Section 2.0 - Loads

ID #1131

Should step pegs and safety climb be included in the Ra calculations for determining the appropriate Cf and additional linear appurtenance wind area calculated for TIA-G? It appears that in the transition from TIA-E to F to G the additional drag to the structure from step pegs and safety climb have already been accounted for.In TIA-E the force coefficients for tubular pole structures were less than those in TIA-F and TIA-G.In TIA-E, foot note 3 states that when step bolts… are attached to the outside of a tubular pole structure the Cf factor must be multiplied by 1.3.TIA-F removed this footnote but increased the force coefficients by 1.3 (except for the subcritical cases for which the force coefficient is 1.2). Presumably the footnote was removed and the values and formulas in the force coefficient table were revised so that the drag of step pegs and safety climb would be considered in the table rather than by footnote. TIA-G is using similar force coefficients as TIA-F (about 1.3 times that of TIA-E and presumably accounting for the drag increase due to step pegs and safety climb) but then state to include linear appurtenances such as ladders… or other similar projections in the Ra calculation. Step pegs and safety climb are not specifically mentioned but would be “similar projections”. If step pegs and safety climb are included in the Ra factor it seems that these linear appurtenances are being double counted when determining the additional drag on the structure.Based on the above discussion, in the calculation of Ra can the area of step bolts and safety cable be ignored?

The force coefficients (Cf), from Table 2-7 for pole structures account for a degree of attachments commonly attached to communication pole structures, therefore, no increase in Cf is required when the ratio Ra is less than or equal to 0.1. Step bolts are considered in the same manner as linear appurtenances since they are generally applied in a uniform manner over a given length. When determining Ra, the projected area of all linear appurtenances (including waveguides coax, step bolts, etc.) in the direction under consideration are intended to be included in the calculation of Ra. This is required to determine the degree of roughness. When the ratio Ra does not exceed the 0.1 threshold value, the force coefficient for the pole section under consideration is not increased. In this manner, step bolts are not double counted. Appurtenances not attached/strapped directly to the pole such as safety cables or ladders are intended to be considered separately for determining wind loads.

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Last update: 2013-08-08 15:09
Author: John Erichsen
Revision: 1.0

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