I have a 10×12 room between a 20×20 garage and the rest of the house. I want to add 4 feet to either side of the 10×12 room making it 18×12. In adding the 4 feet to either side, I want to pour a monolithic slab that will be pinned to the garage, the house, and the exising 10×12 room — so in effect there will be the 10×12 room with these 2 — 4 foot extensiions.
is a 16″x16″ grade beam sufficient with the rest of the area having 6″ of concret? The new wings will be tied into the existing 10×12 room but the slab will not go over the room itself. — Lie in the Wash DC area — tks
Replies
I think your frost depth in the D.C. is greater than 16". On a monolithic pour, what you are calling the grade beam at the edge of each slab is the footing and foundation poured all in one shot. Assuminig you are going to reframe the walls on the new pours the 16" width is probably adequate if the soil has the proper bearing capacity for the load (walls, roof,etc). Since you plan on pinning it to the existing slab and foundation it is imperative that you get it down to the proper frost depth. Frost heave can not only move the new slabs, but could crack the slab/ foundations you have it pinned too.
Dave
Answer to this depends more on existing conditions and soils than anything else.
But you use the term "monolithic pour" inapropriately here. Mono means one. You would effectively have three slabs as you describe here with great potential for failure. You lose the diaphragm and the floatation qualities of a floating monolithic slab by breaking it up and by pinning to existing structures.
You need a proper footing, stem wall, slab, from what I can tell.
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