designing slab on grade monolithic foundations 1

[ssuper]

I need information on the guidelines of designing residential foundations in regards to min. size for footings and, reinforcing steel. It seems the home designers just go about drawing what they think is right from what they seen but no one really seems to have any answers as far as how they came up with the design.Its a common practice here in south Texas to design homes with 12"x24" exterior footings and 12"x18" interior footings reenforced with 4 continous #5 rebars (2 on top 2 on bottom)with #3 stirrups at every 24" O.C. and a 3 1/2" slab with either 6x6 #6 wire mesh or #3 rebar at 18" apart each way. I'm not familiar with ACI 318, so I also want to know if this is common pratice around in the rest of the U.S or are we doing more than the bare minimuns just because thats the way its been done for years.

 

[JAE]

ssuper -
South Texas foundations, like the one you describe, are typically done in southern climates (for houses with no basements) and especially they are done in areas where expansive clays are present.

The idea is to create a stiffened slab-on-grade system...sort of like a waffle slab (two way ribbed slab) system on the ground only with the waffles spaced out between 10 and 16 feet on center.

I do have a design methodology that is at my office...I'll see if I can dig up the reference. I believe it was prepared some years ago for the housing industry and may have been updated recently.

But I know that this method was developed and sort of cross checked against common practice...what you refer to as home designers doing what they think is best. Much of it is just a trial and error approach. I used to live in San Antonio and my own house had 12" wide x 30" deep beams around the perimeter with 2 #6 top and bottom and #4@18 EW in the slab - still moved with changes in moisture and had cracks in my window corners always.

 

[ssuper]

JAE-
Your help would be greatly appreciated. You see the reason I need such information is that I'm a, failrly new to the trade, building inspector in a small town experiencing some growing pains. I've been a home builder for 10 years but now I've jumped the fence and I'm in charge of doing the residential plan reviews. I would like to become more famaliar with the design methodology now that I'm responsible for the newly built homes in my town.

 

[JAE]

Alright - here at my office now. - found the old Slab document I referred to.

It's called "Design of Slab-on-Ground Foundations", August, 1981 (told you it was old)..
Prepared for: Wire Reinforcement Institute, 7900 Westpark Drive, McLean, Virginia 22101 and for the Concrete Reinforcing Steel Institute, 180 North LaSalle Street, Chicago, Illinois 60601.

The method was developed by a Mr. Walter L. Snowden, PE in Austin, Texas. It is an empirical method. There may be an updated version out there somewhere. I would start with CRSI.

 

[BigH]

JAE - not that old - and many of the old papers are best; most of them actually said something!

 

[digrazi]

I have the document that you referenced by the wire reinforcing institute. How do you calculate the cracked moment of inertia Icr? Also for the effective width of slab do you use the ACI 8.10 guidelines for a T beam?