Plain Concrete Pole Foundation Question

[jba970]

Hey all, new to the boards, and looking for some guidance here.

We've been asked to design some pole foundations for a sign company, and the client has requested that plain, un-reinforced concrete be used to save on costs. He provided us with old drawings that another firm has provided him for pole foundations in the past, and there is a note that reads:

"No steel reinforcement is required in cube or auger style footings where the support column is embedded directly into the bottom of the footing." Whoever designed the foundation has the sign pole embedded into the concrete foundation with zero reinforcement.

Is this acceptable? Backed by code? I have looked through IBC and ACI trying to find a justification for this, but no luck. Any help is much appreciated! Thanks!

J

 

[EngineeringEric]

I had a sign guy ask me to review a similar thing. He stated that this is how they used to build them and only used it for up to 15' tall signs. I looked at it out of curiosity (free) and there are numerous concerns. My guys at least installed a few hoops at the top and bottom of the foundation to keep the concrete together. I suppose you could assume the concrete is broken down its height, but if restrained by soil it will have N-amount of confinement which will help. As I wasn't tasked with actually commenting on it i never did any code or real research, but I can confirm know it is done.

In summary, I am not a fan and wouldn't want to use it. The crazy thing is how much does ten #3 hoops @ 1.5' diameter really cost?

 

[jike]

THIS IS PRETTY TYPICAL FOR THE SIGN INDUSTRY.....I HAVE SEEN THEM USE LARGE UNREINFORCED BLOCK FOUNDATIONS.....CONCRETE IS FAIRLY CHEAP.

I RECOMMEND THAT FOR WHATEVER FOOTING CONFIGURATION YOU ARE LOOKING AT, MAKE SURE YOU JUSTIFY IT BY CALCULATION (INCLUDING CONFINEMENT.

 

[jayrod12]

Please stop yelling.

I don't necessarily like it however, if you embed the post down to the bottom of the hole I would consider the concrete the as a sleeve that just widens the effective area pushing on the soil. I wouldn't account for the concrete in a strength aspect but when I was checking the required post depth I would use the radius of the concrete when doing the calculations. ASAE 486 I think.

If that makes any sense,

 

[jike]

sorry for typing in caps....I had a serious injury to my hand and arm and it is difficult at best to type.

 

[jba970]

I think I can just calculate the phi*Mn and compare it to the Mu at the pole embedded depth. ACI 22.5.1 covers phi*Mn for plain un-reinforced concrete. Anyone see any issues with this?

 

[jayrod12]

Eric,

Now I'm not one to side with the contractors typically but I once had a reasonable guy that said to me "Sure what's the cost of that little extra rebar on this one job, I do 1000 of these little jobs in a year, it adds up to me, even if it doesn't add up for one project"

Sure it's maybe $100 in rebar, but if the sign company is busy and profitable that's $1000 every 10 sign posts. If they try to factor that into their pricing they lose the bid.

Depending on the size of the sign I would be willing to consider it at the very minimum.

 

[TehMightyEngineer]

One thing to note is that "plain concrete" does not always mean concrete with no rebar. It means concrete with no reinforcement or less than the minimum reinforcement required to be called reinforced concrete.

Here's a really good article on plain concrete for walls and footings for buildings that summarizes how confused this issue can get:

http://www.buildersengineer.com/assets/images/blog/se magazine jan 20130001.pdf

http://www.buildersengineer.com/assets/images/blog/SE mag 3-13 part 2.pdf

As for a sign pedestal in the ground it appears that ACI 318 shrugs that off with 22.2.2 unless it's precast. I didn't look through the IBC but it sounds like you couldn't find anything there either.

As Eric said; does this really save much money?

Maine EIT, Civil/Structural.

 

[EngineeringEric]

I bet the cost of 10 hoops is much less than the cost to replace one of these. I know we will never convince a contractor this, especially one who has 'been building it this way for 30 years'. I will never say it won't work without running numbers, but I feel confident stating that I don't like it and wouldn't seal that myself today.

I have to say, valid point jayrod12 on the adding up over multiple jobs; however this can be said for many things we specify and require as engineers.

 

[jayrod12]

I agree that we do it all the time, however I believe it is prudent to at least run some numbers and give it some real thought before dismissing construction techniques that have been used and proven themselves serviceable.

Would I prefer to have rebar in the concrete? Of course.

If the sign is small enough will I at least take a look at the possibility of plain concrete? Sure, if the numbers don't work then so be it. But if you can rationalize it then make a contractors day.

Off topic but if you want a tough one to justify, re-analyze an existing pole barn and get the numbers to calc out. They do not, I repeat do not, work on paper in any way shape or form without voodoo. But yet they stand the test of time.

 

[paddingtongreen]

If the bearing stress of the pole on the concrete is okay, and if the stress of the concrete on the soil is okay, and if the spread in width of the effective concrete wedge is okay, what is there to worry about?

I've seen people worry less when the void around the pole is to be filled with tamped crushed stone instead of concrete.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[jayrod12]

My thoughts too Paddington. It's worth checking the numbers.

 

[TehMightyEngineer]

What takes the moment from the sign from wind loads? What happens if the concrete cracks in the pier right below the signs anchorage?

Maine EIT, Civil/Structural.

 

[jayrod12]

The anchorage would not really exist besides embedment. and as noted above the intent would be to embed the post completely into the hole. So as paddington noted the only stress the concrete would see is bearing of the pole on the concrete and the concrete on the soil.

 

[TehMightyEngineer]

Hmmm, so basically the concrete is just acting as grout to fill the gap between the sign post and the hole.

Maine EIT, Civil/Structural.

 

[hokie66]

Yep, the concrete is just soil improvement in this system. If the soil is not good enough to provide the passive resistance required, you just make the hole bigger until it works. Sometimes a collar (concrete of larger diameter) is used close to the surface.

 

[woodman88]

Typically, I have found that poles embedded into concrete drilled piles without reinforcing is done for two reasons. 1) So the contractor does not have to hire people to correctly place the reinforcing. 2) So that some building departments can not require special inspection for the job.
They are also simple to design for moment of the pole to concrete and the concrete to soil. You just have to use enough concrete to make the numbers work. I have seen where engineers have welded (in shop) rebar to the poles to get the design to work easier.

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.