Residential Foundation Design

[Vlaude]

I am curious to get comments on the foundation design provided. 80% of the home builders in the area are constructing a stemwall foundation and 15% are post-tension, with a few other variations. I am looking at beefing up the footings (they are well below the frost line - in the south) and pouring the slab on the footing in a monolithic pour vs two pours as many stemwalls are poured.

This will be a single story home with #4 rebar on 28" with exception to the toe downs/beams where is will be much closer. Plan on doweling in the toe down in the slab to the footing. There will be footings at load points and thickened beams across all load bearing walls. Concrete will be 4,000 PSI no fiber mesh and soils bearing capacity I'm guessing is not great, around 2,000. I will confirm with a geotech. Just curious on general comments... Thanks!

http://files.engineering.com/getfil...97-475f-82dd-8b476189be61&file=Foundation.jpg

 

[oldestguy]

Looks like a heck of a lot of concrete.

Anyhow, are you proposing a brick ledge on that trench filled with concrete? And, that brick has earth backfill against it? and you don't have a termite population there? What does the building code call for as to foundation height above exterior earth?

 

[Vlaude]

I agree a bit of concrete. I believe the code calls for 16" width and 18" depth. There are some houses that have built to code in the development that have had to add piers after some failures. I'd prefer to over design/build than deal with something on the back side. The footing is continuous around the border, the interior piers are just 2 1/2' x 2 1/2' x 2' in depth.

The soil will be below the bottom 2x6 plate. Yes, there are termites, but pre-treated the slab and as I noted the soil will be below the bottom plate. Pretty typical for the areas I have built stick frame. Probably about 3" of clearance to the bottom plate. I can change the grading a bit, just wanted to show positive drainage off the foundation.


I also put fiber and steel with 3500 psi concrete for the driveway. I figure $500 more on a 30 yard pour isn't a large amount when you break it down over, hopefully a 40 year life span.

I appreciate the input!

 

[boo1]

Comments:
Keep finished grade 8" below top slab
Exterior use 16"w x 20" deep with 12"x6" cutout for brick ledge, three #4 or #5 bottom reinforcement (one under the brick ledge). no vert or cages
Slab, use 4" 3000 psi conc w 6x6 on 6 mil Poly
Interior load bearing, just thicken to 8" w two #4
Drive way do not use steel fiber for exterior, 3000 psi mix. Consider adding plasticizer if you want higher strength (so they don't add 30 gallons of water)

 

[bimr]

There is little benefit to the extra cost that you are adding. I agree with boo1's post. $500 now would be worth $3,000 in 40 years at 5% interest.

The failures that you have mentioned are probably related to a Contractor's poor workmanship and the Contractor's skimping on materials, not to the design of the structure. Every dollar that a Contractor skimps on is profit which goes directly to him.

The typical residential Contractor also does not bring with him much expertise except for knowing which end of a hammer to use on a nail. For that reason, the money may be better spent on quality control during construction.

Considering the fact that the south has had some significant tornado incidents in recent years, you might also consider making sure that the structure is adequately fastened together with properly sized nails and clips. Use of plywood sheathing instead of chipboard or styrofoam sheathing would be worth the extra expense.

 

[Vlaude]

Boo1 & Bimr, I appreciate the additional info. I agree on quality of workmanship. I am actually the GC on the project, as I haven't found a builder that I feel I can turn loose that I won't be managing down the road to some degree. That coupled with a lot of builders in the area want the buyer to carry the construction loan, I figure I might as well build it myself. I agree with the investment over 40 years, though I think when I turn the house in however many years I'll get back a portion of that in management fees saved, so not a huge concern. But I do see your point, why throw it out the window?

Out of curiosity, why do you say no vertical cages into the slab? I believe Code requires a keystone to connect the stem to the footing. In this case it will be a monolithic pour on the footing so I was going to dowel in the slab to the footing. The code official noted it would be acceptable in place of the keystone and that it is required in some seismic areas.

Yes, clips and reinforce garage door. A lot of failures due to high winds, specifically roof damage is after the failure of garage doors.

Thanks again!

 

[boo1]

What state are you in the south?
what is the seismic requirement?
Is the area you in a coastal, flood, or high wind zone?
Why use the two part pour? Just do a mono slab.

Consider looking at other typical residential jobs in your area. The details you presented will scare most residential subs. The additional cost, and complexity provide no payback. Typical single story home loads require very little foundation detailing past what the local codes require. Talk you your building dept and a foundation sub. Quality residential concrete is about dimensional accuracy and water control.

Garage door blowout is typically caused by improper garage door track installation (short or wrong screws) or unrated doors.

While inexperience adds to your risk, you can build a small home. Consider getting a small builder or experienced lead man to be a consultant.

What I do for residential mono slab foundations is
clear lot
bring fill in layers to required height
frame foundation
recheck dims, elevation and square
hire surveyor to check lot, setback, elevation (if required), dim and square
bring in remaining fill and compact
rough plumbing, gas, and elect in slab
check above
inspection
dig footers (ext and interior)
place steel
pest treat
poly and slab wire
inspection
place concrete
cut crack control
cover w poly or seal or water