It used to seem a little easier to design a transmission line structure. If you wanted a wood pole, you sized it and ordered it. If something a little stronger was needed, you created a load tree, loading cases and pole geometry, and sent it to suppliers to design and fabricate a steel pole. It was a fairly straightforward process.
However, things change, which is a good thing. Today, most suppliers have catalogs from which line designers can select pre-engineered steel poles. For a number of perfectly valid reasons, these are often used in place of wood poles and are referred to as “wood pole equivalents.” They are not equivalent, for a number of reasons. It is important for line designers to understand the proper application of these products.
Wood poles have both variable dimensions and strength because they are natural products. Steel poles have consistent dimensions and strength characteristics because they are engineered products.
Thanks to the American National Standards Institute (ANSI) 05.1, we have guidelines for wood pole applications. ANSI 05.1 classifies a pole based on a classification load applied two (2) feet from the top of the pole. Pre-engineered steel poles typically are sized such that they will resist this same classification load applied at the same point. However, NESC Rule 250B Grade B load factor for wood is 4.0, and for steel it is 2.5. For NESC 250B Grade B loads, they have equivalent moment capacities because the steel poles compensate for the equivalency factor of 0.625 (2.5/4).
The line designer must be aware the poles are not equivalent for other loading cases, such asline tension, heavy ice, high wind, Grade C, etc. These differences under other loading cases will result in different buckling capacities, deflections, secondary moments and so forth.
The line designer should also pay particular attention to the following in designing and selecting pre-engineered steel poles:
- Guying: wood poles can be treated as struts with the guys bearing the entire transverse load; guyed steel poles should be designed as a complete system with shared load between the pole and guys.
- Using the customary wood pole embedment depth of 10% + 2 ft. may not be adequate for a steel pole. Perform a foundation analysis.
- Connections to pre-engineered steel poles become the responsibility of the line designer, not the fabricator. There has been minimal testing of connections to pre-engineered poles.
- Some traditional wood pole connections with heavy concentrated loads (davit arms, X-braces) can cause local buckling in a pre-engineered steel pole.
- Avoid field drilling of pre-engineered steel poles in welds or bend lines to avoid loss of moment capacity.
- Wood poles have large bearing area for through-bolts. Because of the relatively small bearing area of steel pole walls, bolt strength can be a limiting factor and should be checked.
- Smaller steel pole butt dimensions compared to wood can require the use of additional bearing plates to provide sufficient foundation resistance.
- Wood pole cross-sections are round. Steel poles can be regular polygons, which creates varying section modulus characteristics depending on orientation to the loads.
Pre-engineered steel poles are another useful tool in the line designer’s toolkit. Embrace their use, but do not forget that they are not “equivalent” to wood poles.
For additional details on this subject, refer to ASCE/SEI Standard 48-11, Design of Steel Transmission Pole Structures.