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Placement of adjacent foundation walls

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mattradk

Civil/Environmental
Jul 23, 2016
22
I've been asked to design a two car garage to be placed at the gable end of a raised ranch. The gable end is 24' wide. The owners would like the garage to be 30' wide (wider than the gable end) by 24' deep. The new garage would be flush with the right hand side of the house shown in photo Raised Ranch 1, and would extend 6' beyond the left side of the gable end. The owner would like to hide the utilities from the street. The utilities shown in Raised Ranch 2 would be accessible by walking around the garage since this area would not be enclosed.

Incidentally, the two garage doors shown in Raised Ranch 1 are not functional. A previous owner placed a wall just inside the two doors and finished the garage space as a bedroom. Anything to complicate the situation.

My question: I was considering placing a new 30' poured foundation wall outside of, and immediately adjacent to, the existing foundation supporting the two existing garage doors to serve as the rear of the new garage's foundation. This would place the new stem wall partially on the existing footing and partially on a new foundation. Pins would be placed to secure them together. Shown in figure Footing 1.

I'm planning on using 30' trusses that will run parallel to the gable end, so the load on the rear foundation wall will be relatively small since the truss will carry the roof and snow load at that location. The 24' sides of the foundation will carry the bulk of the load.


The soil is sandy and the frost depth in this area is 48". Snow loads are 40 psf. The rest of the foundation (the 2 sides and new garage's front) would have traditional 8" poured stem walls and footings.
 
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From the info provided, I'd say go ahead and do it. It's obviously an older house. Prior changes to the house no doubt left conditions that are not "brand new" so if some minor cracking occurs, that is easily taken care of during the addition finishing. "Sand" soil tends to have low compessibility and low, if any, frost lifting. That minor added load situation is good.
 
Great, thanks. I hadn't seen it done this way before, so I didn't know if I was out in left field. But it seemed reasonable.
 
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