ACI 318-19 Seismic Shearwall Design - Plastic Analysis to Limit Shear Demand

[sticksandtriangles]

Looking at a tall, high seismic concrete tower with the new ACI 318-19.

The new equations for design shear strength are as follows:

In our specific case, 18.10.3.1 gets capped at 3*Vu, where Vu is the shear that comes from the analysis model.

I do not see any specific out in ACI 318-19 that allows you to pick the minimum of 18.10.3.1 or the shear that is required to induce flexural yielding.

For example, in our building, we are looking at a sample wall, ~4' wide, 22'-9" long and 240ft high.

Vu = 567 kips
3*Vu = 1701 kips

Doing a plastic analysis on this wall, assuming a conservative value on the location of the shear load on the wall, 0.5 * h, the shear to induce flexural yielding = 747 kips. This plastic moment capacity was calc'd with 1.25fy.

Some sample calc's shown below:

Can I limit my design shear to 747kips instead of the 3*Vu value of 1701 kips? There does not seem to be anything in ACI 318-19 that states you can limit Ve to be the plastic shear to induce flexural yielding of the wall, but that seems to be what is at the heart of the code.

Appreciate the feedback!

 

[Deker]

The left side of the inequality in Equation 18.10.3.1 is capping the shear at the plastic moment capacity of the wall (Ω[sub]v[/sub]V[sub]u[/sub]) amplified by ω[sub]v[/sub] for higher mode affects. This value need not exceed 3V[sub]u[/sub], which historically has been the limit for the system overstrength factor (Ω[sub]0[/sub]) in ASCE 7. Perhaps I'm misunderstanding the question?

 

[chris3eb]

You might be chasing your tail here. The reason that dynamic shear amplification was introduced was because for tall slender walls the effective height of the resultant shear force becomes lower once dynamic and nonlinear effects are included. You're assuming that this height is 0.5H, but the code is telling you that they want it to be h_eff = Mp / 3Vu

 

[ggcdn]

Ωv is the ratio of Mpr/Mu, so this already accounts for the plastic mechanism. It still needs to be amplified by higher mode factor.

Research has shown that wall shears are not strictly limited by flexural yielding at the base. You can have higher mode shears that increase beyond the shear corresponding to wall yield.

-JA (working on [link calcs.app]Calcs.app[/url])

 

[Deker]

You might be chasing your tail here. The reason that dynamic shear amplification was introduced was because for tall slender walls the effective height of the resultant shear force becomes lower once dynamic and nonlinear effects are included. You're assuming that this height is 0.5H, but the code is telling you that they want it to be h_eff = Mp / 3Vu

Not sure that 3Vu is meant to capture higher mode affects by limiting the effective height. Do you have a reference? I've understood it to be a practical limit on the design shear so that strict adherence to capacity design principles is not required where deemed to be too onerous.

 

[chris3eb]

Not sure that 3Vu is meant to capture higher mode affects by limiting the effective height. Do you have a reference? I've understood it to be a practical limit on the design shear so that strict adherence to capacity design principles is not required.

I have the same understanding of 3Vu as you. I'm saying that dynamic shear amplification in general lowers the effective height. In general, the effective height is h_eff = Mp / Ve. In OP's case, Ve is 3Vu which so I substituted it in, but that might have confused the issue.

 

[Deker]

Got it, thanks for clarifying.

 

[sticksandtriangles]

You might be chasing your tail here. The reason that dynamic shear amplification was introduced was because for tall slender walls the effective height of the resultant shear force becomes lower once dynamic and nonlinear effects are included. You're assuming that this height is 0.5H, but the code is telling you that they want it to be h_eff = Mp / 3Vu

Thanks for this reference, it helps to keep reading the code, I see the definition of Heff in fig R18.10.3.1.

The way I see it written though, H eff = Mu/ Vu, not Mu / Ve (3*Vu).

This yields the following results:

Using 70ft for Heff still yields about 410 kips of less shear to design for as compared to the Ve = 3*Vu.

Is it acceptable to design to this reduced shear?

 

[Deker]

Is it acceptable to design to this reduced shear?

No, because you're missing the dynamic amplification factor and guessing at the real effective height. You've already found the yield moment M[sub]pr[/sub], which is required to determine Ω[sub]v[/sub] = (M[sub]pr[/sub] / M[sub]u[/sub]). Ω[sub]v[/sub]V[sub]u[/sub] then needs to be amplified by ω[sub]v[/sub] to capture higher mode effects. Once you've found V[sub]e[/sub] = Ω[sub]v[/sub]ω[sub]v[/sub]V[sub]u[/sub], you can determine the actual effective height using M[sub]pr[/sub] / V[sub]e[/sub] (as chris3db described above) and compare it to your initial guess of 0.5H, but it's unnecessary.

The effective height concept is useful for illustrating the shift in vertical force distribution up the wall and perhaps for preliminary design, but that's about it.

 

[sticksandtriangles]

Thanks Deker, that makes sense.

Based on these new 318-19 provisions, both increasing the demand side of the equation and reducing capacity side of the equation, I am not sure these tall concrete shearwall buildings are feasible in high seismic anymore.
This building had about 2' thick shearwalls under 318-14, and with 318-19, we are struggling to get a 3' thick shearwall to work. I had heard the changes were big in the webinars I sat through, I guess it just takes a real project for that to truly sink in.

 

[Deker]

It is a big change on the demand side for this to be required by code, but many engineers in high seismic areas were already proportioning their walls to resist V[sub]pr[/sub] so they could use Φ[sub]v[/sub]=0.75 for shear walls and diaphragms. Some were also already using the ω[sub]v[/sub] factor based on recommendations from SEAOC / Paulay and Priestley.

You may want to check your design, though, as I don't believe shear wall capacities have changed in ACI 318-19.