Ordinary Shear Wall Boundary Zones - No Requirements?

[StLHokie]

Working on a 20-story building in SDC A utilizing ordinary shear walls with pretty low wind speeds (105mph). I have a strong seismic background and am used to the detailing requirements of special shear walls and boundary zones.

With that said, I've got regions in my cores that exceed the 0.2f'c that typically would require boundary zones if the walls were special, however based solely on ACI 318-19 Chapter 11, the only requirement I can find with regards to a requirement for confinement of reinforcing is 11.7.4, where if Ast >0.01AG, longitudinal reinforcement shall be laterally supported by transverse ties, which is pretty vague.

Because of the building use and egress requirements, I've actually got two concrete cores and plenty of wall from a capacity perspective (and low base shear) to the point where right now I can get by with minimum steel, even at the base, for all demands, which means I don't exceed the 0.01AG limit. Drift is driving 12" wall thicknesses. I am also seeing concrete strain limits under 0.003

Part of my understanding is that for seismic zones, buckling of bars is more likely to occur due to the violent and rapid energy transfer during a seismic event, while buildings that experience high wind loads occur over a much longer time period. However even still, it doesn't sit right with me that no confinement of reinforcing would be required.

Has anyone run into this before? Is there any guidance anywhere for boundary zones not in seismic zones? I'm leaning toward putting in confinement reinforcement where 0.2f'c is exceeded, but it doesn't make sense that there appears to be no actual code requirement for it.

 

[KootK]

Has anyone run into this before? Is there any guidance anywhere for boundary zones not in seismic zones?

I certainly agree that this is an annoying dead zone of available guidance.

This is what I've been doing:

1) I put nominal wall reinforcing in all of my shear walls. Even the 8" walls where it kinda doesn't work spatially.

2) If it's a low rise building with squat walls and somebody really wants to not have zones, I'll consider it.

3) For a 20 story building, I'll absolutely have zone reinforcement. And 10" walls minimum.

In a fundamental way I feel that, once one assumes a cracked wall section for flexure, the wall compression block becomes a column of sorts that needs to:

a) Not have it's vertical steel buckle between ties and;

b) Not buckle between floor levels.

I realize that this disregards some of the nuance present when it's truly a wall rather than a column.

 

[OldDawgNewTricks]

Philosophically, it makes sense to me to not require special seismic detailing for SDC A. The purpose of special detailing is to provide sufficient ductility so that the required seismic energy dissipation can occur without brittle failure occurring. But this may not be an issue for some SDC A structures where wind loads are much higher than seismic loads. In the extreme case, say if you designed for seismic loads with a response modification factor, R, of 1.0 then the structure would remain elastic and ductility would not really be a concern.

That being said, the building code provisions are the minimum requirements that are legally mandated for construction. There is nothing to stop you from providing additional seismic detailing if you feel that it is warranted. Just notify the owner's representative that you will be providing this as a betterment above the minimum requirements because in your professional opinion it is warranted for a 20-story building. If it only occurs in a few locations, then the cost will probably be a drop in the bucket for the overall project, and the owner shouldn't have a problem with it.

 

[Deker]

Recognize that the 0.2f'c compressive stress limit is a proxy to evaluate compressive strains due to plastic hinge curvature in the post-yield condition. For SDC A with a wind-governed design, you're not expecting your wall to yield, so your compressive strains won't go beyond the code limit of 0.003 for unconfined concrete. If you go beyond the 1% reinforcement ratio at the jambs, provide confinement based on the column provisions.

 

[milkshakelake]

@Deker I agree with what you said. Just nitpicking a bit - even if seismic doesn't control, the shear walls will probably yield at lower stories due to wind. It might have implications for stiffness and drift. I'd just double check the rupture stress at the tension end.

 

[StLHokie]

In a fundamental way I feel that, once one assumes a cracked wall section for flexure, the wall compression block becomes a column of sorts that needs to:

a) Not have it's vertical steel buckle between ties and;

b) Not buckle between floor levels.

I realize that this disregards some of the nuance present when it's truly a wall rather than a column.

These are two good considerations, thanks.

Philosophically, it makes sense to me to not require special seismic detailing for SDC A. The purpose of special detailing is to provide sufficient ductility so that the required seismic energy dissipation can occur without brittle failure occurring. But this may not be an issue for some SDC A structures where wind loads are much higher than seismic loads. In the extreme case, say if you designed for seismic loads with a response modification factor, R, of 1.0 then the structure would remain elastic and ductility would not really be a concern.

That being said, the building code provisions are the minimum requirements that are legally mandated for construction. There is nothing to stop you from providing additional seismic detailing if you feel that it is warranted. Just notify the owner's representative that you will be providing this as a betterment above the minimum requirements because in your professional opinion it is warranted for a 20-story building. If it only occurs in a few locations, then the cost will probably be a drop in the bucket for the overall project, and the owner shouldn't have a problem with it.

I wasn't planning on going as far as detailing the boundary zones to match what would be required for a special wall system, I just feel like there should be *some* level of boundary detailing, I'm just having issues quantifying what level that should be because I don't quite know what level of plastic deformation my system will develop.

Recognize that the 0.2f'c compressive stress limit is a proxy to evaluate compressive strains due to plastic hinge curvature in the post-yield condition. For SDC A with a wind-governed design, you're not expecting your wall to yield, so your compressive strains won't go beyond the code limit of 0.003 for unconfined concrete. If you go beyond the 1% reinforcement ratio at the jambs, provide confinement based on the column provisions.

@Deker I agree with what you said. Just nitpicking a bit - even if seismic doesn't control, the shear walls will probably yield at lower stories due to wind. It might have implications for stiffness and drift. I'd just double check the rupture stress at the tension end.

Touching on both of these, I end up getting tensile stresses that exceed rupture so they will behave at least in the cracked state, but it's a good point about the 0.2f'c limit representing a true plastic limit state that includes concrete strains greater than 0.003 rather than an elastic one. Thinking about R values, if you multiple the R value of a special shear wall bearing wall system (5) by the 0.2f'c to get the actual force applied, you end up with the 1.0f'c when the concrete stress limit is exceeded, so this begins to make more sense.

I would think then in that case as long as my stress limits remain under the 1.0f'c when wind controls (R value equivalent of 1.0), I can get by without the boundary zones, since the concrete will retain confinement of the bars at that point. I will probably still throw in some confining steel for sleep at night but I believe I had a logical path forward now.

 

[Deker]

@milkshakelake - Do you mean that the shear walls will crack at the lower stories? It sounds like you may be talking about cracking and not yielding. My understanding is that the building MWFRS is expected to remain elastic for the design wind event. There is some work in its infancy that is looking at allowing limited inelasticity in the MWFRS under wind loading that would be particularly beneficial at sites with large seismic loads (Link). The elements experiencing inelasticity would require additional detailing to achieve the toughness required for excursions into the plastic range.

@StLHokie - A compressive stress of 1.0f'c and above would exceed the design strength of the concrete, right? A traditional design using the Whitney stress block would limit the compression stress to 0.85f'c, so you shouldn't be experiencing stress higher than that for wind load.

 

[milkshakelake]

@Deker You're right, I meant cracking. That would have implications for stiffness/drift, not yielding. And I've never seen this publication before, thanks for sharing.

 

[KootK]

My understanding is that the building MWFRS is expected to remain elastic for the design wind event.

That is not my understanding for ULS wind design. Or, at the least, I think that it needs additional nuance. I see it like this:

1) When it is said that the code requires lateral systems to remain elastic for design wind events, I think what they mean is that inelastic energy dissipation is not part of the program for ULS load resistance. We're not counting on deep, cyclical excursions into seriously inelastic behavior. And this is consistent with our usual treatment of wind as a pseudo static load.

2) When a designer works out her ULS wind load and then specs shear wall boundary reinforcement via Whitney stress block etc, that exercise implies reliance on the plastic cross sectional bending capacity of the shear wall and yielding of the flexural reinforcement. I believe this to be the sense in which MSL referenced "yielding". And I feel that it stands as a valid point.

 

[Deker]

I appreciate the nuance. However, it's certainly not a forgone conclusion that plastic design methods will lead to MWFRS yielding in the design wind event when you consider conservatism in analysis and proportioning, strength reduction factors, material overstrength, etc.

I believe MSL meant to reference cracking and not yielding, but I do agree that if limited yielding were to occur, standard detailing would be sufficient to meet ductility demands. Seismic detailing would be excessive, and the use of seismic provisions to evaluate the need for boundary elements to resist wind loads would be inappropriate.

 

[KootK]

However, it's certainly not a forgone conclusion that plastic design methods will lead to MWFRS yielding in the design wind event when you consider conservatism in analysis and proportioning, strength reduction factors, material overstrength, etc.

Agreed, but then that's the case with most things that we design.

I would argue that, once something is proportioned such that it relies on plastic section capacity for ULS design, that assumption should logically be carried downstream through the rest of the structure design regardless of whether or not we suspect that the thing we're designing will really plastify in the wild. Is this not just, fundamentally, how we do ULS design? I doubt that we disagree on this point.

Were I to summarize what I understand the crux of you arguments here to be, it would be as follows.

High ductility seismic shear walls need to go deep into the inelastic range cyclically whereas a shear wall designed plastically for wind doesn't need to venture into the inelastic range any further than what would be required to yield the rebar. The latter represents a far less taxing condition and this is why the detailing requirements for high ductility shear walls may not be applicable to wind governed walls.

And I agree with all of that.

 

[Deker]

So is your argument that because yielding may occur under wind load, standard detailing provisions are not sufficient?

Or are you just pushing back on my statement that the MWFRS remains elastic for wind loading? If that's the case, I'll revise my statement as follows:

My understanding is that the building MWFRS is expected to remain essentially elastic for the design wind event.

We use ULS design because it provides adequate factor of safety against collapse. That's not to say that we expect a ULS designed element to go fully plastic when it meets its design load. It's possible, but it's more likely to experience no yielding or onset yielding given the targeted probability of failure in the design standards. I believe this is considered by the code committees when it comes to driving the anticipated behavior of buildings under wind load. I also agree with you that part of what defines the MWFRS as "elastic" is the practice of not relying on inelastic energy dissipation in its design.

Edit: Yes, I like your summary. What's more, it gets back to the main point I was trying to make to the OP.

 

[KootK]

So is your argument that because yielding may occur under wind load, standard detailing provisions are not sufficient?

No sir. That said, I do feel as though low-seismic shear walls do exist in somewhat of an underserviced vacuum code wise. Are they nothing fancier than just ordinary RC beams? For the most part, the code and standard practice seem to suggest that. It doesn't feel great to me though which is, partly, why I've selectively applied some of the seismic stuff to non-seismic designs. So I feel for OP in this respect.

Or are you just pushing back on my statement that the MWFRS remains elastic for wind loading?

That one. Thanks for the clarifications.