Stem wall held back by slab on grade
Stem wall held back by slab on grade
(OP)
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?






RE: Stem wall held back by slab on grade
Don't forget to shore the wall until the slab has achieved enough strength to brace the top of the wall.
RE: Stem wall held back by slab on grade
Therein lies the problem. This is a residential project and they do not want to reinforce the slab. I was thinking of running the bars the whole way, but the slab is 30'-0" wide and I need most of it to resist my horizontal force.
RE: Stem wall held back by slab on grade
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.
RE: Stem wall held back by slab on grade
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.
RE: Stem wall held back by slab on grade
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Stem wall held back by slab on grade
How did/do you arrive at that spacing. Any numbers? Or is it more a rule of thumb?
RE: Stem wall held back by slab on grade
Or possibly fan the bars out at an angle?
EIT
www.HowToEngineer.com
RE: Stem wall held back by slab on grade
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.
RE: Stem wall held back by slab on grade
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.
RE: Stem wall held back by slab on grade
The cracks in the slab are what I was concerned about. I am going to run the bars out the entire width of the slab that I need. I need about 30' so the reinforcing will run the full 30' width of the slab.
RE: Stem wall held back by slab on grade
From running ther numbers in the last on various projects.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com