Design of Grade Beam Foundation without Drilled piers or spread footings underneath

[oengineer]

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/getfil...-36b8-4e8e-930f-5e9c0001e891&file=S4.03-4.pdf

. 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.

 

[JoelTXCive]

The Wire Reinforcement Institute has an free empirically based design document for slabs on grade. It was originally published in 1981 and then updated in mid 90's.

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

 

[oengineer]

@JoelTXCive - Thank you, I appreciate the information.

 

[bowlingdanish]

This thread is a crime scene

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.

 

[dik]

@OP:
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

 

[Stenbrook]

The way to determine the moment in the beam is quite simple. You can set up the beam as a simple span beam, with a support at each end. You then apply the loads and assume that there is a uniform load acting in the opposite direction (this is the support from the soil). You then solve for this uniform load so that the reactions at the end of the beam are equal to 0. Once you know what the uniform load is, you can draw a shear and moment diagram from the forces on the beam and then design the beam for the maximum forces.

You should find that the forces will be minimal.

 

[structuralsteelhead]

Sorry, Nope. Wall/strip footing. Check allowable bearing/width of footing. Use code minimums allowable for your site soils if you are qualified to know this and you have no geotech on board. Call the building department for advice on site soils if no geotech. Is it a commercial job? If so, why no geotech? Was it a prepared pad by for a development, there must be something you can hang your assumptions on.

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.