Grade Beam on Drilled Shaft (caisson) detail

[lutein]

I am designing a grade beam-drilled shaft foundation system for a 4 story condo, and was wondering how would the foundation details look like for this type of system. If you could shed some lights into this, i would appreciate it.

Detail 1. 18"x24" grade beam spanning between drilled shafts and poured continuously over drilled shaft. I guess the dowels, matching column verticals, will extend beyond the grade beam and lap with column verticals. certainly, the grade beam will be formed to match column size over drilled shaft. Is this a standard detail based on your experience?

Detail 2. Typical detail at slab on grade edges. We were being told to have grade beam continuously around perimeter of slab on grade. basically to prevent settlement at slab on grade edges. I imagine the grade beam is pretty much just supporting it's own weight, probably minimum reinforcing 0.0033 governs. Does this sound correct?

Again - Thank you in advance.

 

[TXStructural]

Grade beams usually carry brick veneers, exterior walls, and slabs. It is a beam spanning between piers and is not "resting on soil." Reinforce it symmetrically top and bottom and do not vary the reinforcement even if spans are different. It is not worth the hassle to complicate these details. Minimums may control for closely spaced piers, but that is your call based on loads and forces. I was instructed by my first principal to increase size until I did not need more than minimum stirrups (at 24" o.c.), otherwise you end up with irregularly laid out stirrups at 24", D/2, or closer. Again, this isn't really a problem for people who do it every day, but may be unnecessary work on your part to schedule something that saves no money in construction.

Always try to make beams and columns different widths so that corner bars in beams and columns are not in the same plane. A 3 or 4 or 6 inch difference (depending on sizes) should allow the placers to work around conflicts and stay within tolerances. Form beams wider than columns. On some buildings, the architect wants the exterior face of exterior columns and beams in the same plane - in this case, consider how the reinforcement will run and specify how you want it in your details.

 

[dik]

The beam doesn't have to be as wide as the caissons/piles... You can bend the caisson reinforcing into the grade beam or use dowels. I'll see if I can dig up some drawings.

With an 18" wide grade beam you can put lots of reinforcing in it. You might want to carry 2-20M (#6) Cont Bot (or whatever) and extra rfg between piles as required. You might want to carry 2-15M (#5) Cont Top plus added rfg over the piles as required.

In some jurisdictions, the beam width may have a bearing on the spacing of stirrups and a more narrow beam may allow greater stirrup spacing if adequate.

Dik

 

[TLHS]

If you're in an area where you have expansive soils or you're not below the frost depth, make sure you put a void form or something similar beneath your beam. It'll prevent some exciting jacking situations you'll want to avoid. I generally install void form in all cases, just so that I get a nice load path, but that's more of a judgment call. I've seen it done both ways.