Utility wood pole soil embedment design 2

[gasma1975]

Hi,

How do you verify the ground stability for an utility wood pole ? I know that the rule of thumb is 10% of the lenght plus 2 feet. But is there a way for more accurate design based on the type of soil and bearing resistance ?

gasma1975

 

[BAretired]

Calculate the wind moment to be resisted in your locale. With soil properties known, consider the pole doing a rigid body rotation, then check the permissible bearing stresses against the soil.

BA

 

[woodman88]

If using the 2009 IBC look at Section 1807.3 Embedded posts and poles?

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[JedClampett]

You could engineer it. But I learned a few weeks back, through a RedVector course, that there is a National Standard (makes sense) on wood poles. It's ANSI 05.1-2002 (this might not be the latest). And it's common to embed wood poles at 10% of the length of the pole, plus 2'-0" or 5'-6" minimum.
As a side note, I was just trying to get some pdhs for license renewals and I thought the course would be easy. But it turned out there's a lot more to wood pole design than I thought. The course was involved and the test was very challenging, with a lot of problems to work.

 

[TLHS]

10% + 2ft is a typical minimum for utility poles. At some point I tried applying the IBC method to this case, but the embedments ended up being higher than what's usual in the utility world. That was a long time ago though, so it might have been specific to that problem.

I'd suggest getting Bulletin 1724E-200 (

http://www.rurdev.usda.gov/SupportDocuments/UEP_Bulletin_1724E-200.pdf

). Chapter 12 is pretty consistent with what seems to be standard practice.

I've got a good recent study that modifies some of that, but I can't figure out where I've put it.

 

[TLHS]

But if it's a relatively short low class pole 10% +2' in average soil will generally get it done.

 

[gasma1975]

I have 2 wooden mast 90 feet apart and a cable. At mid span I have a small flasing light 50 lb. I will have some tension in the cable because the maximum sag is near 4 feet. I'm trying to avoid any guyed cables for the wooden mast. How can I evaluate how deep the embedment will be and the pressure by the base on the soil?

 

[TLHS]

This is a really annoying problem because you have to get fairly specific to control all the install variables:

First decide how you're telling them what they need to install. Are you giving them a cable length to install? An installed horizontal tension? A variety of installed tensions depending on the temperature?

Check your loading based on the cable weight, the weight of the attached load, the weight of any icing, and wind. Some people like putting bird allowance on too. I mostly just like saying bird allowance.

In addition, you have to account for temperature variations. You'll install at some temperature and then later it will get warmer and colder. The former increases your sag and decreases your tension while the latter does the opposite. For a distribution line you'd normally do this with a sag chart. You have a chart that tells the installer what sag and/or horizontal tension it's supposed to have at a given install temperature so that at the hot and cold temperatures the sag and tensions are what you expect them to be. In general, you'll set all your variables like this:

Determine your maximum allowable sag. This occurs after creep and at your hot temperature. So you set your tension or cable length based on this condition and based on your sag. If you're using steel rope or something, creep isn't as big a deal as it is with overhead conductors. With this case, check where your wind is going to push your cable to horizontally if there are clearance issues. This will be your biggest swing case.

Now that you've got your start point, look at it with your minimum temperature, ice, attached load and some/all the wind. This generally gives you your worst case design load.

Take that load, apply it to the pole. Check the pole structurally and for deflection. Get your baseline moment. Design your embedment to take that moment as per the reference I provided. Note that utility pole designs tend to deflect by amounts that would be crazy in most situations structural engineers would be involved with. If those aren't acceptable in this case, you may have to look closer at your embedments.

That reference actually goes into utility pole design in general, so you probably want to look through it. In standard installs it's pretty easy because everything's generally been tabulated by someone, but when you start trying to do anything even a little non-standard there's a lot to get your head around.

 

[TLHS]

Guying it is generally going to be cheaper and significantly more reliable (you won't get the long term deflection you'll get out of an unguyed wooden pole)

 

[BAretired]

How about putting a concrete collar around the pole just below grade? That could create a sizable area to bear against the soil.

BA

 

[TLHS]

It's generally pretty easy to spread load below grade. You can also over-auger and fill the hole with gravel to a certain degree and you can sleeve the gravel with a culvert or something similar as you get bigger. It ends up costing more mostly because of excavation. It's generally cheaper to go deep with an auger rig than it is to come and excavate a hole. Obviously that depends on the equipment they have to mobilize to do other things though.

 

[paddingtongreen]

The rules of thumb for utility poles are used with the expectation of returning and resetting a small percentage of them. If one starts to tip, the wires hold it from falling completely. It should not be used except where there are lines to hold the tilting pole. It is a cost/benefit calculation.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[gasma1975]

Thank you for the help

I saw in some documentation that is not allowed to put concrete around a wooden pole, is it true ? normally what is the detail construction when you embed a wooden pole in the soil ? I guess you put gravel at the bottom, what is around the pole ? just the soil or can I add some compacted crushed stones around it ? I cannot find any detail on the web...

 

[BAretired]

So far as I am aware, it is okay to put concrete around a wood pole, but not okay to embed a wood pole in concrete, the reason being that there is no drainage channel for water to escape.

BA

 

[msquared48]

BA:

I agree, and that's why I put 3 to 6" of gravel at the base of the hole, install the pole, and pour concrete around that for pole buildings. With that, you do get the drainage you need, assuming you are above the water table.

For fence posts, I jus use a dry mix concrete, hand tamped with the gravel at the bottom. My PT fence posts have been in the ground for 30 years and are still very serviceable.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[dik]

The wood pole could rot at the top surface of the concrete, maybe... my experience with concrete embedded fence posts...

Dik

 

[Splitrings]

ASABE EP 486, Shallow Post Foundation Design is a decent reference for embedded poles.