Masonry wall and foundation stem wall construction 1

[jeffhed]

I am designing a masonry addition that is sitting on a concrete stem wall. The masonry is fully grouted with vertical bars spaced 32" o.c. I can't space the stem wall vertical spacing that far apart because of the concrete minimum spacing requirements. So with my masonry bar spacing @ 32" o.c. and my concrete stem wall spacing @ 18" o.c. they won't ever meet up. I could reduce the spacing in the stem wall to 16" o.c. to lap with the masonry wall spacing and need to so it is an 8" increment. Is there any reason why I need the stem wall bars to lap with the vertical bars in the masonry? The bars coming out of the stem wall provide shear resistance at the base of the wall for out of plane and in plane forces. The vertical bars in the masonry wall are developed enough past the midpoint of the wall and shouldn't need to be lapped with the stem wall bars. I realize my stem wall spacing needs to be at 8" increments but other than that if every vertical bar in the masonry wall doesn't lap with a bar in the stem wall is this really a problem or is it just good practice?

 

[msquared48]

For the stem wall bars that don't line up in a cell, I would still use them, just grout the additional cells they intersect for the length of the stem bar.

I would field bend the bar slightly or locally slightly chip out the intermediate rib if the bar hit a rib to get the bar into a cell, or just abandon the bars where they hit the intermediate CMU ribs if the shear from the other bar connections is sufficient.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[hokie66]

I think that depends on how the walls work in bending. If the concrete wall is capable of carrying the horizontal reaction imposed by the reinforced masonry wall, then lapping would not be required. On the other hand, if the whole wall spans vertically with a joint in the middle, then the bars should lap.

 

[jeffhed]

Thanks Hokie and msquared, I always appreciate your more experienced input. The top of the concrete stem wall will be the same as the top of the slab, so no hinge point in the wall. Rebar will only be for shear transfer for out of plane and in plane loading.

 

[hokie66]

Just curious...what does the term "stem wall" mean and what is its origin? In my years in the US, I can't recall hearing it, and it seems from these forums that it is only used in North America. Of course, I may be wrong about that.

 

[jeffhed]

In this case there is a grade drop between the interior slab and exterior finished grade of about 2'-0". So there will be a footing then a 2'-0" stem wall that will be poured on top of that to bring the top of foundation grade up to the same grade as the foundation where no grade drop exists. At least thats how our firm uses the term.

 

[dhengr]

Why not space the vert. bars in the stem wall at 16" o/c, that’s one more bar every 12'. Then every other bar can be lapped with the wall bars. I would like that little moment continuity at the joint btwn. the CMU wall and the poured wall. You do want (hope for) all of the found. bars to line up with the conc. blk. cores without a bunch of bending and messing around. And, you should warn the contractor to pay attention to this in placing the steel. I would also leave the top surface of the poured wall slightly rough so that first mortar bed and the grout keys with the poured conc.

Hokie.... I don’t know where the terminology ‘stem wall’ came from, maybe a short stem on top of the footing. Usually, the term ‘stem wall’ connotes a shorter height wall on top of a footing, as opposed to a full ht., story high, found. wall as in a full height basement. The stem wall may have soil on both sides as with a slab on grade; or only on one side as with a crawl space under the first fl.; often with lighter rebar too. Our footings are down at 48" for frost cover, so we commonly have a 3 or 4' high stem wall up to the sill plate or slab on grade. You see some wild variations of ‘stem walls’ when you get in the DIY’er. and small home builder arena.

 

[jeffhed]

dhengr,
I could do that, its not that big of deal. I gave some rebar spacings, but my question was really more about if the rebar really needed to be lapped or not. If my stem wall spacing was 24" o.c. and my masonry wall spacing was 32" o.c., then it may be more of an issue.