pascal)
ρ = Average air density, 0.0809 lb/ft^{2} or 1.29 kg/m^{2}
V = Average wind speed in ft/sec or m/sec
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Further simplification:
q = x · V^{2}
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 50year 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

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

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 ½mile (805 m) or more from the site. This category includes flat open country, grasslands, and bodies of water under 1mile (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 1mile (1,609 m) in width. Exposure D extends inland from the shoreline ¼mile (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 50year 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

Related documents:
References
International Building Code (IBC) 2000
American Society of Civil Engineers (ASCE)
Minimum Design Loads for Buildings and Other Structures Revision of ASCE 798
American Association of State Highway and Transportation Officials (AASHTO)
AASHTO, Guide Specifications for Structural Design of Sound Barriers (1989)
FHWA Highway Noise Barrier Design Handbook
The Civil Engineering Handbook, W. F. Chen, (1995)
Reinforced Concrete Mechanics and Design, James G. MacGregor (1988)
NRCCNRC, National Building Code of Canada, (1995)
CAN/CSAS600, Canadian Highway Bridge Design Code
S6.100, Commentary on CAN/CSAS600, Canadian Highway Bridge Design Code
Manual of Steel Construction, Load and Resistance Factor Design (1986)
First Edition for Beam Diagrams and Formulas in Static Load Conditions
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