footing for post

[JimMorrissey]

Haven't been on for a bit.....

I'm helping a friend add a couple of support posts to a main beam in his basement. His living room floor is bouncy and he wants to firm it up. Problem is the slab is pretty thin, so I'm going to cut a square in the slab, dig out the dirt and fill it flush with concrete to strengthen the area for the posts. I'm thinking 12"x12" about 6" deep should do it. Anybody have any spicific parameters for depth and size of the footing?

 

[Dusty]

Your formula is correct, double the width as to the depth. I would go 18" square and a 9" thick pour. Don't forget to tamp the bottom of the hole and to wet the ground around the inside of the hole and the surrounding concrete. You don't want the footing to dry out too quickly. After it is poured, cover with a piece of plastic to slow the drying and to improve the cure strength.
Dusty

 

[EddieWalker]

I like the idea of the footing being allot deeper. You're probably alright at 6 inches, but if it was me, I'd go down at least a foot. It's such a small pour that it won't cost very much more to double it.

Eddie

 

[Birdhunter1]

Instead of wetting the dirt down inside the hole you could also put plastic along the edges of the hole in the dirt

 

[Buckeye_Jim]

I recommend you look at the whole structure. Get some idea of how much floor area and roof area might bear on this new footing. That is the determinate of the footing size. The suggusted 18" square is 2.25 square feet and if your soil can bear 3000 lbs per sq. ft. then you can carry 6750 lbs on that footing. A matrix of rebar may also be a good idea.

It could also be very likely that the existing structure is already carrying the loads adequately, so your new footing is unlikely to ever fail.

Good Luck,

Jim

 

[rdln]

My last experience similar to yours was approved by inspector in a crawl space, two or three concrete blocks on a 24x24x8 pad used to support the middle of an i-beam on a 20' wide addition

 

[JimMorrissey]

Thanks. I think I'll just go big with 18x18x9 pads so I won't have to question it....maybe throw some rebar in for good luck and go with fiber reinforced concrete. Material costs won't be too much and the base is sand so it'll be easy digging with high bearing.

 

[Highbeam]

Are you going to use one of those screw jack type adjustable columns so that you can tighten it up just right? I think those are super.

 

[schmism]

you might consider the prefabracated plastic footing forms.

most of the math has been worked out for you. set it flush with the basement floor and id say your covered... the website even has calculators to help you figure the right size based on load.

Square Footing Form Do It Yourself Information

 

[RobJ]

I'd go 24x24x at least 12" you only want to do it once. Rebar and the works. But a lot depends on your soil. I watched a foundation company work once. When the hydraulic jack, they were pushing 8" round peers (flat on the bottom) into the ground like it was nothing. The guys says he watches the pressure gauge on the lift to let him know when he hit the solid clay. 2000-2500psi or somewhere in that area. Some places in this house got 3' down, some got as much as 5' down. Much deeper then the bell bottom peers that had previously been put in a failed.

It was neat to watch.

 

[schmism]

The guys says he watches the pressure gauge on the lift to let him know when he hit the solid clay.

this is how they drive pile for foundations. The soils engineer specs out what the movment per hit should be. once they reach that they stop

 

[CurlyDave]

Haven't been on for a bit.....

... Problem is the slab is pretty thin, so I'm going to cut a square in the slab, dig out the dirt and fill it flush with concrete to strengthen the area for the posts. I'm thinking 12"x12" about 6" deep should do it. Anybody have any spicific parameters for depth and size of the footing?

I have never seen a footing for a post base, designed by an engineer, which was less than 12" thick.

I have read a few soils reports, maybe a dozen or so, and the lowest load bearing strength I have seen for any soil is 2000 psf.

Bottom line is I think you want a minimum of 24" on a side and 12" thick.

 

[CurlyDave]

this is how they drive pile for foundations. The soils engineer specs out what the movment per hit should be. once they reach that they stop

Pilings work on an entirely different principle than footings.

A footer transfers force to the ground through simple down pressure, a piling works through friction on the sides of the piling.

 

[schmism]

this is how they drive pile for foundations. The soils engineer specs out what the movment per hit should be. once they reach that they stop

Pilings work on an entirely different principle than footings.

A footer transfers force to the ground through simple down pressure, a piling works through friction on the sides of the piling.

ummm entirely diffrent principle? ahhhhhh the ground is still supporting the load of the building.

how you transfer the load to the ground can occur though diffrent methods i agree, but at the end of the day, (how ever you do it) the ground still holds it up.

 

[CurlyDave]

ummm entirely diffrent principle? ahhhhhh the ground is still supporting the load of the building.

With footings the ground directly under the footing is under a compressive load.

With pilings, the ground around the piling is under a shear load.

These are entirely different principles and the design calculations are entirely different.

Many people are under the mistaken impression that the bottom of a piling rests on something solid and this is what supports the load. This is usually untrue, although there are rare special cases where the piling, more or less accidentally, hits a big rock.