Demo A Foundation Under A Radiant Heat Slab Without Disturbing The Slab...?

wackman

We got a remodel we're doing where we're removing a bunch of walls, The entire house sits on a slab and part of it is radiant heat. The engineer is calling out for a large grade beam to go under the three remaining posts that will support over half of the home. Part of this home used to be outside and has, in a past remodel, been made interior space and that new slab doesn't have radiant heat. The posts are right at the juncture between the two slabs. So the grade beam is supposed to split the two. We were going to cut the non radiant side and excavate about a foot under the radiant slab and have a 27" wide grade beam, half under the heated side.

The problem is, as we were cutting the slab we discovered that there is an actual exterior foundation under there, footing and all, (this is in the middle of the living room) and no one expected this. I had the foundation radared and it has no rebar so the engineer says it's gotta go too.

So now we have to bust out this rather good sized foundation, through a 13" wide trench that's 14' long that's sitting directly under a 3" to 4" heated radiant slab without disturbing it at all. It's at a funky angle and we can't really bang on it that hard.

I kicked it back to the arch and eng to see if they had any ideas and they're still scratchin their heads.

So what do you guys think, got any ideas?

Thanks,
Wack

ch0mpie

have you determined the width and thickness of the existing footing? If these are larger then the proposed grade beam, it may be able to carry the new loading althought it may not meet building code requirements.
Also, I'm guessing that the reason your engineer feels the rebar is critial is because of the concentrated loading from the new columns. This concentrated load could be reduced by increasing the size of the cross-section of the column (spreading the load over a bigger area of the footing).
If you engineer shoots all this down. My only other suggestion is to get the extra capacity from the existing footing with mini-piles.

wackman

It's hard to know how wide the footing is because the heated slab goes partially over the top of the foundation so we really don't know how wide the stem is but it looks like the footing is about 12", maybe 14".

Could you elaborate on the mini-piles, I'm not sure what you mean.

Thanks.

ch0mpie

Mini piles are used to either underpin or gain capacity from an existing footing. A mini pile is constructed by drilling a hole through the footing into the soil below. Rebar would be placed in the hole, then you would fill it with grout, ussually under pressure. Your engineer would have to decide how deep you need to drill, what diameter hole, what size rebar, grout mix and pressure, how many piles, pile locations etc. From my very limited knowlegde of this topic I would guess it would be something like one 6 to 8 inch hole, less then 8 feet deep, under each column with maybe one #11 rebar. But thats really just a guess, there are a lot of other issues, someone with experience doing this would have to evaluated the situation in detail. I'll look to see if I have any tech articles about this.

Don't be surprised if your engineer looks at you like you have three heads when you mention this to him.

wackman

I thought maybe you meant something like pilings for hillside foundations. That would certainly be easier than trying to break it out.

I called him yesterday and today but he hasn't called me back yet. I'm tempted to suggest to the customer to use a value engineer and see if he's way overbuilding it.

That's something I'm going to have done on some of my house plans. It costs about 2K more but if it saves me $500 a house and I build the same plan 50 times...New Toy Hauler!

ch0mpie

I'm interested to see what he tells you. 27" sounds like it may be overdoing, but what are you soil conditions? Worst case I would think a 27"x14' footing would give you around 30tons. You said 3 columns, so 10 tons per column? Must be a really big house. If you look hard enough you can find an engineer to sign off on almost anything

denick

wackman,

I am curious as ch0mpie as to how big the house is? What kind of weight is bearing on this. What area of the country are you in?

When you said the footing looks 12" to 14" did you mean in depth or width? From the face of the stem wall you see how much footing showing?

__________________
Nick

"Every day I remind myself that my inner and outer life are based on the labors of other men, living and dead, and that I must exert myself in order to give in the same measure as I have received and am still receiving" Albert Einstein

wackman

I'm in Portland Oregon.

No soils done.

I, personally, think it's extreme overkill but I'm not going to get into a pissing match with this guy, you spec out-I'll build out.

It's a one story modern style from the mid 60's. It's got a slab through out the home which is odd around here. It's also got a flat roof over 80% of the house with the roof sheathing being t&g 2x6. The sheathing is supported by beams and it's all exposed, all being very uncommon here. Re-wiring/re-plumbings gonna be a bitch with the radiant slab on the bottom and everything exposed up top plus there's a bunch of floor to ceiling windows. They're still working with the arch on the kitchen design.

This 80% used to have 3 columns plus a 30' "L" shaped wall right in the middle and we're taking out the wall, swapping out all the beams that landed on that wall, and all that weight runs back to the 3 posts. All in all the area is about 1200 sqft (30'x40') with exterior walls all around but only those three posts in the field.

From the area we opened up I can see 3" of stem width (the rest is covered by the radiant slab), 14" of stem height, then 4" of footing width and 6" of footing depth.

I'm thinking, what if we cut a chamfer in the non radiant slab directly over the stem and seated a steal I beam on top of it? It's gonna be covered with hardwoods... He made it sound as if it'd be good to go if it had re-bar in there...

denick

Seems like a good way to proceed. Good luck! If you had a chance post a picture I would like to see the problem.

__________________
Nick

"Every day I remind myself that my inner and outer life are based on the labors of other men, living and dead, and that I must exert myself in order to give in the same measure as I have received and am still receiving" Albert Einstein

wackman

Engineer said no to the I beam idea. Said it will just rust, as he said that I was thinking "well what do you think rebar does" but oh well.

So now we are about to proceed with carefully removing the foundation. I'll take some pictures to post.

denick

Wack,

The thing I'm not clear on is if the stem wall is typical of the foundation holding up the rest of the house and has no sign of failure. why is it unacceptable to carry the load? If it an issue of some posts concentrating the load could you not cut some 3' x 3' holes in the non radiant side where the posts go and cut through the existing wall and pour a pier of sufficient size to support that post. Very large structures are sitting on 2' x2' footings and 1' x1' piers?

__________________
Nick

"Every day I remind myself that my inner and outer life are based on the labors of other men, living and dead, and that I must exert myself in order to give in the same measure as I have received and am still receiving" Albert Einstein

ch0mpie

I'm not surprised that your I beam idea got shot down. Steel reinforcment really needs to be on the bottom of a footing because thats typically where the tension is.

Without any soils report or being able to see much of the existing footing, wack's engineer is probably assuming all of the worst cases. If this is really a big deal and you feel it will save you and your customer a lot of money I would have a soils engineer look at it. I would think that if you could expose a decent size section of the existing footing, on the non radient heated side, down to the bottom of foundation that this would be sufficient for inspection.

wackman

I suggested that very thing but the engineer seems annoyed and the home owners don't want to press the issue. I just said "OK then, here is the price to remove that section of foundation" and they took it.

I think the foundation would have held fine from my personal experience but I'm no engineer so I could totally be wrong. We are removing a 12' wall and running all the weight it supported back to these three columns so it has significantly more weight now than it used to.

We're just going to take it slow and steady with some good sized rotohammers and chipping guns. I figure it'll take a few days for two guys to get it all out. Since it has no rebar or mesh of any kind it should break out in nice sized chuncks.

ch0mpie

If the ho doesn't want to make an issue, what can you do. Good luck.

denick

Yes good luck wack,

Don't forget to let us know what you found.

__________________
Nick

"Every day I remind myself that my inner and outer life are based on the labors of other men, living and dead, and that I must exert myself in order to give in the same measure as I have received and am still receiving" Albert Einstein

wackman

Ok, we demoed the rest of the slab and then attacked the foundation underneath and it went surprisingly well.

I'll take some pictures before we pour it. It went really smooth but we had to take it slow. I thought it would take 2 days with 2 guys but it really only took 1 day with 2 carpenters and a laborer.

The radiant was only 2 inches from the edge of the slab and was completely exposed on the underside so when we were digging all the dirt out we had to be very careful not to hit a tube (plastic) with a shovel.

The edge of the slab wanted to crumble if you bumped it so I had to put up a temp wall over it to make sure no one accidentally put weight on it.

We cut out the slab like normal but used an electric 60lb jack instead of a bigger one and then used chipping guns to seperate the top of the stem from the radiant, it came out smoother than I thought it would because it had a weak cold joint. From there we went in layers taking down the stem and then broke the footing out in big chunks.

I was prepared for the worst and charged an extra $4975. just for the foundation removal and accomplished it in 7 hours so I feel pretty good. After all associated costs and overhead still profited about $3400. My cousin pointed out that if I could put together a years worth of days like that I'd make a mil a year. Heheh.

mdshunk

Maybe, but that's still a scary freaking job. You'd be a nervous wreck of a millionaire.

wackman

Very true, I spent more time thinking about this one aspect of this one job for the last week than the spec I'm finishing, 3 other bigger remodel jobs, two land purchases, one land division and the preparations for a custom home combined.

wackman

Hey guys, here are some pics of this thing after we got it all out and ready for pour. Notice that is some spots you can see the radiant underneath and in a couple places the radiant slab was only 1/2" thick, if you sneezed on it it'd break.

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