Friction Force of Pipe embedded into concrete ? 2

[StructuralEngineerTX]

Hello all,

I'm designing a 12ft tall sign. The sign company says the steel Pipe column of the sign actually embeds into the concrete pier all the way down to the bottom of the pier (3" clear from bottom). I'm concerned about the Bond between the steel pipe and the concrete since this would feel like it would be almost the same as putting smooth rebar and having no friction.

But, I just "feel" the skin friction between the steel pipe and the concrete pier is OK to hold gravity and uplift loads if I'm going down 10ft, I just don't know how to prove it and now the sign company is getting impatient because they've been doing it like this for 30 yrs.... I just can't prove them right or wrong.

Any thoughts?

Thank you in advance!

 

[BAretired]

You do not need friction between pipe and concrete. The moment can be resisted by horizontal forces between the pipe and the concrete, one near the top, the other near the bottom. Where is the problem?

BA

 

[jwilki]

How big is the pier diameter relative to the steel hollow section? Can it be said that the concrete around the steel helps prevent corrosion in the steel, but effectively it does nothing to support/stabilise the sign? The steel will be taking the moment at ground level and below regardless if the steel slips relative to the outside concrete of the pier. As long as the CHS takes the bending forces and the pier is deep enough to resolve the forces into the ground then it should be fine.

 

[StructuralEngineerTX]

Appreciate the comments.

I'm not concerned with the moments. The real problem is the weight of the sign. It has masonry on both sides. It's pretty heavy. I don't want the weight to shear through the bottom of the pipe because there's no base plate (the bottom of the pipe is hollow so that concrete flows inside and around the pipe). So, I'm relying on the friction between the steel and the concrete (if you want to be exact about it, then there is some bearing of the pipe wall thickness in the concrete, but it would shear through without a base plate or without relying of some bonding between the walls of the pipe and the surrounding concrete).

The Pipe is 12" dia. Schedule 40, while the pier is 30" dia.

 

[Ron]

There are two components to consider. One is contact friction if there is no bond between the concrete and the steel, just contact. This is the same as any steel to concrete interface. The second is a chemical bond that exists between the concrete and steel. You might lose this last one right at the top of the pier because of concrete shrinkage, but shouldn't be more than a foot or so of loss.

Considering that, I usually assume about 25 psi bond strength between the concrete and the steel. You have a lot of bond area, so that is likely sufficient.

 

[StructuralEngineerTX]

Thanks Ron, how did you come up with the 25 psi? If I can get 25 psi then with all that surface area I would have plenty of capacity.

 

[Ron]

SETX...from load tests on aluminum tubes embedded in concrete. Tubes were isolated from concrete by layer of paint so the bond was to paint. Bond to steel would be at least as good. Our results were quite a bit higher than 25 psi, but for conservative value, that's what I use.

 

[BAretired]

Going back to ACI 318-63, the allowable bond stress for plain bars was one half that of deformed bars but not more than 160 psi. I'm not sure that the bond stress of a pipe would be the same as that of a small bar, however.

For deformed tension bars, the allowable bond stress was 4.8√f'c/D. For f'c = 2500 and D = 12, this would be 20 psi. Half of that would be 10 psi which is a bit more conservative than Ron's value.

A sudden cooling down in temperature would tend to shrink the pipe faster than the concrete, so it would seem to be good practice to provide a few studs welded to the pipe to ensure mechanical bond.

BA

 

[Ron]

BAretired...excellent suggestion on the studs. Easy way to solve the problem.

 

[azcats]

I've used short pieces of pipe or TS welded to the pipe close to grade in lieu of studs. They're less apt to get bent in shipping and a sign fabricator probably has drops laying around.

 

[TLHS]

I've meant to track down this paper and some of the references for a while, because I feel it would be useful for the design of pipe piles, but still haven't gotten around to it.

http://www.isope.org/publications/proceedings/ISOPE/ISOPE1999/papers/I99v4p024.pdf