Uplift Capacity of Plain Concrete Pier

[zurch1818]

I'm currently in the process of analyzing a concrete foundation of a multi-leg water tower that was designed in the 1936. The plans have survived time and show a typical pad and pier foundation. The one thing that is odd to me is they call out rebar in the bottom pad, but all they show in the pier is the two anchor rods. I believe the pier has is plain and has no rebar in it. I'm hoping that they poured the pad and pier at the same time to prevent a cold joint, but I really don't know the order of how it was actually formed. As seen in the photo, the foundation has already showed some significant cracking in ways that make me believe there isn't any steel besides the anchor rod in the foundation.

When I turn to section 22.5.3 of ACI 318-08, it gives a way to calculate the bending capacity of plain concrete. However, with the loads I have, I don't really have much bending. It is concrete that has to support pure tension which seems weird to use the combined bending and axial compression equation. Does it still make sense to keep my stress under the the 5*sqrt(f'c) value or do I just have to fail the foundation because it doesn't have minimum temp/shrinkage steel in it?

Thanks for your opinion.

 

[KootK]

Does it still make sense to keep my stress under the the 5*sqrt(f'c)

I think that it does as the value quoted is essentially the reliable modulus of tension ruptutre. I'd multiply that stress by the area of the pier to estimate the pier tension capacity. Of course, whether or not I would actually want to rely on concrete tension capacity is another matter altogether. I'm not sure that I would.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ron]

Have you checked overturning with the small piers? If you don't have that, the other issues are somewhat moot.

As small as those piers are, you could drill and epoxy rods in any direction you need.

 

[hokie66]

Your photo doesn't show the anchor rods. If the anchor rods go down and are developed into the footing, and the footing is designed for overturning, it may not be so important that the plinths themselves are reinforced. If the cracking is an issue, you could perhaps wrap the plinths with fibreglass, and repair the cracks.

 

[zurch1818]

I did the calculations and it appears I am under the 5*sqrt(f'c). I'm not sure I want to rely on that capacity but it could explain why the foundation has lasted as long as it has. Most of the time this foundation is in compression because the tank is not empty and therefore in compression.

There are 8 piers that were designed to act in pure uplift/compression, so there isn't a whole lot of overturning. I only showed a picture of one of the piers. Also I didn't even realize that the anchor rods aren't shown in that picture. There are two smaller plates on each side of the leg that each has one anchor rod in it.

 

[KootK]

How far down the the anchor bolts extend? Similar to Hokie's comment, you ought to be able to engage at least all of the concrete above the bolts.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[zurch1818]

It makes sense what hokie and Koot are saying. However, I have no idea how far the anchor rods extend down. The plans do not call it out. The one thing that is called out is a hooked anchor rod of an unknown hook length. Given my experience with appendix D, these don't usually perform that well either. Does a f'c of 3000psi make sense for concrete in 1936? Is there a good source that shows historical f'cs that were specified over time...ie 1900's all the way to now?

I have finalized my numbers and have determined that the tower is at 120% and the anchor rods are at 115% in stress and 190% in pullout (pending what f'c to use). The anchorage was designed using the full dead load to resist uplift. The .6DL load case is killing the anchorage. Given the age of the structure, I'm going to fail the structure and foundation and call it a day. It just makes me too nervous.

 

[Wallache]

On the other hand, 80 years of service is a decent indicator that the design was adequate. I doubt there is a historical structure out there that will stand up to the latest codes.

 

[KootK]

I think that it does as the value quoted is essentially the reliable modulus of tension ruptutre.

I need to retract this statement. I was reading through my new, Australian concrete book this morning and it seems that the tensile stress available for pure tension applications is about 0.6 x the allowable flexural tension stress (value from ACI I believe). Apparently, the strain gradient in a flexural thing makes a significant difference. At this point, I'm not sure what the ACI value should be for direct tension but I'm not confident that it should be 5xSQRT(f'c).

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.