Tension Pile Reinforcing

[RonRoberts]

Has anyone addressed the issue of a drilled concrete shaft, highly loaded in tension (tower crane leg uplift) and the type of reinforcing laps required under ACI 318?

Specifically, should a 30 inch diameter drilled shaft, loaded to 1200 kips tension be considered a tension tie, thus requiring mechanical couplers rather than lap splices under ACI 318 Section 12.15.6?

Thanks for the help

 

[JAE]

I designed some tension piers for a series of large hold downs for a stay-cable supported roof for a stadium project using belled piers with post-tensioning.

I didn't trust the idea of mild steel for a couple of reasons. One was the required depth of the piers - going down about 80 feet - required a number of lap splices and this made me nervous. I know that a properly designed lap splice theoretically carries the same tensile capacity of the bar itself. The issue was that I just felt better with a redundant system in place in addition to the mild steel.

Second reason was in high tension I felt that the piers would be cracked and allow moisture into the section, possibly deteriorating the mild steel over time - even with epoxy coating.

So we used full depth tendons in the piers, prestressed them such that the concrete would not go into tension.

 

[RonRoberts]

Thanks JAE,

Our situation is a little different, we have an intermitently high tension load of a somewhere between a few minutes and an hour duration probably occuring many times a week over the next 3 years. Corrosion won't be a factor but the presence of a bundled bar lap splice of 100% of the bars, immediately below the pile cap has me concerned.

I found a paper in ACI Structural Journal Nov/Dec 2005 that says that though it's not permitted under ACI 318, it has been done safely (with some constraints such as staggering the splices, and providing quite a bit of transverse reinf.)

I'm search for someone that has found any other research or has experience with that particular issue of treating a tension pile as a tension tie member.

 

[structuresguy]

Hmm, interesting discussion. We typically don't require anything beyond a lap. But generally don't have very significant tension piles. Though I am just finishing a parking garage project, where some piles have fairly significant tension, up to about 40 kips or so.

 

[swivel63]

just bars that go all the way down. with laps of course.