Section 2.0 - Loads

ID #1176

Section 2.6.9My question is regarding how to determine the wind loading on a triangular shroud that is surrounding all equipment on a monopole. This question encompass all shrouds of this nature, but the shroud in question for this particular case was 6’ tall by 13’ wide on a 48” diameter round pole. There were a few methods discussed based on our interpretation of the code. These interpretations are presented below. Please advise if which method(s) are recommended by the TIA.For the first case we considered the shroud as a truss type mount with a solidity ratio of 1.0. The code specifies to determine the wind area as though the platform were a section of a latticed structure in accordance with section 2.6.9.1. Since we have a solid face, using this equation with a solidity ratio of 1.0 will give us a Cf of 2.1. There was an argument that this section was for lattice tower and would not apply to monopoles, but we do not believe this is the intent of the code. Section 2.6.9.2.3 mentions no shielding shall be considered for the supporting structure, but we neglected the EPA of the pole structure shielded by the shrouds based on the limit stated in 2.6.9/2.6.9.1. For the second case we considered the shrouds as appurtenances. Using a Ca = 1.2 and Ka = 1.0 (per section 2.6.9.4 we cannot use a Ka less than 1.0) we came up with front and side EPA for the shrouds and modeled one off each face. We did not consider the shrouds to shield each other and although there are is not a reference for shielding the pole for appurtenances, we considered both methods above to shield the pole section within the shrouded region.The last method, which we did not agree with, proposed to treat the shroud as an appurtenance, but to allow shielding to the appurtenances (shrouds) on the other faces along with using a Ka=1.0. This gives an EPA of 94 sf. We believe this to be very aggressive and would recommend using one of the methods described above. Example calculations for this particular shroud are shown below. The shroud is 6’ tall by 13’ wide on a 48” diameter round pole.Pole EPA: 0.6x4’x6’ = 14.4 sfShroud as Mount:Cf: 3.4(1.0)^2 – 4.7(1.0) + 3.4 = 2.1EPA: 2.1x6x13 = 163.8 sfEPA minus pole = 149.4 sf Shroud as Appurtenances (did not consider side area):Ca: 1.2EPAn: 1.2x6x13 = 93.6 sfETAt: 0Ka= 1.0EPA = 1.0 [93.6 + 2 x 93.6 x Cos(60)^2] = 140.4 sfEPA minus pole = 126 sfShroud as Appurtenances (shielding allowed):EPAn: 1.2x6x13 = 93.6 sf

The intent of the Standard is to calculate the effective projected area of a discrete appurtenance in accordance with Section 2.9.6.2. The effective projected area for both the normal and transverse directions are intended to be calculated in accordance with Table 2-8 based on the aspect ratio of the projected area of the discrete appurtenance for each direction. The leeward faces of the appurtenance under each wind direction may be considered shielded in accordance with Section 2.6.9.4. For a triangular discrete appurtenance as described in the question, the back faces may be considered to be 100% shielded when determining the EPA for the normal and transverse directions.

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Last update: 2015-11-18 18:04
Author: John Erichsen
Revision: 1.0

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