Putting a new spread footing over an thin spread footing 1

[RustyTools]

When the house was built in 1992, the builder ran a 2' wide spread footing down the center of the basement (70' long) but only made it 6" thick. He located colums about every 6' on this footing. He poured a 4" thick unreenforced slab over the footing and the entire floor. From my calculations the loads on the columns are 2000 to 8000 lbs. I am putting a column between these columns on that same spread footing. It will be bearing 23,000 lbs.

It is too difficult to cut out the existing footing. I am uncomfortable with simply putting 23,000 lbs on that footing. My first choice would be support the column by pouring a reenforced 4' x 4' footing 1' thick on top of the slab. I realize it is kind of big, especially since the soil can support about 3000 psf, but it would cover the hole that was cut to find out what footing were there already. This section of the basement isn't used so the loss of floor space is not a big deal. So now you would have a 4'x4'x1' footing, sitting on top of a 4" slab, sitting centered on a 2' wide x 70' long spread footing.

Another option would be to dig underneath the 2' wide footing, pour a 3' x 4' x 1' thick footing UNDERNEATH the footing and have the 23,000 lbs rest on the old slab. (I would use a 1' x 1' x 1/2" thick steel plate to spread the load) So it would be 1/2" plate, 4" slab, 6" old footing and 1' new footing. This option would leave a level floor but require more work.

In another spot I have a 9000 lb load. I feel comfortable the slab + old 6" footing could take this load.

I have never run into a footing that is only 6" thick. Any thoughts on this matter?

 

[GeoPaveTraffic]

Unless the 6-inch footing is tensioned, I wouldn't worry about cutting it. That would allow you to cut the floor and footing and install the new footing however you would like.

Unless you are in an area with expansive soils, I can't figure out why they would have installed a continuous 6 inch thick footing.

 

[RustyTools]

A few builders in the area have continuous footings. They do it because they are not sure exactly where they are going to put the columns.

It is difficult to cut the old footing because I can't find an electric saw that goes 10" below the slab. You are talking gas powered when you get that big and it isn't that safe to use gas in a basement.

I think I am stuck with the two options I outlined. The digging one isn't looking so bad. It wouldn't be a big deal to dig out underneath the footing, & sneak the rebar in. The new footing would be 3' x 4' x 1' thick under the old footing and come up on both sides of the old footing, level with the slab. This would look more professional than going on top.

Thanks for your help.

 

[eric1037]

You could probably call a concrete cutting company to cut the slab and/or foundation. It usually isn't prohibitively expensive.

 

[boo1]

Residential construction typically places interior thicked section (12" thk total), we call them grade beams. I think that placing the rebar under the beam is the way to go.

 

[jheidt2543]

I think eric1037 has the solution, call and get a price from a concrete sawing company. They would have an electric saw large enough to saw out the floor and footing to the size you need for the new footing.

 

[RustyTools]

Thanks for your feedback. I just put the 3' x 4' x 1' footing under the existing spread footing. Due to its high viscosity, it was quite difficlt to get the concrete under the the existing footing. In retrospect, cutting the old footing out probably would have been easier.

In terms of strength, I think the two stacked footings are at least as strong as the single 3 x 4 x 1 footing. First, the two together are thicker, bringing it further down to undisturbed soil, which was significant in this case. Second, the top footing extends past the bottom footing, increasing the bearing surface. For calculations I ignored the top footing because I couldn't be sure of its strenght. The bottom footing still had to be 1' thick because I couldn't be sure of the bond between the stacked footings.