ACI Shear Wall Shear Capacity Ch11 vs Ch 18 318-14 vs 19

[bookowski]

I'm updating an internal concrete shearwall design spreadsheet and want to make sure I'm not missing anything. There's some inconsistencies between Ch 11 and Ch18 in 318-14 (my locally adopted version of 318). I see that 318-19 acknowledges this and has updated Ch 11 but there still seem to be some oddities.

Summary:
[ul]
[li]318-14 (and earlier 318s), CH11: A fairly involved calc for wall shear capacity that considers axial force (with tension reducing the Vc) and Mu/Vu etc[/li]
[li]318-14, CH19: For special structural walls gives a much simpler formula for shear capacity that does not consider axial tension or Mu/Vu etc. It's a very simple formula that gives between 2 and 3 x sqrt(f'c) for the concrete contribution[/li]
[li] Since '14 is my currently adopted code I am using the above formulas, separating out wind vs eq checks. But it seems odd that for my seismic checks I can ignore net tension and use a much simpler formula (and often gives a higher Vc)[/li]
[li]318-19, CH11: Revised shear capacity formula that now matches the simple formula from 318-14 Ch 18, with the commentary acknowledging that it has resolved previous inconsistencies. The CH11 formula includes a simple factor for tension (as net tension approaches 500psi Vc goes to 0)[/li]
[li]318-19, CH18: Repeats the CH11 formula exactly except that it does not mention the factor for tension.[/li]
[/ul]

It seems clear that '14 and earlier had some issues that they've addressed. But why did they go through the effort to reconcile these and a) bother to reproduce the formula in CH18 vs say "refer to CH11 formula xzy", I don't know of any other formula is repeated verbatim in 318 and b) leave out the factor for tension in CH18. The fact that they bothered to repeat the formula and not include the tension factor makes me think that this is intentional. I am mostly wondering if I am missing something in my above understanding. I'm updating our spreadsheet and may just use '19 vs '14 since it's simpler, but it feels weird to have a provision in there for my EQ combos that allows me to ignore net tension. The EQ checks do have the limit of 8 vs 10 for the aggregate wall system so maybe that makes up for the free 2 in a way, but it still seems odd. Why not have one consistent formula that considers tension and avoid the confusion?

 

[lexpatrie]

Read the commentary, of course, but also look in the references section and see if there's any research that looks like it might explain the situation.

 

[bookowski]

I could read the research papers - but I'm hoping that someone on here is designing special walls often enough that they can confirm my understanding, or point me to what I'm missing.

 

[bookowski]

Bumping myself up

 

[Deker]

Thinking out loud...a wall won't experience net overall tension, but a coupled wall will resolve it's moment into axial load in the piers. While the pier in tension gets a reduction in shear capacity, the pier in compression gets an increase in shear capacity. At the same time, the tension pier experiences a reduction in stiffness and the compression pier experiences an increase in stiffness, causing the shear demand to redistribute in lockstep with the shear capacity. Consider also the short term cyclical nature of seismic loads and the proportioning / confinement requirements for special shear walls, and perhaps ACI deemed that the effect of axial load on shear capacity could be neglected?

 

[bookowski]

@Deker I understand your logic, but then why leave it in for wind and bother removing it for seismic? It's also not always the case that the tension/compression coupled walls are symmetric and would balance out. If nothing else it seems stupid just for the added complexity of calcs/bookkeeping to arrive at a less conservative capacity.

 

[bookowski]

Bump.

No one out there with a spreadsheet for shearwalls that has come across the same question?

 

[lexpatrie]

Pave the path, maybe. Post the code excerpts so nobody has to open four versions of the code to even consider your question.