Convert Pier and Beam to Slab foundation

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Old 01-12-17, 08:58 PM
G
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Convert Pier and Beam to Slab foundation

Due to undersized foundation (5" wide x 12" deep concrete wall with no extra footing), low exterior grade to siding clearance, and little to no crawlspace access, and a bad floor that needs replacing anyway, a contractor friend suggested that we temporarily support/level the sills/walls (2.5" square tubing with 4"x4" bearing plates between existing sill and existing stem wall 23" oc on exterior bearing walls, and 3/4" all thread to concrete block on interior partition walls, remove the floor joists, and float in a 5.5 inch slab. It is about a 20' by 25' single story wood frame area, 4 layers of exterior wood siding so LOTs of temporary shear in place, with some extra bracing to prevent kick in/out. Concrete slab to be floated into place in Mono pour connected to and capping existing footing. See the picture for details. The reason for leaving the existing to make it easier to temp support the structure. We think we have enough steel in the design to support the cap part of the slab? Northern Cal, so we are not in a zone where freeze/thaw is an issue, and in 60-years the existing undersized has settled only 1/2" in one corner due to poor downspout management which is being fixed.

Does this approach (working around and over the existing footing in this manner), seem adequate, or should I be focusing on an approach that totally removes the existing footing?

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Old 01-12-17, 09:45 PM
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Looks reasonable, provided your building department approves it. They are the ones that may say the original foundation needs to go. (the notch created by the stem wall does create a weak point in the pour where stress cracking could occur) You will also have to comply with their footing depth even though it may be or seem excessive. There are usually local code limitations (sq/ft size) to a monolithic pour such as you propose, so that will also be a potential sticking point as yours is over 400 sq ft. And they will probably not approve the all thread tie downs as it will likely need to be 1/2" j-bolts or better. The rebar looks good although they may require vertical reinforcement between the slab and the footing every x/ft. There may also be seismic requirements that we don't know about.
 
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Old 01-12-17, 11:24 PM
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Well that's confusing, location in your profile says MA, then you say your in northern CA.
Which is it, two different issues.
 
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Old 01-13-17, 12:30 AM
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Calif...updated profile from default. Also I'm not in super high earthquake zone, up by Chico. I know corner straps are going to be little interesting, but I can easly notch existing to get more bite for those. There is verticle/horizon 90 deg rebar tying the slab plane to the footing that isn't shown in the diagram.
 
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