Dead Loads to be included in Drilled Pier design for Slab-on-Grade Foundation
Dead Loads to be included in Drilled Pier design for Slab-on-Grade Foundation
(OP)
When designing a Bell & Shaft drilled pier for a warehouse with a mezzanine floor, is it too conservative to include the weight of the grade beam & pier caps in the dead load for the pier design? Is it enough to just include Roof & mezzanine floor dead load? My thoughts are that even though the grade beams is resting on the soil, the grade beam needs to be designed as spanning from pile to pile. Since the grade beam is spanning from pier to pier, the pier needs to be designed to include the weight of the grade beam and cap. The entire span of the grade beam is not included, just the tributary length of the grade beam effecting the pier. Any suggestions/ comments are appreciated.





RE: Dead Loads to be included in Drilled Pier design for Slab-on-Grade Foundation
If soil concerns are settlement and piers thereby necessary - the grade beams must eventually span all the way from pier to pier should the soils move down.
If soil concerns are expansive clay pushing upward, then the grade beams should have permanent voids constructed beneath and they then span all the way from pier to pier as well.
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RE: Dead Loads to be included in Drilled Pier design for Slab-on-Grade Foundation
RE: Dead Loads to be included in Drilled Pier design for Slab-on-Grade Foundation
Also - any load imparted onto the grade beam needs to be included also.
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RE: Dead Loads to be included in Drilled Pier design for Slab-on-Grade Foundation
RE: Dead Loads to be included in Drilled Pier design for Slab-on-Grade Foundation
In some cases for deep (30 ft. to 40 ft.) piers with bells we would work with something > 10,000 psf.
Otherwise I'm not sure why the numbers are so low. It may be that the original designer did not include the grade beam weight in the design, which probably isn't correct.
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RE: Dead Loads to be included in Drilled Pier design for Slab-on-Grade Foundation
So would excluding the dead grade beam weight cause structural/ foundation failure? Or does the safety factor provided in the allowable soil bearing pressure given by the geotech engineer take care of any failure?
RE: Dead Loads to be included in Drilled Pier design for Slab-on-Grade Foundation
I do agree with removing the soil weight of for the displaced section of the piers and caps although would the displaced weight of the soil where grade beams are poured be subtracted? Since the grade beam weight would only be imposed on the piles when the soil settles I don't believe the weight of the soil displaced by the grade beam should be accounted for since as the soil settles the entire weight of grade beam would be transferred to the pile.
Could another option be to model grade beam and pile and to assume the soil under the grade beam to be a spring. With the expected settlement of belled piles included for the calculations of the spring constant, this value may be easily obtained from the geotech on the project. I would suspect in many cases that the grade beam not be fully transferred to the piles and the settlement of the native soil under the grade beams be fairly low in comparison to the settlement of the piles themselves, although this would depend on exact site soil conditions.
I further agree with JAE that 3200 psf is pretty low for end bearing piles and there may poor soil conditions. If this is the case the gradebeam will need to accounted for.
RE: Dead Loads to be included in Drilled Pier design for Slab-on-Grade Foundation
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RE: Dead Loads to be included in Drilled Pier design for Slab-on-Grade Foundation
Houston has some pretty weak clays, so the 3200 psf may be appropriate.
The clays are often expansive, and that is the first consideration. If the soil may heave under the grade beams, void forms are necessary, and as JAE said, the grade beam weight will go to the piers.
If this warehouse will have live loads of several hundred psf on the slab-on-grade floor, the soil could settle out from under the grade beams. Again, the beam weight would go to the piers.
For a lightly loaded building, like a school, on non-expansive soil, with estimated pier settlements small, I might assume the grade beams act as their own footings. They need to be constructed like footings, on a trimmed, undisturbed bearing surface.
RE: Dead Loads to be included in Drilled Pier design for Slab-on-Grade Foundation
RE: Dead Loads to be included in Drilled Pier design for Slab-on-Grade Foundation
RE: Dead Loads to be included in Drilled Pier design for Slab-on-Grade Foundation
RE: Dead Loads to be included in Drilled Pier design for Slab-on-Grade Foundation
A few things about this building seem odd:
22 kips design column load seems pretty light for a roof plus mezzanine.
3200 psf is on the order of a footing bearing pressure. Perhaps the designer is using piers to get below the depth of seasonal shrink/swell, not to get to stronger soil. Again, if shrink/swell is an issue, perhaps void forms under the beams are needed. I would also look at the floor design; is it allowed to float or connected to the grade beams?
Are the walls supported on the grade beams?
RE: Dead Loads to be included in Drilled Pier design for Slab-on-Grade Foundation
RE: Dead Loads to be included in Drilled Pier design for Slab-on-Grade Foundation
If you have the geotech report, does it mention potential shrink/swell of the soil?