Wall Shear (Out of Plane)

[strguy11]

I have an 8" thick concrete wall where I am having problems getting the shear to work. The shear is out of plan of the wall. My Vu is greater than 1/2 Phi Vc, however Vu is less than Phi Vc.

Based on what I can determine, it appears I need to provide shear reinforcement. Any ideas? I was thinking of a #3 bar stirrup horizontal around the vertical reinforcing. Is this correct? It apears that 6" oc spacing is the minimum based on a 5.625" d dimension.

 

[JAE]

The [φ]Vc/2 limit is for beams, not for slabs or walls.

If Vu < [&phi;]Vc then you are OK.

 

[strguy11]

What if it would be greater? Can you add stirrups then? Is it feasible?

 

[hokie66]

Stirrups are not practical in an 8" wall. In that case, you should consider using higher strength concrete or thickening the wall.

 

[JAE]

Yes, I'd agree with that. I did note that there is some sort of new provision in the new 318-09. Apparently there is something about being now able to use headed stud assemblies as shear reinforcement for slabs and footings. I don't know if this would also apply to walls but it might.

I've never used stirrups in a slab or wall.

 

[hokie66]

The headed stud assemblies are probably what are usually called "studrails", although that may be a brand name. These are generally used for increasing punching shear capacity. I don't think they would be practical for the type shear strguy11 is talking about.

Strguy11, is this a retaining wall? Must be for you to have such high shear values. If that is the case, I think you need to worry more about the joint at the bottom of the wall than the shear within the wall. I don't believe I have ever actually had out of plane shear control wall design.

 

[strguy11]

No, its a wall in a building, and I have a concentrated force on the wall and a thin width section where the shear is kind of high. All of my forces Vu are less than Phi Vc, so I am ok.

One other question that I have is in relation to this but say for an exterior wall, with only reinforcment in the center. Say d=4". For an exterior wall case, the wind loads can be positive or negative. If the moments are greater than the cracking moment Mcr, wouldnt the concrete crack on both side of the rebar (Ext face and int face) Would there really be any concrete left to take the shear our of plane? Or are most people using a double layer of reinforcing for exterior walls, say 8" thick?

 

[JAE]

For out-of-plane shear in a concrete wall, you still use d=4 and calculate [&phi;]Vc. Yes it may crack in both planes but under any one specific lateral load you would always have a compressive stress condition on one side or the other of the wall and thus have some [&phi;]Vc value occurring.

Also, don't forget to include P/A in your Mcr calculation. Many times the axial load increases the Mcr.

Per ACI, Mcr = f_rI_g / y_t

This is from f = My/I

But with axial load, f = P/A +/- My/I

So by inserting this into the Mcr equation you get:

Mcr = (f_r +/- P/A)I_g / y_t

Just be sure to watch your sign convention on the P/A by remembering that f_r is a tension stress.

 

[NS4U]

I don't see why adding cross ties would be a problem for out-of-plane shear. The only challenge is generating good design drawings to show how they go together.

They're not exactly labor intensive to place, and depending on your load pattern may be more economical than thickening your wall.

 

[JAE]

I think cross ties are extremely labor intensive in a wall.

 

[hokie66]

Me too.

 

[NS4U]

I'm curious why you think they are more labor than say stirrups in a beam?

The only hgue draw back I see is that there is a lot of reinforcing steel in your cage

 

[JAE]

A beam is a discrete member where the steel can be fabricated in a single long basket (stirrups and long. bars).

The wall would have to have one mat tied up, placed on one side of the wall. Then the second mat tied up and stood up on the other side. Then you'd have to have your steel workers climb up the mats of steel and tie individual little double hook stirrups all across the wall face at the specified spacing. Very time consuming, difficult to support the workers hanging off the mats of steel., all the while the two mats of steel are not usually supported well until the other form is placed and the mats cross tied to the wall ties. Just a difficult scenario.

 

[NS4U]

good point. I agree, I actually never have spec'd cross ties in walls(I normally just increase the thickness) but my thinking was along the line that if you only need 1 or 2 feet of shear reiforcement at the base of the wall it would be easy to install, and possibly less costly then all of the additional concrete.

 

[JAE]

Well, that might be true - if the ties were limited to a small area.

 

[civilperson]

The cost effective increase in shear strength comes from; 1) higher strength F'c, 2) thicker wall at the base, either full height, tapered or with a fillet.

 

[VoyageofDiscovery]

If this is an existing wall with a new concentrated load how do other engineers distribute the load effects for the wall? I have done FEA and found this not that helpful as the stress concentration is extremely local and I believe that the wall will distribute the stress through the steel mats.