Calculating Pole Embedment

[bridgebuster]

I’m a bridge engineer and occasionally I have to design a utility pole.

We’re working for a contractor and he needs to hang traffic signals between two poles. The span wire will be 34’-6” above grade. The signal designer came up with Class H-1 poles embedded 9’. The electrician can't get 45' H-1 poles on short notice (and they have to install them next week), but he has plenty of Class 1 poles. The signal designer said he didn’t know how design reinforcement for the pole. We’re going to reinforce the Class 1 pole by bolting a smaller length Class 1 pole to it (7/8” threaded rod at 9”). My original idea was to guy the pole or install the Pole Enforcer, which would’ve been much simpler than bolting two poles together; they don’t want to do it.

The person who checked the pole reinforcing design stated that 9’ embedment is too little; it needs to be 16’ per the IBC formula. I’m aware of the 10%+2’ etc rule used in pole design; I also know it’s not always accurate. Anyway, I was asked to check the embedment and used the method in USDA 1724e 200, which gave me about 8 ½’. Our geotech said the USDA method isn’t valid and he used Bromm’s method and came up with 13’ embedment.

I’d like to know how people who know what they’re doing calculate pole embedment. Also, if you have any thoughts on pole reinforcement, I’d be happy to hear them too.

Thanks

 

[Lomarandil]

For transmission line and substation work, I used Brom's.

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The name is a long story -- just call me Lo.

 

[bridgebuster]

Thanks

 

[msquared48]

I've always heard from others that embedment used by utility companies for their poles will never figure under the IBC.

I checked it once and found the embedment needed to be much greater under the IBC than the standard 8 to 10 feet.

Perhaps that's why I see so many leaning utility poles.

As for a quick reinforcing fix, why not use a galvanized pipe pile set in an augered hole partially fill with gravel in the interior, set the pole in the pipe pile, then backfill on the outside with a flowable grout mix?

Mike McCann, PE, SE (WA)

 

[bridgebuster]

Thanks Mike. I read one time that the embedment rule of thumb can be highly inaccurate. Somehow it must work. A few moths ago when this problem arose, someone suggested setting the pole in a pipe but the electrician didn't want to buy the pipe.

 

[jwkilgore]

The 10%+2' is low for anything engineered. It's semi-adequate for short distribution-grade wood poles, generally Class 1 or 2 or below. Not really for low-strength steel, definitely not even close for higher strength.

For design, either Broms method or we use MFAD software (by FAD Tools). It takes into account the annular space, which can be made of compacted native soil, gravel, or concrete.

For your situation, one way to make 9' work is to auger a larger hole and fill with compacted gravel, CLSM, or even concrete. Instead of a 3' hole, auger one 4' or more. Without a pipe (or reinforcing in the concrete) you won't get to count the full diameter of the larger hole, but it will be wider than just a bare pole against dirt.

Another option would be adding something to increase the pressure width. Dig a trench and bolt a length of wood pole sideways just below the frost line. If you're in a non-corrosive area maybe use a steel HSS or WF (flange flush against and bolted to pole). It's harder to make the base wider if you also need that, but possible.

A third option to help the foundation would be a "ground guy". I'm assuming normal guys at the top of the pole aren't wanted because of ROW or maintenance problems. But you can install a helical guy anchor such that the top is right at the pole at the ground line.

None of these help the "two poles bolted together" ugliness, but they could allow a shallower foundation. Otherwise it looks like you'll be ordering a 50-ft pole.

BTW, is this a temporary or permanent installation?

 

[bridgebuster]

Thanks jwkilgore. I like the idea of a larger hole. I'm sure they're going to complain about 13'. I forgot to mention this is a temporary installation; about 6 months. I don't know why they didn't want to use a guy; bolting two poles together was the electrician's idea and I suppose his mind is made up

 

[bridgebuster]

The contractor sent an email this morning complaining (surprise). A 13' deep hole is excessive & bolting the shorter pole to the main every 9" is impractical. Now he's asking if he can bolt two 45' poles together every 9' or guy the poles. Another reason why I "love" design build jobs.

 

[msquared48]

Guying may require further Right-of Way easements... Be careful on that.

(Why do they call it guying instead of galing?)

Sounds like a wider hole with a larger auger would be the best bet here.

Tell the contractor to quit acting like our politicians in Washington and compromise. That should get you some bennies!

Mike McCann, PE, SE (WA)

 

[bridgebuster]

Institutional sexism & patriarchy

Meanwhile, the electrician told the GC he has his own idea what to do; should be interesting.

 

[BigH]

I know I am way late on this - but what kind of soil are the poles to be embedded into?

Now, the poles have been installed, obviously, given the original post.

What was actually done?

What type of pole, depth, etc. and are they performing properly or did any require remediation?

 

[oldestguy]

I' do what ever is convenient for a temporary job. So it leans, but it won't fall down, just tighten the cables.

 

[bridgebuster]

They ended up using three guy wires to mitigate the bending but the design PM still made them go with 13' embedment, which I didn't think was necessary.

 

[oldrunner]

Mayby they had really bad dirt? One time a geotech told me that the passive pressure started at 10 feet! I usually design pole footings per the latest AASHTO Luminaries code. I also ALWAYS use a geotech. (Former clients: Tower Structures and U.S. Flag and Flagpole) So who is taking professional responsibility for this project? I hear the 6 months but I've got a 6 month temporary inground 20 foot diameter wooden hot-tub like structure still being used (I think) that was constructed in 1988!