I am designing a 6'-0" CMU stem wall for a garage. We are running the rebar out of the top of the CMU wall and bending them over into the slab to resist the lateral force at the top of the wall and avoid a large cantilevered retaining wall footing. The slab provides enough frictional resistance to resist the lateral force at the top of the wall. My question is this, how far should I run the bars into the slab? My development length is 36" but it feels too short. How do I determine the correct length to run the bars?
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Stem wall held back by slab on grade
- Thread starter jeffhed
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- #3
Well, you can't rely on the concrete in tension to resist the force. If you reinforce the slab, you will both brace the wall and perhaps produce a better slab.
Actually, building the wall as a cantilever may be more economical than propping it for an extensive time and having to develop the reaction in the slab. I don't know, sounds like you have a penny pinching client.
Actually, building the wall as a cantilever may be more economical than propping it for an extensive time and having to develop the reaction in the slab. I don't know, sounds like you have a penny pinching client.
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- #5
Hokie66,
You are right. I dont want to use the tension of the concrete, if the slab ever cracked I could lose the whole wall. I have tied the slab to the wall with #4 bars @ 24" o.c. For the bars that go all the way across the slab I could probably space them out farther than that. How could I decide what that spacing should be? Once I have developed the bars out of the wall into the slab it is simply a question of ensuring I am dragging the slab with the wall. I don't think I would need bars at 24" o.c. to do that.
You are right. I dont want to use the tension of the concrete, if the slab ever cracked I could lose the whole wall. I have tied the slab to the wall with #4 bars @ 24" o.c. For the bars that go all the way across the slab I could probably space them out farther than that. How could I decide what that spacing should be? Once I have developed the bars out of the wall into the slab it is simply a question of ensuring I am dragging the slab with the wall. I don't think I would need bars at 24" o.c. to do that.
msquared48
Structural
I normally space the bars at 4 to 6 feet in this circumstance.
Mike McCann
MMC Engineering
Mike McCann
MMC Engineering
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- #7
Could you figure out the length of slab needed for sliding resistance (based on weight). Say this is 6ft. Then assume a 45deg distribution or crack that would start at the end of the rebar and work back toward the wall. If you spaced the bars 4' oc the crack would travel 2' (half way) before intersecting another crack. Therefore if you made the bars 8' long the cracks going back toward the wall would intersect at 6' from the wall. I think I might be stretching here.
Or possibly fan the bars out at an angle?
EIT
Or possibly fan the bars out at an angle?
EIT
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- #9
RFreund,
I did figure out the length of slab I needed to retrain the top of the wall. It is the majority of the slab, which is where I was thinking that I could space the bars out more where I am just trying to drag the slab with the wall. The 45 degree rule is a good idea. I think I will go ahead and use that. Then at least I can save the home owner a little money using less bars.
I did figure out the length of slab I needed to retrain the top of the wall. It is the majority of the slab, which is where I was thinking that I could space the bars out more where I am just trying to drag the slab with the wall. The 45 degree rule is a good idea. I think I will go ahead and use that. Then at least I can save the home owner a little money using less bars.
If you need a majority of the slab, then I believe you will have to provide some reinforcing thru the entire slab. If you're engaging a majority of the slab, that means that for any bit of the slab you need, that portion will be in tension. I never rely on concrete in tension for any purpose.
Reinforcing does not necessarily have to be bars. It could be welded wire fabric.
If you need a 36" develoment length, I'd probably extend the bars another foot or so to be conservative and then reinforce the slab using welded wire fabric, lapping over the bars their entire length (36" minimum). To me, this engages the slab.
Just from experience, if you have a slab with no reinforcing, and its 30' wide, you're going to have cracks in it at about 10' to 12' on center. The finisher will probably cut a joint in the slab at about the center....so really, you've only got about 15' of the slab that your depending upon 30' of.
Reinforcing does not necessarily have to be bars. It could be welded wire fabric.
If you need a 36" develoment length, I'd probably extend the bars another foot or so to be conservative and then reinforce the slab using welded wire fabric, lapping over the bars their entire length (36" minimum). To me, this engages the slab.
Just from experience, if you have a slab with no reinforcing, and its 30' wide, you're going to have cracks in it at about 10' to 12' on center. The finisher will probably cut a joint in the slab at about the center....so really, you've only got about 15' of the slab that your depending upon 30' of.
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msquared48
Structural
jeffhed:
From running ther numbers in the last on various projects.
Mike McCann
MMC Engineering
From running ther numbers in the last on various projects.
Mike McCann
MMC Engineering
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