Drilled Pier Foundations EQN 18-1

[CBlaker114]

Hello all,

I'm a structural engineer in North Dakota and I'm designing drilled piers that will support softball scoreboards. I'm running into an issue when I'm trying to call out the depth of my drilled piers using EQN 18-1 from section 1807.3.2.1 in the 2012 IBC. The geotechnical engineer has dictated that the first 5 feet from grade be ignored for passive resistance. Equation 18-1 does not seem like it allows you to specify an "ignored" depth.

My first thought was to increase my "height" of my scoreboard so that I effectively treat the -5ft elevation as grade level and whatever result I get from EQN 18-1, I just add 5 ft. However this seems really conservative to me. I get ignoring the first 5 feet for passive bearing, but this approach negates any vertical effective stress above -5 feet, effectively shifting my bearing stress triangle down 5 feet.

I feel I should be able to use the original allowable passive pressure stress triangle, but I would just truncate the part of the triangle above -5ft. Does anyone know of a method to account for this ignored depth using EQN 18-1 or any other approach?

Thanks,

 

[dcarr82775]

Your idea of ignoring the top 5ft of passive resistance, but accounting for the benefits of that 5ft is reasonable. I have never used the IBC equation. Many texts have solutions to cantilever piles/piling depending on cohesive or non-cohesive soils. These can easily be adjusted to ignore the 5ft at the top.

 

[JedClampett]

Why don't you call the Geotechnical Engineer and see if he meant that for that application?

 

[msquared48]

Calculate the OTM and resisting moment by hand until they equate using the prismatic shape determined by knocking off the top five feet of soil, and taking the applied soil pressure to the depth of pile assumed.

But first, do as Jed says.

Mike McCann, PE, SE (WA)

 

[msquared48]

Oh, and you will have to factor the OTM for the factor of safety desired.

Mike McCann, PE, SE (WA)

 

[DaveAtkins]

I think your geotech is being terribly conservative. Light pole bases are done all the time, which don't even extend 5' into the ground. Also bases for chain link fences.

DaveAtkins

 

[bridgebuster]

I typically ignore the top 1 foot, sometimes 2 but never 5

 

[CBlaker114]

Thanks for the thoughts all.

I'd contacted this geotech specifically about the allowable passive pressure for designing these piers, so yes that recommendation was for this specific application

msquared48- I'd previous designed my piers using the method you suggest which makes me feel better about my logic, so thanks.

Our frost depth here in ND is 5 ft, so I kind of understand where the geotech is coming from with a slightly larger ignored depth, but I agree that 5 ft. seems extremely conservative. Anyway, I'm going to get another opinion about the depth I need to ignore from another geotech and see if I can't reduce my required depth a little.

Thanks all,

 

[KootK]

Just north of the border near you, 2100 mm is a very common frost depth for exterior applications. The 5' business doesn't surprise me at all. For lateral pile resistance, I think that your proposed method is quite reasonable..

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.

 

[dcarr82775]

If you want to keep it simple (case 1 of attached), you can sum your moments about A assuming the passive on one side only including a factor of safety. Then you multiply that depth of embedment by anywhere from 1.3 to 2 depending on the soil conditions to 'account' for the reversal. The passive pressure distribution in case 2 or more correct and involves more algebra and iterations to solve.