Shear Strength of a Concrete Drilled Pier

[dcarr82775]

Anyone have a reference that discusses the shear strength of a straight shaft reinforced concrete drilled pier (caisson)? ACI-318 specifically excludes them, but I have always used it anyway. Anyone know of another source?

Thanks

 

[Splitrings]

ACI 336 "Design and Construction of Drilled Piers". It has equations and nomographs for laterally loaded piers.

 

[Lion06]

That's interesting that ACI 318 specifcally excludes them. That's what I use. If you're referring to 1.1.5 - I read that as ACI 318 not legally governing teh design of caissons, not that the provisions of ACI 318 don't apply to caissons. It says that (except for regions of high seismic risk) caissons embedded in ground are regulated by the general building code (which requires some % of steel in the top 1/3 of the caisson, I believe - going from memory).

I believe that, especially for caissons supporting lateral resisting elements and where shear and moment diagrams are provided by a geotech, ACI 318 is perfectly valid to use for the design of caissons.

I use ACI 318 to design caissons for shear and bending.

 

[Splitrings]

ACI 336 Section 3.3 Strength Design of Piers states, "....may be designed using reinforced concrete in accordance with the provisions in ACI 318...sections 7.10, 10.2, 10.3, 10.8.4, 10.9, and 10.15..."

 

[Splitrings]

ACI 336 Section 3.3 Strength Design of Piers states, "Design of plain concrete piers is governed by the provisions in ACI 318.1."

 

[dcarr82775]

My problem is the piers are in the ground and were designed with no shear reinforcing (ties less than minimum shear steel), yet have large horizontal shears that exceed 0.5*2sqrt(F'c) limit in ACI 318 Chapter 11 for beams. I am trying to find a way around the 0.5 requirement for beams so I can use the full 2*sqrt(F'c) or some other manner of getting higher shear capacities. I have already included the axial loads but they are not high enough to get me where I need to get.

 

[Lion06]

Is there no shear reinforcing at all or just less than ACI's minimum?

How close are you? Would a slightly higher f'c get you where you need to be? Do you have access to the break information for the caissons so you can see if the strength came in higher than spec'd?

 

[dcarr82775]

The ties might as well be nothing at all, too small and too far apart to realistically count for anything.

If I take everything into account that can help me I am around 2*sqrt(F'c) so bumping up the F'c even another 1ksi isn't helping enough.

 

[Den32]

What about Shear Friction if there is longitudinal reinforcing?

 

[JAE]

Den32 - I don't think that would work since the longitudinal reinforcement would have to be fully developed past the shear plane to allow shear friction to work. The shear plane, in this case, can be 3 inches below the top of the pier.

 

[Lion06]

I agree with JAE regarding the shear friction not being appropriate here.

I would even go a step further and say that even if the bar were able to be fully developed in that 3" that shear friction doesn't apply to an inclined shear crack. Shear friction assumes a shear plane at some assumed crack location. An inclined shear crack is not a shear plane, it's a principle tensile stress plane.

It's the same reason you can't use longitudinal beam reinforcement at the ends for shear friction.

 

[PMR06]

This might be more of a thought experiment than a practical solution, but is it possible to core thru the center of the pile, install a prestressing bar (grout the bottom, top unbonded), and PT the entire pile so you get to use the prestressed shear values?

 

[dcarr82775]

PMR06, that is what we plan on doing, but am trying to find a legitimate engineering way around having to do it.

 

[ishvaaag]

If the piles end 1 or 2 levels under street level you may try to pass the horizontal forces to passive push in the corresponding wall. A thorough reexamination of model and path of loads might be required.

If just at ground level you can still count slippage against the friction of the soil if a mat on the piles, or some passive push against the pile caps, if embedded ... of course if appropriate and permissible, that might not be.

You may also try to pass the shear as pure friction, i.e., a coefficient of friction to be developed between pile cap and pile. That of course would be forfeiting entirely the customary practice and usual intent and detail in RC construction, but the mechanism could be considered just to have some other appraisal of what available. Then it might be considered, would the piles stand the applied friction forces? A dynamic coefficient of friction would need to be considered if from earthquake. Everything here adventurous since not really a friction mechanism present, but of shear, and no clear real surface available for the friction.

 

[ishvaaag]

Of course if the same shear needs to be passed even through a conceptual mechanism of friction, you are in the same problem for the laterally loaded piles. Only the cases that alleviate the applied shear should be useful to then see as valid a lower available shear strength.

 

[ishvaaag]

An even when passive soil stands to restrain the lateral forces the possibility of the earth being naturally or artificially removed needs to be considered.