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Re: Eccentric Load on Drilled Pier

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Rich,
 
        I don't much like large moments on single pier foundations, particularly for cranes, because it often requires fairly large pier deflections to mobilize the moment resisting capacity of the soil.  I think it's almost mandatory to tie the pier top to the building slab to prevent lateral movement of the pier otherwise you may have difficulty maintaining the alignment of the crane runway beams.
 
        Another thing to remember is that the tolerance for pile location is about 6" and it's not uncommon to have them more than 6" out of position.  You should probably increase the design moment to allow for this possibility.
 
Regards,
 
H. Daryl Richardson
----- Original Message -----
From: Rich Lewis
Sent: Friday, February 23, 2007 7:46 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