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Re: Eccentric Load on Drilled Pier
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Eccentric Load on Drilled Pier
- From: "Mark D. Anderson PE" <mark(--nospam--at)alaskaengineer.com>
- Date: Fri, 23 Feb 2007 06:31:18 -0900
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
----- Original Message -----
Sent: Friday, February 23, 2007 5:46
Subject: Eccentric Load on Drilled
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
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
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