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Re: Eccentric Load on Drilled Pier

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It shouldn't be any problem to design the single pier for the moment if you have a reliable way to counter the lateral thrust at the top of the pier(s) for all the load combinations.
 
Do you have the option of utilizing a sub-slab continuous tie to provide lateral restraint at the top of pier?  That would be better than using some sort of tension/compression tie to the perimeter foundation wall to provide lateral restraint at the top of pier. 
 
It sounds like a tie between the piers of opposing crane columns (corresponding to the span of the crane) would be in compression due to the effects of the crane column loads acting on the combined pier, acting to tend to reduce the outward acting thrust of the building columns if the building frame is clear span.   Maybe solving the lateral thrust problem at the top of the combined pier is no worse than solving the problem of the outward ward acting thrust that would act on the building column piers if they were alone?.
 
Mark D. Anderson
Anchorage
----- Original Message -----
From: Rich Lewis
Sent: Friday, February 23, 2007 5:46 AM
Subject: Eccentric Load on Drilled Pier

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