Augercast Piles & IBC 2012 Seismic Detailing Requirements

[Tankie]

Greetings,

First of all, a disclaimer: not much experience with deep foundations, I'm more of a design-above-the-foundation type of engineer. But on several recent projects where augercast piles have been used, my colleagues and I have been at odds over the rebar cage length needed to satisfy IBC 2012 Section 1810.3.9.4.1. Would appreciate any feedback you can offer based on your own experience and/or knowledge of the IBC Code.

The structures are assigned to IBC Seismic Design Category C. The augercast piles are generally 50 to 70 feet long, 16 inch diameter, carry a large vertical compressive load (80 tons to 100 tons), have zero uplift, and a lateral load of around 4,000 lbs per pile. Sometimes wind shear controls the lateral loading, sometimes it's seismic shear that is greater.

The augercast piles are designed for lateral loads using LPILE, and more often than not, the bending moment in the pile goes to zero within 10 feet (+/-) of the pile top.

Here's where the difference of opinion begins. The foundation/pile designer will design the longitudinal bars, and select the length of the longitudinal bars, on the basis of the LPILE analysis, with no regard for the minimum length requirements of IBC Section 1810.3.9.4.1, which would require that the rebar extend a minimum of one-third of the pile length (that's the controlling criteria from this section of IBC).

I'm wearing the black hat and saying, essentially, that it doesn't matter one bit if LPILE says that the rebar can be terminated 10 feet (or 12 feet, or whatever) from the pile top. If the augercast pile is 60 feet long, and you're in SDC C, the rebar needs to be at least 60 / 3 = 20 feet long. Period. There's no code provision in the IBC that I can find that permits deviation from this requirement.

So, my question to you experienced foundation designers -- do you run into this same issue, and if so, what do you do? Follow the IBC and extend the rebar to the minimum depth specified in Section 1810.3.9.4.1? Or is there some clause or wording squirreled away somewhere in IBC that allows deviation from the minimum rebar length requirement?

Thanks for any feedback you can provide.

 

[jayrod12]

I am not overly familiar with the IBC nor with seismic requirements however, the code is not the law as I have understood it. The code is considered guidelines that if followed, require no additional evidence to prove that the design works. You as an engineer are permitted to deviate from the code at your discretion however you must be prepared to provide evidence that your deviation will not result in failure.

But that is my $0.02

 

[WinelandV]

Hey Tankie,

I'm reading Section 104.11 of the 2012 IBC. It seems to indicate that deviations from the IBC are possible, but they must be approved by the building official. This may or may not help as you're at the mercy of the building official.

Jayrod, it's my understanding that the IBC is the law if the local jurisdiction has adopted it as such. Of course, you also get to try and figure out if they adopted it wholly, or if they amended it/excluded certain portions, etc.

I will be the first to point out that I am not an expert in the IBC, however, so please take everything with a grain of salt.

 

[jayrod12]

As you state in your first sentence. Deviations are allowed at the discretion of the building official. If you can provide enough evidence and have a building official with some common sense (not likely) then you should be able to persuade them into letting you do whatever it is you are proposing.

As I said, you need evidence to make your case if you are going to deviate from the code.

 

[Tankie]

I appreciate the feedback, guys. The IBC is the in-effect building code in the project location, so we are bound to follow it.

Thanks for pointing me at Section 104.11, I suspected that something like that lurked somewhere within IBC. I suspect that being at the mercy of the local building official to approve deviations from the code, especially as they affect seismic detailing requirements, would not be an easy or quick process.

 

[jayrod12]

Try asking them. You'd be surprised sometimes what they're willing to accept with proper documentation.

Prove to them that there is zero moment below 10 feet (for example) then provide some buffer like extending the rebar 3 feet past the point of zero moment to account for some inflection and see what they say.

I've been pleasantly surprised albeit not as often as I have been disappointed at their reluctance to even consider looking outside the code.