Letral Load on Drilled Pier and Grade Beam Foundation

[00Z]

I need to design a drilled pier and grade beam foundation for a 1,000sf single family home. I have a soils report that provides the allowable skin friction which I will use to determine the minimum depth to support vertical loads (toe bearing is being ignored). To determine the minimum depth required to resist lateral loads I would like to use the constrained formulas provided for the design of embedded posts found in IBC 1807.3. My question is...is this an acceptable method of determining lateral load capacity for this type of foundation? Has anyone used this method for this type of foundation.

 

[Teguci]

http://www.fhwa.dot.gov/engineering/geotech/foundations/nhi10016/nhi10016.pdF

I've used L-pile, a Geotech analysis, ACI has a manual on drilled piers which discusses lateral loading designs and chapter 12 of the enclosed publication. I recommend using the enclosed simply because it is free and appropriate for drilled piers under lateral loads.

Closely spaced piers will shadow each other and a pier in the shadow of another will not resist as much lateral load. Also, for short piers, you get a rigid rotation of the pier where long piers go into a sinusoidal bending pattern.

If you want to keep it simple then you could probably get away with just using the embedded post calc but have a read through of the enclosed just to check some of your assumptions.

 

[KootK]

I'll probably incur some criticism for saying this out loud. However, this is fundamentally safe space for telling the truth so that's what I'll do. Engineers in my area, including me, wouldn't normally bother with checking lateral pile effects on any structure that would fall into the "house" category. Instead, we make up a somewhat plausible story about soil friction on the sides of the grade beams and/or slab on grade friction getting the job done. Most small project geotechnical reports around here won't even include the required information for lateral pile design.

That said, I'd have no problems with the IBC method.

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.

 

[Stenbrook]

I have used design criteria for embedment of piers laid out by Czerniak. It is a pretty straight forward approach. It does involve making an estimation as to your soil strength though. I've attached a handy chart.

 

[azcats]

If you do use the IBC formula, I'm not sure the constrained method is the way to go. That method assumes you have something resisting the force at grade and the pier rotates about that point and that's not really the case in your scenario. If I had soil data available, I'd run an Lpile analysis.

I don't do pier & grade beam design - do you assume fixity at the top of pier or is it a pin?

 

[00Z]

Thanks you all for the responses.

Teguci thanks for the FHWA reference I will check it out.

KootK I could see how some experienced designers might just use there engineering judgement and be able to assume the drilled piers are adequate for the lateral loads on a small home like this. Unfortunately the last time I designed piles was for a bridge 15 years ago so I just don't have the experience with this type of foundation to do that. I do expect the loads to be relatively low so I was hoping to find a simplified design method.

Stenbrook thanks for the reference I will check it out. It looks like that may be the simplified method I'm looking for.

Ascats I don't usually do pier design either, that's the problem. I dont have Lpile so I'm going to have to do some hand calc's. I planned on using the constrained method to account for the fact that the piers are all tied together by a concrete grade beam and I would assume they are fixed at that point. My thinking was that designing them individually as apposed to as a group would be conservative. Of course the pile spacing would play a part in this. I planned on starting my design with a spacing controlled by the maximum concrete beam span I could get away with without stirrups.

 

[BigH]

Where are your lateral loads coming from? Seismic? Wind?

I've seen hundreds of geotechnical reports for structures - homes, low commercial buildings, etc. - and seldom did any of the reports ever discuss lateral pressures - and the ones that did, talked about the grade beam/strip footing's lateral resistance.

A 1000 ft2 home - why are you using drilled piers? Unless your soil is really poor - or your frost penetration is really high, why would you not just use strip footings? If you are using strip footings (or your grade beam) - you will have passive pressure resistance to lateral loads - the beam/footings would be buried in the soil for frost protection or seasonal moisture change - and they have a width and thickness that would develop passive pressure. A single story 1000 ft2 home is not a bridge.

 

[00Z]

BigH- the lateral loads will be coming from Wind and Seismic loads. The site has expansive clay soil and that is the reason the soils report is calling for drilled piers.

Stenbrook- do you know the name of the book that Czerniak figure came out of?

 

[BigH]

Then the drilled piers are there solely for getting the foundation below the zone of potential expansion/contraction. Still, the grade beam should be able, I would think, to handled the limited lateral pressures induced by wind and seismic. The embedment of the piers would help and would be counted on but I doubt that it be necessary to actually compute the lateral capacity of the piers. Unless this is a very unique structure, it is still a house. The only time I've seen houses really be considered would be if the soil liquefies . . .

Check out for excel foundation programme - it has Czerniak's analysis. I've not used it, though.

http://www.calculatoredge.com/structural/polefdn.htm

 

[FSS]

Just ask the geotech for the number. Should take them all of about 5 minutes to answer since they did the analysis to come up with vertical loads already.