**Wood Equivalent Steel and Concrete Poles**

| Borrowers are cautioned that there should not be a    direct substitution of wood poles with steel poles or concrete poles if the original line    design was based on Grade C construction. Most manufacturers standardize on steel or    concrete poles such that the poles have a wood pole equivalency based on Grade B    requirements in the NESC. Table 1 below summarizes the loads used to design poles:

Table 1

Pole Class
Wood Pole        Tip Loads (lbs.)
Steel or        Concrete Pole        ‘Equivalent’ Load (lbs.)
1
4500
28001
2
3700
2300
3
3000
1900
4
2400
1500
5
1900
1200
6
1500
950
An example will be used to demonstrate the confusion    when using standard class steel or concrete poles in distribution lines.

Example:

A cooperative wishes to build a distribution line meeting Grade C    construction. The design calls for 40-4 wood poles but decides in one section to use steel    distribution poles. The transverse working load (load without overload factors, [OLF]) is    1200 lbs. two feet (.6 m) from the top of the wood pole.2 The common    practice is to substitute a standard class steel pole, designated as a 40-4, for the wood    pole. From Table 1, the ultimate load for the equivalent steel pole would be    1500 lbs.

But, this is where the problem occurs. The ‘ultimate’ load,    to which the steel pole needs to be designed, should be: 1200 lbs. (working load) x 2.2    (OLF3) = 2640 lbs.

In the table above, the 2640 lbs. ‘ultimate’ load corresponds    to a ‘class 1’ steel pole. 

If a 40-4 steel pole with a tip load of 1500 lbs. were selected,    it would be under the required NESC strength by approximately 75 percent. This is    where the potential confusion comes into play.

If you would like more information or have any questions, please call    H. Robert Lash, Chief, Transmission Branch, at (202) 720-0486 or Harvey Bowles,    Chief, Distribution Branch, at (202) 720-1979.

1 Ratio of Grade B steel OLF 2.5 to wood OLF 4.

2 Class 4 tip load of 2400 lbs. divided by Grade C wood OLF 2.0.

3 Overload factor for steel poles not at a crossing, Grade C construction.
 

[Abbreviations] [Returnto Table of Contents] |
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