PMP 450 Wind Loading

Products PMP
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Ensure that the site is not prone to excessive wind loading. Antennas and equipment mounted on towers or buildings will subject the mounting structure to significant lateral forces when there is appreciable wind. Antennas are normally specified by the amount of force (in pounds) for specific wind strengths. The magnitude of the force depends on both the wind strength and size of the antenna.

Calculation of lateral force (metric)

The magnitude of the lateral force can be estimated from: Force (in kilograms) = 0.1045aV2

Where:

     a is the surface area in square meters and

     V is the wind speed in meters per second

The lateral force produced by a single PMP 450 at different wind speeds is shown in Table 26 and Table 27.

Table 26 Lateral force – metric

 

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Calculation of lateral force (US)

The magnitude of the lateral force can be estimated from:

Force (in pounds) = 0.0042Av2

Where:

     A is the surface area in square feet and

     v is the wind speed in miles per hour

The lateral force produced by a single PMP 450 unit at different wind speeds is shown in Table 27.

Table 27 Lateral force - US

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Capabilities of the PMP 450 Series

The structure and mounting brackets of the AP are capable of withstanding wind speeds up to:

  • 190 kph (118 mph) – 5 GHz Sector Antennas
  • 216 kph (135 mph) – 2.4 GHz, 3.5 GHz, 3.6 GHz Sector Antennas

Ensure that the structure to which the AP is fixed to is also capable of withstanding the prevalent wind speeds and loads.

The structure and mounting brackets of the SM are capable of withstanding wind speeds up to 190 kph (118 mph). Ensure that the structure to which the SM is fixed to is also capable of withstanding the prevalent wind speeds and loads.

Wind speed statistics

Contact the national meteorological office for the country concerned to identify the likely wind speeds prevalent at the proposed location. Use this data to estimate the total wind loading on the support structures. Sources of information:

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Hi Ray, I just wanted to confirm that the implied drag coefficient stated by these estimation equations make sense to you.

If lateral force can be calculated by the drag equation:
Drag = 0.5 * density * drag_coefficient * area * velocity^2

And your estimate is:
Drag = 0.1045 * area * velocity^2

Then we can set them equal to each other to solve the implied drag coefficient:

0.5 * density * drag_coefficient * area * velocity^2 = 0.1045 * area * velocity^2

Cancelling terms:
0.5 * density * drag_coefficient = 0.1045

Solving for drag_coefficient with density of sea level of 1.225 kg/m^3:
drag_coefficient = 0.1045 / (0.5 * density) = 0.1045 / (0.5 * 1.225) = 0.1706

Given that a flat plate has a coefficient of 1.2, are we under-estimating drag with the estimation equation? An implied coefficient of 0.1706 seems a little low to me, especially given that I find that this equation is being stated in User Guides for devices that aren’t necessarily bullet-shaped, and closer to the flat plate estimate.

Under-estimating the drag using those equations can give installers false confidence of whether the tower is over-loaded laterally, and I think this should be addressed sooner rather than later. I’ve also noticed unit inconsistencies in some tabulated figures applying these equations.

I’ve been scratching my head as I’m trying to come up with realistic loading on a tower, and the datasheet only speaks to survivability instead of authoritatively stating a loading figure, as seems to be standard with other manufacturers. Thanks in advance for any insight on this.

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