Concrete shear wall boundary elements 1

[StructuralMike]

Are you permitted to have just a single layer of reinforcing at the boundary elements. The code requires confinement steel if compression exists. So providing confinement is really not possible with just 1 layer of reinforcing. I believe you must always have 2 layers with confinement ties. Does anyone know if a single layer can be detailed?

 

[KootK]

Great question. I struggle with this myself when trying to use 8" walls as shear walls. Ever try to put a column tie in an 8" wall? It's ridiculous. Basically, if I need confinement, I'm forced to bump up to a 10" wall which is equally ridiculous for small stuff. Ugh.

You don't always require confinement at your boundary elements. If you don't need it the sure, one layer. But if you do need confining then I agree, you'll need two layers (at the boundary element at least).

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[mike20793]

Are these special concrete shear walls? If so, you need to check the shear requirement for using one layer of reinforcing. In special boundary element, the reinforcing can actually buckle out of the concrete under high compressive stresses, thus tight spacing of confinement reinforcement is required. Typically, if you're at this level of compressive stress, you cannot use one layer of reinforcing.

If it is ordinary concrete shear walls, one layer can certainly be used. I use the horizontal bars with a 180 degree hook around the boundary steel for confinement.

 

[KootK]

I use the horizontal bars with a 180 degree hook around the boundary steel for confinement.

Clever. I'd never thought of it this way. Does that mean that you limit your boundary steel to single vertical bars? Or maybe a bundled group of two or three?

I'm not sure how much of a code issue it would be but, with the confining steel being placed in the middle of the wall, I'd be comfortable with 90 degree hooks. And that means that I could just use standard corner bars lapped with the horizontals. Contractors would love that...

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[mike20793]

I guess I should elaborate a bit. It really depends on the thickness of the wall. If the wall has low enough loading to only require one layer and it has an unsupported edge, I will use two layers at the very edge of the wall and a #3 hairpin at the same spacing as the horizontal bars. If it occurs at an intersecting wall, I use corner bars lapped with the horizontals like you mentioned. I really only use the one layer with a 180 degree hook on the vertical bar in 6" walls. That's only because there isn't enough room to fit the bend diameters in the wall.

 

[KootK]

Six inch shear walls? I want in on that. Is this how it works:

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[mike20793]

Yes, I show the bars come in towards the horizontal bar, but I doubt it's ever constructed that way. The U-bar is more likely what they install since it easier to bend.

 

[KootK]

I bet that they place the hairpin around the boundary bars and then twist it as vertical as they can so that it can be tied to the horizontal reinforcing. Come to think of it, that would work nicely in place of the 180 bends in six inch wall as well.

The question that I have here is the extent to which this arrangement constitutes confinement in the direction perpendicular to the wall. Normally, you're tying a bar on one face of a wall to another bar on the opposite face, engaging most of the wall thickness. Here that's a bit more questionable. Both bars might just buckle laterally together which would make tying one against the other pointless.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[mike20793]

If my compressive stress were high enough that buckling might occur (greater than 0.2f'c), I would probably have two layers and would be able to get crossties in or increase f'c to 5 or 6 ksi. A few engineers at my firm have used 8 ksi in concrete shear walls before. I've only had compression issues happen in special concrete shear walls where I have ties and crossties at 4" in the special boundary elements. If it were a special boundary element, I don't know if one layer is permitted (I don't have ACI 318 near me to check). It is something to keep an eye out for in the future, though.

 

[StructuralMike]

Thanks for the information posted. What I have been able to determine so far is if you don't need boundary elements then one layer is fine. Once the stress is too high and boundary elements are needed, you really need 2 layers and tie them together. My case is not a special shear wall.

What types of software do either of you use to analyze shear walls? We just started using RAM Elements and RAM Concrete and trying to figure this all out.

Thanks.

 

[KootK]

I've mostly been using ETABS and S-Concrete. S-Concrete is awesome for shear walls.

I try not to take this stuff too seriously. In the US, you calculate your limiting compressive stress for confinement based on the uncracked concrete section. And that means that it's completely wrong and really nothing more than an index.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[mike20793]

Any shear wall has boundary elements at the end, whether it's concrete, wood, or masonry. In concrete, the boundary elements don't typically require additional detailing of extensive confinement until the compressive stress is large enough that the very slender reinforcement may buckle out of the concrete. This can happen if the wall is slender (tall relative to width causes large overturning moment) or the overturning moment is large due to high shear). Concrete walls use the same theory as beams for design and analysis, so it isn't that difficult to find the compressive and tensile forces at the outermost reinforcing bars.

That being said, I've used ETABS, RAM, and RISA to do shear walls. For ordinary shear walls in fairly regular buildings, RAM is awesome and easy to use. For special concrete shear walls or shear walls in irregular buildings, you may want to consider ETABS. RISA seems to handle both ordinary and special shear walls fairly decently.

 

[bookowski]

Mike - What size/type of buildings are you using 6" shearwalls on?