Design of Grade Beam Foundation without Drilled piers or spread footings underneath
Design of Grade Beam Foundation without Drilled piers or spread footings underneath
(OP)
I am designing grade beams sitting on a soil with 1500 psf allowable bearing capacity for total load. The beams are bearing directly on the soil and are NOT supported by drilled piers or spread footings. The total length of the beam runs 64'-8" LF. I have 34'-8" LF of the grade beam supporting a 13'-4" tall CMU wall, a trib. width of 10'-8" carrying DL =25 psf & LL=20 psf. I have attached a plan view of the foundation layout: http://files.engineering.com/getfile.aspx?folder=8... . Currently I am designing it as a simply supported beam, but I am getting that the beam is failing as a 12x24 grade beam with (3) #6 bars. I have been only been able to get it to work using 18x36 w/ (4) #8, 18x36 w/ (5) #7, and 12x36 w/(4) #7. How should I go about designing the grade beam? Do I design it as a simply supported beam? Any suggestions and/or comments are appreciated.






RE: Design of Grade Beam Foundation without Drilled piers or spread footings underneath
RE: Design of Grade Beam Foundation without Drilled piers or spread footings underneath
RE: Design of Grade Beam Foundation without Drilled piers or spread footings underneath
RE: Design of Grade Beam Foundation without Drilled piers or spread footings underneath
RE: Design of Grade Beam Foundation without Drilled piers or spread footings underneath
RE: Design of Grade Beam Foundation without Drilled piers or spread footings underneath
Dik
RE: Design of Grade Beam Foundation without Drilled piers or spread footings underneath
With the applied loading you have (roughly 1400 plf assuming partially grouted masonry) there's not much capacity left for a reasonably sized grade beam that's only 12" wide.
As for the reinforcing in the grade beam, in your case it's more like a footing than anything. You can put T&B bars and stirrups if you want. It makes sense to be sure it has some spanning capability in case there are any soft spots below areas.
RE: Design of Grade Beam Foundation without Drilled piers or spread footings underneath
RE: Design of Grade Beam Foundation without Drilled piers or spread footings underneath
RE: Design of Grade Beam Foundation without Drilled piers or spread footings underneath
RE: Design of Grade Beam Foundation without Drilled piers or spread footings underneath
IT"S FULLY SUPPORTED AT THE BASE! There's minimal moment or shear applied to the grade beam.
RE: Design of Grade Beam Foundation without Drilled piers or spread footings underneath
RE: Design of Grade Beam Foundation without Drilled piers or spread footings underneath
Design the reinforcing to span some extent of soft bearing material. How far you choose to design it to span is up to you, but something like 5-10ft is like more than adequate. If your grade beam has to span further than that over a soft spot, you're going to exceed your bearing capacity beyond the soft area.
RE: Design of Grade Beam Foundation without Drilled piers or spread footings underneath
RE: Design of Grade Beam Foundation without Drilled piers or spread footings underneath
RE: Design of Grade Beam Foundation without Drilled piers or spread footings underneath
RE: Design of Grade Beam Foundation without Drilled piers or spread footings underneath
RE: Design of Grade Beam Foundation without Drilled piers or spread footings underneath
RE: Design of Grade Beam Foundation without Drilled piers or spread footings underneath
RE: Design of Grade Beam Foundation without Drilled piers or spread footings underneath
All you really need are a few soil parameters and the climate zone to design a slab. If you follow their procedure and charts, it will give you a required steel area for your beams.
We use it at our firm for generator pads and other low-risk type slabs. You can design an entire slab in under 30 minutes with it after some practice.
Here is the design document: TF 700-R-07 (WRI/CRSI 81): Design of Slab-on-Ground Foundation
They have a design example in the back of their Design Manual that you can follow.
Google Wire Reinforcement Institute.org
RE: Design of Grade Beam Foundation without Drilled piers or spread footings underneath
RE: Design of Grade Beam Foundation without Drilled piers or spread footings underneath
oengineer - what if you dropped the notion of beam, and replaced it with 'ground lump of concrete'. Then you'll see that it is just in fact a lump of concrete sitting on the ground. Your only job then it to try make it not sink in to the ground too much so it doesn't ruin those 7 toilets sat right next to it. That won't end well for anyone!
For what it's worth I'd still prefer a wider footing. But those residential construction codes that spec ground beams are tried and true, and I think mainly they are used because they don't typically have geotechnical reports or competent people to compact the ground, so they span over 'soft spots'. Be careful though, the ones I've read have a typical scope of max 2 stories. If you're supporting a concrete slab and roof, you may have to come up with an 'engineered solution' as they say in the residential code.
RE: Design of Grade Beam Foundation without Drilled piers or spread footings underneath
Do you have any geotechnical consultant involved to properly determine the bearing capacity and possible settlement. If he's happy with it... loads are small and soil is good, then no issues.
Dik
RE: Design of Grade Beam Foundation without Drilled piers or spread footings underneath
You should find that the forces will be minimal.
RE: Design of Grade Beam Foundation without Drilled piers or spread footings underneath
Bearing on sound/native material over excavated if necessary to sound material and backfilled with LSM (or compacted fill only if compaction verified by that guy/geotech). Install minimum footing reinforcement (top and bottom) per code (ACI code)assuming you own one. Verify frost depth if applicable which may/may not get you to sound bearing.
If the soil is that bad or concerns you for some reason, you'd better get a geotech involved, they are nice people dispute what you might think and their questionable public reputation. Might have something to do with having dirty fingernails. Apologies, I digress, anyway, if the soil is that bad, better start thinking post tension slab with your 12" wide wall footing. Anyway, It's a wall footing, turned down, or independent, it's a wall footing uniformly support by soil/springs. Not a believer,....you can model it if you wish. You will be modeling a concrete sandwich. I suspect the only reinforcing requirement will be governed by material steel/concrete minimum reinforcement ratios. But practice will say reinforcing bars at top and bottom to cover the soft spots and negative bending. Depths and width's of some footings will exceed building code minimums with regards to minimum reinforcing required to be there, not material code minimums where min. steel/conc. ratios will govern. Building code minimums are not designs. Never have been. oengineer,...hope you've got a sense of humor,...I can get a little slap happy! Good luck! and,....."Wall footing". I'm hungry, going to lunch with a Geotech.