Interaction Diagram Concrete Wall

[EDub24]

This might seem a silly question but when calculating an interaction diagram for a concrete bearing wall for out-of-plane forces what is the Pn,max? The ACI code points you to the columns section which says that your Pn,max is a portion of your Po. The reduction is based on whether you have ties or spirals that conform to code requirements. Because I'm looking at a wall I don't have ties or spirals. Should I use the full Po with no reduction or be conservative and use the smallest value (0.80Po)? That doesn't seem right but I can't find anything specific. Thanks.

 

[KootK]

I'd use the value for ties. For wall-ish reinforcement ratios (<1%), there's no concern for bar buckling but you also do not have the confinement afforded by spirals.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[WARose]

I've always had my φPn as being the limits for plain concrete (currently eq. 22-14 or 22-5 in ACI 318-11). I've always felt this is a more realistic upper bound for the axial load than doing it like a tied column. (Because it isn't.)

Most of the time you wind up with it being controlled by the flexural reinforcement anyway. (I.e. the (compressive) axial load is just gravy.)

 

[MotorCity]

Horizontal bars would serve as "ties" to prevent any buckling of the vertical bars out of plane from the wall.

 

[WARose]

[blue](MotorCity)[/blue]

Horizontal bars would serve as "ties" to prevent any buckling of the vertical bars out of plane from the wall.

Not sure I'd want to rely on that connection (i.e. between the 2 bars; or it even being there in some cases). But I guess it just depends on how daring one wants to get.

 

[EDub24]

Thanks for the feedback. My coworker's opinion was similar to KootK's response and to use the worst case (i.e. the 0.80 for ties). I'm inclined to agree with this although realistically I doubt the axial load on a wall will likely never reach that point.

 

[Celt83]

Horizontal bars would serve as "ties" to prevent any buckling of the vertical bars out of plane from the wall.

similar to WARose not sure I would count on this and in some instances based on flexural demand the vertical bars may be the outer layer.

I usually take WARose's approach and limit the upperbound axial to plain concrete which is similar to how the masonry code looks at the axial strength ie the contribution of the reinf. is ignored if it is not tied.

 

[WARose]

[blue](Celt83)[/blue]

I usually take WARose's approach and limit the upperbound axial to plain concrete which is similar to how the masonry code looks at the axial strength ie the contribution of the reinf. is ignored if it is not tied.

The masonry analogy is a good one. A lot of times when checking a wall after I've drawn my crude/conservative, lower bound interaction diagram (i.e. consisting of 2 straight lines connecting 3 points (max. P, pure max. bending capacity, and tensile strength based on vertical re-bar capacity)), I've back checked using a working stress design approach. (Similar to masonry.)

 

[Deker]

ACI 318-11 does allow you to use untied compression reinforcing in walls when designing per chapter 10 (see section 14.3.6). That said, the testing that resulted in the empirical method of section 14.5 showed that reinforcing ratios had no effect on the wall capacity.

 

[MrHershey]

@Deker: Would caution on using that clause in ACI 318-11 for compression reinforcing. I agree with your reading, but if you look at the exact same clause in ACI 318-14 (now located at 11.7.4.1) it now reads: "If longitudinal reinforcement is required for axial strength or if Ast exceeds 0.01Ag, longitudinal reinforcement shall be laterally supported by transverse ties."

So you'd have to tie your longitudinal rebar if you need it for compression under ACI 318-14, even if it's under 1% steel.

 

[Deker]

Thanks MrHershey. I hadn't read the updated clause.

 

[KootK]

The version in 318-14 is a typo. It should read as shown below. It wouldn't make sense for there to be no way at all to utilize compression steel in an untied wall.

If longitudinal reinforcement is required for axial strength or AND if Ast exceeds 0.01Ag, longitudinal reinforcement shall be laterally supported by transverse ties.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[MrHershey]

@KootK:
That was my assumption as well. I assumed that in 'fixing' the double negative from ACI 318-11 and prior, they accidentally changed the meaning.

Been trying to clarify with an ACI 318 committee member and code consultant and at least his first response seemed to double down on current wording of ACI 318-14, that the OR is correct.

 

[KootK]

Frightening. Thanks for the update MrH.

As it stands, a basement wall with >1% I.F. reinforcing would need to have its verts tied. Our contractor friends would love that. I'd say no, like Ofred refusing to stone Ofwarren to death in The Handsmaid's Tale. Sometimes my wife forces me to watch what turns out to be really good television.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.