How to Calculate Wind Load

Wind is an almost horizontal flow of air moving from an area of high pressure to an area of low pressure. ^{[1] X Research Source} Strong winds can cause great damage because it puts pressure on the surface of the structure. The magnitude of this pressure is called the wind load. The effect of wind depends on the size and shape of the structure. Wind load is a must-know parameter to design and build buildings with better safety and resistance to wind, and to install objects on the roof of buildings such as antennas.

Table of Contents

Calculate the wind load using the general formula

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Define the general formula. The formula for calculating wind load is F = A x P x Cd , where F is the wind force or wind load, A is the projected area, P is the wind pressure, and Cd is the drag coefficient. ^{[2] X Research Source} This equation is useful for estimating the wind load on a given object, but does not meet the requirements in the building codes for the design of a new building.

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Find the area of projection A . This is the area of the two-dimensional surface that the wind is blowing on. ^{[3] X Research Source} For a more accurate analysis, you must repeat the calculation for each face of the building. For example, if the west side of the building has an area of 20m ² , substitute that value in A to calculate the wind load on the west side.

The formula for calculating the area depends on the surface shape. For flat walls, you use the formula Area = length x height. Calculate the approximate column surface area using the formula Area = diameter x height.
In the SI system, you need to measure A in square meters (m ² ).
In the British system, you need to measure A in square feet (ft ² ).

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Calculate wind pressure. The simple formula for calculating wind pressure P in the British system (pounds/square feet) is

P

=

0

,

00256

DRAW

2

{displaystyle P=0.00256V^{2}}

P=0.00256V^{2}

, where V is the wind speed in miles per hour (mph). ^{[4] X Research Source} To find the wind pressure in the SI system (Newtons/square meter), you use

P

=

0

,

613

DRAW

2

{displaystyle P=0.613V^{2}}

P=0.613V^{2}

, and measure the velocity V in meters per second. ^{[5] X Research Sources}

This formula is taken from the American Society of Civil Engineers standards set. The factor 0.00256 is the result of a calculation based on typical values of air density and gravitational acceleration. ^{[6] X Research Source}
Engineers use a more precise formula that considers factors such as the surrounding topography and type of building. You can find the calculation formula in the ASCE 7-05 series of standards, or use the UBC formula below.
If you don’t know what the wind speed is, look up the highest wind speed in the area according to the Electronic Business Association (EIA) standards. For example, most of the United States is in Zone A with wind speeds of 38.7 m/s, but coastal areas are in Zone B (44.7 m/s) or Zone C (50 m/s).

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Determine the drag coefficient of the object under consideration. Resistance is the force of the wind on a building, which is governed by the building’s shape, surface roughness, and many other factors. Engineers usually measure drag directly through experimentation, but if you want an estimate you can look up the typical drag coefficient for an object’s geometry. Example: ^{[7] X Research Source}

The standard drag coefficient for long cylinders is 1.2 and short cylinders is 0.8. These coefficients apply to antenna holders on many buildings.
The standard drag coefficient for flat slabs such as building faces is 2.0 for long flat slabs, or 1.4 for short flat slabs.
The drag coefficient has no units.

Calculate the wind load using the E-Business Association’s formula

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Define a formula developed by the Electronic Business Association. The formula for calculating wind load is F = A x P x Cd x Kz x Gh , where A is the projected area, P is the wind pressure, Cd is the drag coefficient, Kz is the contact coefficient, and Gh is the system. wind gusts. This wind load formula considers a few more parameters, and is commonly used to calculate the wind load acting on the antenna.

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Learn the variables in formulas. To use this formula effectively, you must first understand what each variable means and its units.

A , P and Cd have the same meaning as in the generalized formula.
Kz is the contact coefficient and is calculated from the height from the ground to the midpoint of the object. The unit of Kz is the meter.
Gh is the recoil factor and is calculated over the entire height of the object. The unit of Gh is 1/m or m ^-1 .

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Determine the projected area. The projected area of an object depends on its shape and size. If the wind blows against a flat wall, the projected area is easier to calculate than a round object. The projected area will be approximately equal to the area that the wind is exposed to. There is no formula for calculating the projected area, but you can estimate it with some basic calculations. The unit of area is m ² .

For flat walls, using the formula Area = length x width, measure the length and width of the wall where the wind is blowing.
For cylinders or columns, you can approximate the area by length and width. In this case, the width is the cylinder or column diameter.

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Calculate wind pressure. Wind pressure is calculated by the formula P = 0.613 x V ² , where V is the wind speed in meters per second (m/s). The unit of wind pressure is Newton per square meter (N/m ² ).

For example, if the wind speed is 31.3 m/s, the wind pressure is 0.613 x 31.3 ² = 600 N/m ² .
Another way to calculate wind pressure at a particular velocity is to use standards for wind speeds in different geographical areas. For example, according to the Electronic Business Association (EIA), much of the United States is in Zone A with wind speeds of 38.7 m/s, but coastal areas are in Zone B (44.7 m/s). ) or Zone C (50 m/s).

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Determine the drag coefficient of the object under consideration. Resistance is the part of the wind force acting in the direction of blowing on the surface of the object. ^{[8] X Research Source} The drag coefficient represents the resistance of an object in the fluid, and depends on the shape, size, and roughness of the object.

The standard drag coefficient for long cylinders is 1.2 and short ones is 0.8, commonly applied to antenna masts on many buildings.
The standard drag coefficient for flat slabs such as building faces is 2.0 for long flat slabs, or 1.4 for short flat slabs.
The difference between the drag coefficient of the flat plate and the cylinder is approximately 0.6.
The drag coefficient has no units.

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Calculate the contact coefficient Kz . Kz is calculated using the formula [z/33] ^(2/7) , where z is the height from the ground to the midpoint of the object.

For example, if you have an antenna 1 meter long and located 15 meters above the ground, z will be 14.5 meters.
Kz = [z/33] ^(2/7) = [14.5/33] ^(2/7) = 0.8 m.

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Calculate the wind recoil coefficient Gh . The wind gust coefficient is calculated by the formula Gh = 0.65+0.6/[(h/33) ^(1/7) ] , where h is the height of the object.

For example, if you have an antenna 1 meter long and located 15 meters above the ground, Gh = 0.65+0.6/[(h/33) ^(1/7) ] = 0.65+0, 6/[(16/33) ^(1/7) ] = 1.32 m ^-1

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Calculate wind load. Using the values found above, you can now calculate the wind load using the equation F = A x P x Cd x Kz x Gh . Substitute values into variables and perform calculations.

Let’s say you want to calculate the wind load acting on an antenna with a length of 1 meter and a diameter of 2 cm, with a wind speed of 31.3 m/s. The antenna is placed on the roof of a 15m high building.
Start by calculating the projected area. In this case, A = lxw = 1 mx 0.02 m = 0.02 m ² .
Calculate wind pressure: P = 0.613 x V ² = 0.613 x 31.3 ² = 600 N/m ² .
For short cylinders the drag coefficient is 0.8.
Calculate the coefficient of contact: Kz = [z/33] ^(2/7) = [14.5/33] ^(2/7) = 0.8 m.
Calculate the wind recoil coefficient: Gh = 0.65+0.60/[(h/33) ^(1/7) ] = 0.65+0.60/[(16/33) ^(1/7) ] = 1.32 m ^-1
Substitute into the equation: F = A x P x Cd x Kz x Gh = 0.02 x 600 x 0.8 x 0.8 x 1.32 = 10 N.
10 N is the wind load acting on the antenna.

Calculate the wind load using the formula of the UBC-97 standard (Uniform Building Code)

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Determine the formula of UBC-97. This formula was developed in 1997 in the UBC (Uniform Building Code) set of standards for calculating wind loads. The formula is F = A x P , where A is the projected area and P is the wind pressure; but this formula has a different way to find wind pressure.

Wind pressure (N/m ² ) is calculated according to the formula P= Ce x Cq x Qs x Iw , where Ce is the combined coefficient of the height, exposure and recoil of the wind, Cq is the pressure coefficient force (equivalent to the drag coefficient in the two equations above), Qs is the stagnation pressure of the wind, and lw is the important factor. All these values can be calculated or looked up from the respective tables.

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Determine the projected area. The projected area of an object depends on its shape and size. If the wind blows against a flat wall, the projected area is easier to calculate than a round object. The projected area will be approximately equal to the area that the wind is exposed to. There is no formula for calculating the projected area, but you can estimate it with some basic calculations. The unit of area is m ² .

For flat walls, using the formula Area = length x width, measure the length and width of the wall where the wind is blowing.
For cylinders or columns, you can approximate the area by length and width. In this case, the width is the cylinder or column diameter.

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Determine Ce , the combined coefficient of height, exposure and wind gust. This value is looked up from table 16-G in UBC and considers three types of terrain-related exposure, with different heights and Ce values for each.

“Exposure type B is terrain with buildings, trees or other unevenness, covering at least 20% of the surrounding area and extending for 1.6 km or more from the considered location.”
“Contact type C is flat and generally open terrain, extending 0.8 km or more from the site of consideration.”
“Exposure type D is the most severely impacted terrain, with an average wind speed of 129 km/h or higher, and a flat, unobstructed terrain, surrounded by open water.”

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Determine the pressure coefficient of the object under consideration. The pressure coefficient Cq is similar to the drag coefficient Cd . Resistance is the part of the wind force acting in the direction of blowing on the surface of the object. ^{[9] X Research Source} The drag coefficient represents the resistance of an object in the fluid, and depends on the shape, size, and roughness of the object.

The standard drag coefficient for long cylinders is 1.2 and short ones is 0.8, commonly applied to antenna masts on many buildings.
The standard drag coefficient for flat slabs such as building faces is 2.0 for long flat slabs, or 1.4 for short flat slabs.
The difference between the drag coefficient of the flat plate and the cylinder is approximately 0.6.
The drag coefficient has no units.

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Determine wind stagnation pressure. Qs is the stagnation pressure of the wind and is calculated similarly to the calculation of wind pressure in the previous equations: Qs = 0.613 x V ² , where V is the wind speed in meters per second (m/s).

For example, if the wind speed is 31 m/s, the stagnation pressure of the wind is 0.613 x V ² = 0.613 x 31.3 ² = 600 N/m ² .
Another way is to use standards for wind speeds in different geographical areas. For example, according to the Electronic Business Association (EIA), much of the United States is in Zone A with wind speeds of 38.7 m/s, but coastal areas are in Zone B (44.7 m/s). ) or Zone C (50 m/s).

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Determine the critical factor. lw is an important factor and can be looked up from table 16-K in UBC. It is a multiplier used to calculate the load taking into account the use of the building. If the building contains hazardous materials, the critical factor will be higher than that of a building for normal purposes.

Calculations for a standard use building will have an important factor of 1.

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Calculate wind load. Using the values found above, you can now calculate the wind load using the equation F = A x P = A x Ce x Cq x Qs x Iw . Substitute values into variables and perform calculations.

Let’s say you want to calculate the wind load acting on an antenna with a length of 1 meter and a diameter of 2 cm, with a wind speed of 31 m/s. The antenna is located on the roof of a 15 m high building in an area with Exposed Type B terrain.
Start by calculating the projected area. In this case, A = lxw = 1 mx 0.02 m = 0.02 m ² .
Determine Ce . According to table 16-G, using the height of 15 m and the terrain of Contact Type B, we can find Ce is 0.84.
For short cylinders the drag coefficient or Cq is 0.8.
Calculate Qs : Qs = 0.613 x V ² = 0.613 x 31.3 ² = 600 N/m ² .
Determine the critical factor. This is a standard building so lw = 1.
Substitute into the equation: F = A x P = A x Ce x Cq x Qs x Iw = 0.02 x 0.84 x 0.8 x 600 x 1= 8 N.
8 N is the wind load acting on the antenna.

Advice

You should know how wind speed varies at different heights from the ground. The wind speed increases with the height of the structure and the closer it is to the ground, the more erratic it is, because it is affected by structures on the ground.
It should be remembered that it is this erratic variation that will reduce the accuracy of the wind load calculations.

Steps

Calculate the wind load using the general formula

Calculate the wind load using the E-Business Association’s formula

Calculate the wind load using the formula of the UBC-97 standard (Uniform Building Code)

Advice

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