spColumn - confinement for modelling shear walls 1

[KZSteel]

Hello All,

I am currently using spColumn to analyze some shear walls to develop interation diagrams. I'm analyzing them as irregularly shaped "columns" within the program. My issue is with the confinement properties that I believe spColumn is applying to the wall - is it allowing distributed vertical reinforcement that is not confined in a zone to act in compression?

KZ

 

[PicoStruc]

1) spColumn do not increse concrete strength in confined zone in the calculation.
2) The confinement setting are only used to define material resistance factors and define ties size.
3) In core walls, distributed reinforcement between two concentred zone might be in compression even without confinement... where is the problem ?
In my understanding, the confinement are required in corner and wall end or to embded horizontal rebars.
It's not required in distributed zone. if it would be the case, everything in core wall would be confined !

Any tought ?

 

[MrHershey]

You can alter the capacity reduction factor to get what you want. If you go to Input>Reinforcement>Confinement, you have the option of tied, spiral, or other confinement. The Axial compression (a) parameter refers to the leading coefficient you see in ACI equation (10-1) and (10-2) in ACI 318-05. You could theoretically decrease this factor to account for lack of confinement if you wish. HOWEVER, one thing that I would note is ACI 14.3.6 (again in 318-05). You don't need lateral ties for wall reinforcement if EITHER the vertical reinforcement area is less than 1% of the gross concrete area OR vertical reinforcement is not required as compression reinforcement.

Hope this helps.

 

[KZSteel]

PicoStruc: My issue is that I believe if not properly laterally braced (via a tie of some sort) I can't rely on bars to act in compression. I'm thinking that spColumn knows this, but erroneously believes there to be ties in my shear wall model, thus allowing all those bars to be activated in compression.

MarkHirschi: That's a very interesting clause, regarding the <1% Ag. I'm based in Canada, so I wasn't aware of the ACI clause. I don't know if there is a similar clause in the Canadian standard. The capacity reduction factor may be exactly what I'm looking for if I don't trust those bars to be laterally braced.

 

[JAE]

We do exactly what MarkHirschi stated - alter the capacity reduction (phi) factors down by 0.1 and also keep wall reinforcement less than 1%.

 

[PicoStruc]

Confinement paremeter (a) is used for pure compression only. (Pr = 0.8 or 0.85 Pro for the canadian code)

THIS IS NOT RELATED to confinement of rebar in compression caused by wall or column bending.

My understanding is that the general practice FOR WALL is to confine ends and corner of wall and design wall tickness accordingly. The resistance calculation where compression rebar is included as effective even if not restrained by ties (like in distributed zone) are common practice.

If you disagree, please provide references.

 

[MrHershey]

Pico-

Understand that (a) is not related to confinement. This much is spelled out in the commentary to ACI, which informs that this factor is to account for accidental eccentricity. But it is a factor you could reduce to attempt to account for lack of confinement instead of altering the phi factors. As long as you're under 1% steel then no adjustment would need to be made.

Generally speaking, unless our walls are designed as special shear walls or greater than 1% steel, we do not use lateral ties to confine the walls at any location. Including ends/corners. For low levels of stress and required ductility, this is fairly common and allowed by ACI 14.3.6 as referenced above.

KZSteel-

Another idea would be to use an irregular pattern for reinforcement. Only put reinforcement on the tension end of the wall and make sure you're still okay. Obviously won't work if you reverse the direction of the load, but would eliminate the concern of spcolumn using the steel as compression reinforcement.

 

[PicoStruc]

I might be wrong, but because OP use spColumn for shear wall bending analysis and talk about confinement tie, I assumed we were talking about seismic design of shear wall.