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Converting standard deck to heated concrete+stone top deck. 1

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DoubleStud

Structural
Jul 6, 2022
464
We are redoing a drip through deck into concrete topping deck. We are going from 15 psf DL to 65 psf DL. We are redoing everything including lowering the ledger (to accommodate concrete/veneer thickness) and adding blocking between studs at ledger elevation to increase capacity.

The question I have is with the foundation. Currently the piers are 16" diameter sonotube on 2.5'X2.5 spread footing (from the existing structural drawing). I do not have a soil report. In the past the building department has been OK with me assuming 1500-2000 psf soil bearing pressure on small deck projects. I am not sure how to go about it with this project. Would I be able to reuse the foundation? Can I justify a higher bearing capacity since the soil below it has taken under load for years? Do I make them dig it up and extend the footer? Below the deck they also have a stone patio. So digging up the existing piers will mess up the patio floor finish. Assuming 2500 psf to 3000 psf will certainly give me the capacity with the extra DL. What would you do?
 
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I would use the assumption typical for the area. If 1500psf is the norm, use that. Don't try to stretch it. If they want extra capacity, have them get a boring and get a geotech to sign off on it.
 
phamENG, I was just thinking maybe I could assume higher since the soil has gone through the initial settlement.
 
It's gone through initial settlement for the 15psf dead load, not the other 50psf. Total, final consolidation probably won't be substantially different then if they had applied the 65psf in the beginning.
 
I don't think initial settlement = increased bearing capacity. I'd get a soil penetrometer test done at the bearing level. Otherwise, you wouldn't have a leg to stand on if the added weight causes soil failure during a college party and differential settlement cracks the finishes.
 
In NJ I've never seen anything less than 2000 psf used. Anything less than that we go to piles. So if it's in a shady soil area I would use 2000 psf. Or if you know that area has pretty decent soil overall I would have no problem assuming 3000 psf. Recommending a geotech and using the minimum otherwise is never a bad option for us structural engineers IMO.
 
It's possible to build out the footings to get more bearing area, use helical piles, or add more support points with new footing to reduce the new load on the existing footings to a tolerable level. With prices the way they are these days, a geotech report could offer significant value, but sometimes the report gives the same bearing pressure (1500) or perhaps worse.
 
Here in the swamp, 1500psf is the norm for shallow spread footings. I've had a few sites come back 1000psf from a geotech. Usually works for a house, but anything bigger and those sites go to piles. Worst one was a house sitting on a 9foot 'crust' of reasonably stiff soil, followed by 89feet of WOH muck. Yikes.
 
Yea in my area we don't have that issue very often. I can't recall ever seeing a geotech report come in lower than 1500psf. Where I have seen geotech reports be 'worse' than the IBC presumed values is in the lateral soil loading. One friend of mine designed a basement wall footing and was taking heat from the contractor, so he recommended a geotechnical report, and after the report came in he had to make the footings bigger. I see it as a win because he didn't underdressing the wall but the homeowner and contractor probably were not all that pleased.
 
Anything less than 1500 psf might as well be soup IMO. I think most building codes limit to 1500 psf anyway right?

One project I received a soils report that said the soil was so bad it had no bearing capacity at all and that the site was at risk for liquefaction. And this was not in an area of the state known for bad soils generally. Good thing we got a report for that one, I thought liquefaction only existed in textbooks.
 
In my neck of the woods in New York, anything less than 3000 psf will need piles or mat foundation. Most of the time, it's 4000-6000 psf and regular footings are used. The building officials are pretty stringent about bearing pressure. I guess it's a different market here, because 1500 psf bearing material is considered unsuitable fill and can't even be used for mat foundations.
 
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