How to design footings?

[tonkota]

I am planning on building a small 24'-0" x 24'-0" x 10'-0" building. I would like a 6" curb for the walls to sit on (standard 2x6 construction) and radiant floor heat in a 6" slab.

Anyone here know how to design the footings?

EDIT: This building will be on a very flat lot. Soil is slightly sandy, no big rocks.

 

[splitshot]

Might consider going with a monolithic pour (footing and slab are poured in one unit). Then use a course of 8-8-16 block for your stem walls.
The size and depth of the footing trenches depends on soil type and building load.

 

[RavenTai]

Spokane is east WA correct? the ground freezes in the winter right? that will be anoter factor in your footer, the footer need to go deeper than frost could ever be expected to in your worst winter, frost heave can destroy a foundation if it can get under your footer, you need to take a look at the local code and see what it specifies,

with radiant heat you will also want to insulate the slab and footer,

 

[EricG]

Depending on your area, you'll also need a soils report to figure the bearing load, rule out expansive soils, etc. If you're area has building codes, you'll typically need the soils report and have to have the foundation design signed off by a licensed engineer.

 

[fj40charles]

Depending on your area, you'll also need a soils report to figure the bearing load, rule out expansive soils, etc. If you're area has building codes, you'll typically need the soils report and have to have the foundation design signed off by a licensed engineer.

X2.... When I built my building, the city wanted the concrete slab to be designed and signed off by the licensed engineer. What is really dumb is the fact that any PE (professional engineer, licensed in state) could design and sign off on the drawing. I could have gotten a ME or EE to sign off as long as they were a PE.

 

[goschbrandon]

Check the building codes/permitting. Is it a garage/shop? Depending on local codes, you may only need a thickened edge slab of some specified thickness instead of a fully designed footing.

 

[haystax]

I don't have codes or inspections to worry about out here - we usually just go overkill to make sure we don't have problems to begin with.

My garage foundation that we put in last year consisted of a 6" x 24" footer in an excavated trench, 8" poured stem wall and a floated, sloped 6" slab inside the stem wall. Framed structure is 26x42x12 w/ 6/12 wood trusses 2'oc. Framing is all 2x6 24"oc with trusses resting directly above each stud. Two doors 12'wx9'6"h under the eave with 6' of double sheared wall on either side of the doors and between as well. Built up header w/LVL + 2x12 + plywood above doors.

No tubing in the garage but I laid tubing in the basement pour, that wasn't very difficult or expensive but since I already own a 60'x80' heated shop with all the goodies I wasn't too worried about a super nice radiant heated garage...but the tubing doesn't affect the slab details much. I didn't use mesh in the garage and used it in the basement simply to attach the tubing to.

Backfill and compaction before the slab is the main issue with this method, it takes a ton of time to go this route. The alternative is to build up your pad and then trench around the perimeter to get below frost. Just about anything will do for the trench, no need to get fancy. Leave that really rough and pour a slab right over the top. Not nearly as much time excavating and backfilling.

The slab on grade method will leave you with a flat floor, no drainage out the doors for snow/ice to get out of the garage. You can prevent a driving rain from getting under the doors if you float the slab and the garage doors seal to the stem wall and then slope the transition to the approach apron.

No engineering, no code approval, no inspection. So far so good, that's about all I can offer is the fact that this will work for at least one year...whether an engineer stamps it or not it should last a lot longer.

Good luck.

 

[bkfj40]

Be careful of the information you get on this board...there are lots of states represented and some are in quite a different area than you. If you have a building inspections department...go in and talk with them.

Here in Virginia, the building department will help residents with basic design. If the garage slab is going to be placed on more than 2' of fill, a retaining wall over 3', or if the house has some unusally large loads, the building department usually makes a recommendation to go see an engineer. The same was true when I worked in Wyoming.

You aren't looking at anything unusual. The footing will need to be placed below frost depth (if applicable) and the minimum size is dictated by soil types.

If you are going to hire a contractor...have them tell you what they plan on doing. Get a licensed contractor and get a building permit...you'll be much happier (assuming you don't want to do it yourself)


You don't need an engineer for this unless the building department makes you. And hey, I make my living by being a civil and structural engineer.


bk

 

[D'Animal]

I strongly agree with checking with the local codes.


When I built my shop I had to go with a thicker slab and a double reinforced concrete, wire mesh as well as re-bar since I installed 14' doors. The county code engineer caught this and changed the work order. The reason is that I could portentially get big equipment in and out of the shop. The biggest I have had in my shop was a Peterbilt with a grain trailer.

 

[tonkota]

Thanks guys for the answers. I am an engineer, just a mechanical engineer and I know nothing about footings. You guys bring up some great general points that I had not thought about.

Thanks!