Pole Embedment in Concrete 1

[abusementpark]

How do you determine the required embedment of a steel or wood pole into a concrete drilled shaft (assume unreinforced)?

See the attached sketch for scenario I am envisioning. What would be the potential failure modes?

 

[abusementpark]

OK, for some reason, the attachment link is not working.

 

[abusementpark]

Here is a link from the web since Eng-Tips won't let me attach it.

http://i.imgur.com/wwfjF.jpg

 

[Ron]

abp...here is a similar condition...principle is the same

 

[Ron]

ok....you're right. Link doesn't work.

Try this..

http://imgur.com/8XZyV

 

[dik]

Ron:
Your attachment is somewhere in cyberspace...

Dik

 

[apsix]

I don't think that the principle is the same.

For an unreinforced drilled shaft the critical failure mode is likely to be the flexural capacity of the concrete at the bottom of the embedded pole. Design the embedment for that.

 

[Ron]

dik...the second attempt works...

 

[dik]

You can use the same approach for concrete to soil as Ron's diagram (I use a slight variation) and neglect the top foot or so for passive earth pressure to take hold...

You can check the plain concrete section for flexure at the base of the post...

Couple of cautions... the pole or wood post should be protected against corrosion/rot at the concrete/steel/wood interface, the top of the concrete should slope away from the wood/steel to provide free drainage, the top of the concrete should be 6" or 8" above surrounding grade.

Dik

 

[msquared48]

You need to check the cracking moment at the base of the pole, and the unreinforced shear on the reduced area at the base of the pole.

My only question is why would you want to do this?

It's just a simple pole footing and you are complicating it.


Mike McCann
MMC Engineering

 

[Ron]

Here's a pretty good article on pole embedment from the USDA...

(Public Domain Article)

http://imgur.com/qDCxk

 

[dik]

I liked Ron's explanation of how he designed these pole footings...

Dik

 

[Ron]

Thanks, dik. I took my approach to pole/post embedment and applied it to handrail embedment, now back to pole embedment. I've rationalized that there's little or no difference, so applicability prevails.

 

[abusementpark]

How would verify that the concrete shaft is adequate against breakout? Assume a breakout shape with a conservative allowable stress?

Also, what about the vertical component of friction on the pole?

 

[Ron]

Steel and wood are treated differently with regard to their interface with concrete. Steel actually has a bond strength with the concrete. It gets a little better with age, due to some initial corrosion/contact, but then remains (as far as I know) unless there is significant corrosion of the steel and it exfoliates.

Assuming no significant deterioration, I use between 25 and 50 psi for shear-bond strength (depending on surface prep of the pole...25 psi for no prep, 50 for near-white grit blasting). I compute the pull-out resistance based on the surface contact area of the post/pole, then apply a safety factor to the result, usually at least 2 but not more than 4. If the pole is open at the bottom such than concrete or grout can flow into the pole, you obviously get more contact area, but perhaps not as much as around the outside. I consider that the contact area inside is never more than 1/2 the outside area.

For wood, it is a bit different. Initially there is bond to the wood, but as the wood shrinks, so does the bond. I'm not much of a fan of wood embedded in concrete or grout, so I have no real numbers to apply to this. Check with the Forest Products Lab of the USDA...they might have something. I know that you can epoxy coat the wood and get close to the steel values I gave, but I would keep that on the conservative side.

As for breakout, this is affected only by the moment at the pole's fixity. Pullout does not create a shear-cone as an embedded anchor would. It will fail in shear at the interface bond or in direct tension of the concrete annulus. Reinforce accordingly with spirals and vertical rebar.

 

[apsix]

I use steel dowels through the pole to transfer tension to the concrete.