Cantilever Retaining wall schedule

[COEngineeer]

Sometimes we have so many retaining walls on one project so we do a schedule so we don't have many details. So usually we just put rebar for shrinkage and temperature for the horizontal bar on the wall. My question is, a lot of the retaining walls jogs. Don't you think the horizontal rebar need to be increased since the wall is restrained on the side(s) (fixed)? One engineer thinks if the cantilever design works, it doesn't matter if you jog the wall. I think it will crack the wall vertically, what do you think? I am still a little green.

Never, but never question engineer's judgement

 

[cvg]

how about just calling out for a construction joint at the bends? That will take care of the cracking problem

 

[COEngineeer]

Then the other wall will be simply supported on one end, you will still need more steel than temperature and shrinkage.

Never, but never question engineer's judgement

 

[cvg]

I doubt that you are doing a 3-dimensional analysis of the walls, they are designed for a 1-foot strip without any contribution from anchorage on the ends or the jogs. Your buddy is right, no additional reinforcement is necessary

 

[civilperson]

A control joint to provide crack control at center between jogs would be a good detail, (and every 30 feet or so). The jog is usually stiffer and the horizontal steel area needs to be continuous around the jog at 0.002 of gross area.

 

[COEngineeer]

Ok cvg.. lets say you have an elevated slab where you design it as 1 way slab because it is supported only on 2 opposite sides (spanning 20 ft). You put rebar going the other way just enough for T & S. Then they decided to put walls supporting on the other sides 20 ft apart also and now it is like 2 way slab but you only put T&S on one way. Is that ok?

Never, but never question engineer's judgement

 

[civeng80]

My experience as a civil engineer designing these insitu walls and as a builder building them is that even with double the temperature and shrinkage rebar they still crack, so my rule of thumb is to saw crack control joints every 3m ie. 10 ft. A different story if the walls are precast !

 

[cvg]

coengineer - I don't believe I am qualified to advise on the design of an elevated concrete slab (floor?)and I'm not sure I fully understand your new question either. However, it seems that if you are supporting a slab on retaining walls you should have a structural engineer doing the design that can evaluate whether to design as a one way or two way slab. It seems to me that if your slab is supported on all 4 sides by walls that you would want it to be a two way slab design. Perhaps some of the structural engineers can weigh in on this new question and perhaps elicit the additional information to clarify the question?

 

[NKT]

cvg: coengineer was using a comparison to illustrate deflection compatability:

Loads will follow the path of greatest stiffness.
If this doesn't match up with where you added strength to the system, material failure can result. With closely-spaced vertical restraints (jogs in the wall), the path of greatest stiffness is a horizontal span between the "jogs", and the wall will crack under horizontal bending before the vertical reinforcing takes much of the load.

The wall may still be sound, unless the vertical cracking is severe enough to reduce the bond with the cantilever reinforcing steel, but it won't be pretty...

 

[cvg]

i agree and stick by my original suggestion, construction / control / expansion joints should be placed at these locations.

 

[Lion06]

coengineer-
The situation you describe (the elevated slab) will result in unsightly cracking, but the system will not fail since you did provide a load path.
My graduate concrete professor said that you should design it the way it wants to behave, but that as long as you provide an acceptable load path it won't fail. That makes sense.
That being said, I asked the very same question in my first week on the job and was told the exact same thing that you were told. The answer bothered me, but I got real busy real fast and that fell off the radar.

 

[COEngineeer]

Yeah, I know it is not going to fail. I just wanted to make sure I wont have big visible cracks.

Never, but never question engineer's judgement

 

[tngolfer]

COE, if you don't want cracks, just follow cvg recommendations of control joints at the bends so they are not providing support. Your wall will behave as a cantilever and not as a 2-way wall. Min horizontal steel would be fine then.