Ties for flexural steel in RC beams and walls

[Serhiy]

Good day. I'm designing a manure pit wall and it mostly acts as a flexural member resisting lateral loads from manure and soil. it spans vertically as it is supported at the top. The wall is 8" thick and I have reinforcement on both faces due to stress reversal. My question is, do I need to tie the reinforcement which acts in comression even though I don't use it in my calculations? Canadian code CSA A23.3 says that ties are not required for distributed vertical compression reinforcement in walls if area of steel reinforcement smaller than 0.5% Ag. I think that this clause covers my question (my reinforcement area meets this requirement) but what about reinforcement in beams? If I have a beam with top and bottom steel (only because of stress reversal), do I need to tie compression reinforcement even though I don't use it in my calculations? Clause 7.6.6.2 says: "Transverse reinforcement for flexural framing members subject to stress reversals or to torsion at supports shall consist of closed stirrups..." To me this seems like yes I do need the ties for any flexural reinforcement that works in compression but what to do about not tying it in walls? Can I still rely on the clause about 0.5Ag above? Would like to hear your comments. Thanks.

 

[jayrod12]

If you aren't accounting for the steel in compression, then I would think that no, confining steel would not be a requirement.

 

[hokie66]

Confinement for compression reinforcement is only one reason for stirrups in beams. They are mostly for shear. Confinement reinforcement is normally for columns.

A wall in bending would rarely have compression reinforcement. If you need compression reinforcement, you need a thicker wall. And anyway, using ties in an 8" wall is impractical. It is difficult enough to place the concrete with two layers of reinforcement, without the additional impediment of ties. Give the concreter a break. You can draw it, but he has to be able to build it.

For a wall holding manure, I suggest you need a thicker wall, because you need substantial cover to protect the reinforcement.

 

[Serhiy]

Like I said before one layer of my flexural reinforcement will work in compression because I need to design for both mutually exclusive cases: manure pressing on wall from the inside and earth pressing on the wall from outside. It ended up being same reinforcement on both faces. My concern is that when flexural reinforcement works in compression, it may buckle and cause concrete to spall (blow out) even though I don't consider it in calculations. The fact that I don't consider it in calculations doesn't stop it from acting in compression.

Hokie66, I agree that thicker wall would be required but unfortunately that's not an option.

 

[hokie66]

You have a mistaken understanding of compression reinforcement. Just because there is reinforcement near the compression face does not mean that the reinforcement is in compression. The concrete takes the compression. Except in rare cases, reinforcement near the compression face of a wall remains within the elastic tension zone.

 

[jayrod12]

As alluded to as well, keep in mind the large clear cover required for manure retaining structures. It's entirely likely that the "compression" steel is on the other side of the neutral axis and may actually be in tension.

 

[Serhiy]

I totally missed the fact that my compression zone is only about 3/8" so my reinforcement is in tension - thanks guys.

To close off the topic, would you consider tying the top reinforcement (which is not considered in calcs) in case it ended up being in compression be it in a beam or wall? If yes, at what point of stresses/strain in steel would you do it? I remember testing reinforced masonry beam in school and top reinforcement was tied and it buckled locally after the steel started to yield in compression.

 

[jayrod12]

Serhiy,

I was there at that test of the masonry beam you're talking about. The one in Fariborz's class right? Masonry is a different animal altogether, and I don't believe we saw true buckling of the compressions steel until well after everything else had failed catastrophically.

 

[Serhiy]

That's crazy but you are right, it was in Fariboz's class. If your name is Jarred (or any other spelling variations) I remember you. We should meet for a coffee sometimes.