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Re: Eccentric Load on Drilled Pier

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Rich,

Single pier or double pier solution will be okay. As you have remarked, ignore the upper 4' to 5' of soil which is a compacted fill. If you go with the double pier option, use the same size for both piers so that the contractor does not have to work with two augers and bell formers. I hope the soil has enough cohesion for the bell to be formed. If not, you may have to use a pier without bell. Personally, I don't like bells, unless I am working with an experienced contractor.

I would use a 24" thick cap with the pier reinforcement tied in to it and, placing a seperation joint between the cap and the rest of the floor slab.

The recommended pier spacing is 3 times the shaft diameter or the bell diameter, whichever is larger. So, if you use the dual pier option, the cap will have to be long enough to give the appropriate spacing between the piers.

Rajendran


Rich Lewis <seaint04(--nospam--at)lewisengineering.com> wrote:
I?m designing a foundation for a pre-engineered metal building with 75 ton overhead crane.  The overhead crane has independent columns for vertical load.  The center to center spacing of the PEMB column and the crane column is about 5?-6?.  The crane load is 225 kips and the metal building load is 45 kips gravity and up to 46 kips wind uplift, with a load combination uplift of 36 kips.
 
I?m looking at 2 design solutions.  The first is a spread footing.  It works out to about 17?x13?x2?.  The contractor would like to look at a drilled pier option.  He thinks it may be cheaper.  I was initially looking at placing 2 drilled piers.  One directly under the crane column and one directly under the PEMB column.  The crane column would be 24?-30? dia. and have a 6.5 ft. dia. bell.  The other would be an 18? dia. And have a 3 ft. dia. bell.  This put the bell bases about 12?-18? apart at the bearing.
 
I am wondering if a single drilled pier would work.  If I used a single pier then it would have eccentric load and moment at the top.  I would place the pier near the crane column since it has the largest load.  I?m thinking maybe about 12? off center to the crane column, towards the PEMB column.  I?m hesitant about this because the top section of the pier may be in about 4-5 feet of compacted fill.  I don?t think I should count on compacted fill for much lateral load resistance caused by the moment.  I have a 12? thick concrete slab between the PEMB rigid frames to work with.  I can have a tension tie between the columns.  They tell me they will never cut the slab open for pits or trenches.  I?ve never designed a pier with an eccentric load like this before and I don?t have someone else to ask nearby if this is a good solution, so I?m putting this out to the list for some feedback.  Is it wise to have an eccentrically loaded drilled pier in the foundation?  And does having the top in a fill condition make this a dumb move?  My geotech engineer is not much help on this.
 
Thanks for your insight.
 
Rich
 


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