Structure Deflection 2

[EP007]

I am trying to have an appreciation of the numbers I see for single wood pole (tangent) deflections, however, it is not my field. Poles should deflect in strong winds. I also notice that some are leaning without wind, which must be due to foundation, or perhaps pole age. The maximum deflection should be determined at least for clearances and right-of-way calculations. My question is: Is there much that a designer can do in order to control the amount of maximum deflection? Can one company design for 2-4%, and another for 8-10%?

 

[transmissiontowers]

There is not much consideration given to the deflection under load for Distribution class wood poles. The Distribution guys have a 60' exemption in NESC where hurricane or extreme wind from rule 250C does not blow below 60 feet. From a physics standpoint, this is incredibly wrong and the only way they justify the practice is to point out the debris blown around by extreme wind catches on the wires or poles and the sail effect brings the line down. Typically they just go out, stand the poles back up and kick some dirt around the base and walk away.

The other problem is the 10% plus 2 feet embedment for any soil condition rule of thumb should be retired. It might work if you had to blast a hole in rock to get the pole in, but for bad soils it is way too shallow. Distribution systems rely heavily on standards and rules of thumb (at least at my company) where some engineering was applied 40 years ago and the results were put in a Standards Book and nobody has taken the time to check the standard. The leaning poles you see are a result of too shallow embedment in poor soils.

To control deflection, you can switch to steel or spun concrete poles. If you were talking about Transmission class poles, wood poles are sized for stress and the deflection is not considered because wood is a naturally grown material whereas steel poles can be made to control deflection.

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I have been called "A storehouse of worthless information" many times.

 

[MikeDB]

I am not aware of any mandated deflection limits in the US, so yes, utilities are free to adopt different limits or none at all. A 2% limit for some substation structures was used in NEMA SG6, but such a limit would add needlessly to the cost of overhead power lines.

You are correct that deflection should be considered for clearance and ROW calculations, and I believe that is a common practice. Generally there is a lower clearance requirement when considering the conductor's position in the "blown out" condition.

There are a number of things that can be done to limit pole deflection, if that is a primary objective. One could increase the moment of inertia or increase the modulus of elasticity of the pole. One could also reduce the length of the pole or reduce the loads on the pole.

Some poles have constant loads on them, like unguyed taps or line angles, which cause the wood to creep over time, adding to the deflection.

 

[transmissiontowers]

There is a relatively new manual by ASCE (#113) if you are interested in designing substation structures which has some deflection recommendations and ASCE has just published Manual #74 which is a revision of the older version for transmission line loading. I was on the editorial committee for both.

If the OP is in the USA, the NESC specifies a 6 PSF wind for blowout calculations (this is about 48 mph IIRC) and the wire catenary must stay within the ROW and with the modern software (we us PLS-CADD as does about 90% of the industry) you can check the combined structure and wire deflection.

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I have been called "A storehouse of worthless information" many times.

 

[bacon4life]

We don't use any deflection critera for wood poles other than checking blowout clearances.

For steel poles, sometimes our deflection criterea ends up sizing the pole. For deadends we have used 3% with factored loads and all wires in place, or 8% with wires only one side. On tangets we have often used 4% with overloads.

We also specify a raking load case for steel poles so that we can install the bare pole at an angle, then have the typical everyday loads bring it into plumb.