Unfortunate Shear Wall Location 10

[JAE]

We have a project and have come upon a problem - wondering if others have an approach to deal with it.

1. Single story building - generally rectangular in plan with one of the long walls having a jog or offset. There is also a small offset in the short wall near the long wall offset.
2. The building is supported by perimeter load-bearing precast wall panels all around which serve as shear walls as well.
3. The roof is steel joist with metal decking.
4. Wind case perpendicular to long walls.
5. Because of the jog in the long wall, there is a short precast wall segment about 3/4 of the way down the wall.
6. This wall segment acts as an intermediate, short shear wall and with classic tributary wind design attracts a LOT of load.
7. If we model a semi-rigid diaphragm, the shear wall still attracts a lot of load.
8. With the offset in the short wall, the "end wall" of the long rectangular diaphragm has two separate shear walls - an inner wall and an outer wall.
9. The inner wall attracts a lot of the load and the outer wall is actually attracting a small amount of load in the opposite direction due to bending/deflection in the diaphragm.
10. We have included spring supports to model the shear wall stiffnesses as well - this doesn't help all that much.

Is there any approach to analysis where you can minimize the load in these short segment offset shear walls? We thought of having a slip connection between wall and diaphragm to release the shear from it but that is very complicated and of questionable efficacy.

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[bhiggins]

I'm not a tilt up designer, but I take KootK's approach when designing wood shear walls. If there is a shear wall too small to be usable, I'll not use it to brace the structure and design the other walls with a larger tributary. In the case of a semi-rigid diaphgram, my rationale is that the hold-downs will yield and allow for small rotations, or the concrete may crack which will relieve stresses and cause the load to be redistributed to the next brace-wall line.

We see wood structures all the time with jogs and small panel segments. Wouldn't these shear walls fail all the time if they were attracting so much lateral loads?

 

[sandman21]

A foolish and perverse part of me looks forward to my day in court. As long as I'm allowed to say my piece, I feel that I could explain this successfully. I want my Jack Nicholson moment. Lilly livered Joe Publics out there don't know how soiled our hands and souls get trying to keep them safely and economically sheltered. We are the men in the night on the wall. And the gals too of course,

A speech admitting intentional wrongdoing......

 

[KootK]

A speech admitting intentional wrongdoing......

Only from the perspective of the lilly livered Joe Public adjudicating the trial & watching the movie. I didn't get the impression that Jack left the courtroom feeling that he'd done any genuine wrong. A mere technical transgression en route to serving the greater good. And the same would hold true for me.

In the case of a semi-rigid diaphragm, my rationale is that the hold-downs will yield and allow for small rotations, or the concrete may crack which will relieve stresses and cause the load to be redistributed to the next brace-wall line.

Fascinating. I was thinking pretty much the opposite thing just last week. I deal with a lot of wood frame buildings built on 2nd floor precast podium slabs. You end up with hold downs anchored into the ends of hollow core with the shear walls overhanging the slab by some amount. Those might be good for 7K and their failure mode would definitely be brittle concrete breakout. Got me to thinking that, given:

1) Who knows how stiff wood shear walls really are...

2) Mixing and matching perforated and segmented walls probably doesn't aid precision...

3) Varying degrees of shear wall rotational restraint offered by the precast deck in different locations...

4) More possible load paths than stars in the sky...

...how sure am I that my 6K hold down requirement on my 7K hardware isn't really a 9k brittle failure? Not. Too. Sure. I totally agree with your point though. You're average wood building is 1000% more complex than your average precast wall panel building. And, again, life must go on.

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.

 

[JoshPlumSE]

So, if I understand KootK's premise, it would be to design the adjacent shear walls for the extra loading that probably goes to that wall. With the understanding that when this little wall fails (or just becomes flexible enough that it's not very effective) the load properly re-distributes to these outer walls.

I can get on board with that concept, but feel it needs some additional caveats.
a) If I've got gravity only columns in this area, I want to make sure they can take increased diaphragm deflection (and P-Delta effect) associated with the larger diaphragm displacements. Essentially, making sure the gravity load system isn't compromised when that wall goes down and larger deflections occur.
b) The diaphragm and chords (of course) must also be designed as if that unfortunate wall segment doesn't exist so that once it yields / fails, the rest of the lateral force resisting system remains intact.
c) The diaphragm connection at this shear wall should be designed for something close to the full seismic drag load into this unfortunate wall segment. Otherwise, you're going to have diaphragm separation / failure at relatively low loads. Alternatively, you could warn the owner that this are should be inspected periodically for damage and if it's not it has the potential to cause building envelope problems (i.e. leaks).

This is a perfectly rational approach, IMO. It does kind of walk the line between regular code based design and something akin to a 'performance based' analysis of the system. If it were my project, I might want to check out the ASCE 31/41 requirements for elastic analysis forms for performance based design to see if there are any key aspects of the analysis that we've missed. But, it's not (and I'm not a glutton for punishment) so I'll leave that to others.

Personally, I'd prefer to replace this unfortunate wall segment with a stud wall without end hold downs. That way it's clearly not going to behave as a shear wall and no extra justification is required. None of this means there is anything "wrong" about the KootK approach.

 

[KootK]

So, if I understand KootK's premise..

You got it.

...but feel it needs some additional caveats.

For me, personally:

a) Yes, if you're into that kind of thing.

b) Sort of. On these types of buildings, with essentially 360 shear wall, it's pretty easy to argue that you don't actually needs chords. Or, more accurately, that it was ridiculous to really think of them as participating meaningfully in the first place.

c) No. It would be pretty hard to design the dia / wall connection for all the load that it might see. And I'd argue that you could tear the diaphragm right off over the length of the short walls and it would still function in the macroscopic sense. The short wall itself ought to remain stable post rip owing to connections to it's neighbors.

None of this means there is anything "wrong" about the KootK approach.

Meh, it's a little wrong in the Jack Nicholson sense.

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.

 

[sandman21]

Only from the perspective of the lilly livered Joe Public adjudicating the trial & watching the movie. I didn't get the impression that Jack left the courtroom feeling that he'd done any genuine wrong. A mere technical transgression en route to serving the greater good. And the same would hold true for me.

If he truly believed he was right he would have taken responsibility for his actions and not lied in an attempt to have others shoulder his responsibility. His attempted attacked on and bravado towards Kaffee, was an attempt to hide his shame for being caught not living up to his responsibility.

 

[KootK]

If he truly believed he was right he would have taken responsibility for his actions and not lied in an attempt to have others shoulder his responsibility. His attempted attacked on and bravado towards Kaffee, was an attempt to hide his shame for being caught not living up to his responsibility

We must have watched different movies. I saw a maniac who believed in his own logic to the core and lied about it because that was the only practical way to execute his vision. He only outed himself out of frustration that he was forced into lying by the civies. Movie aside, in real life, I think it perhaps a bit naive to assume that:

Folks believing that their right = folks not lying and happily taking responsibility for their actions. Certainly, this is not true of any of my children.

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.

 

[haynewp]

IBC has things about seismic isolation of non-participating masonry and concrete elements or detailing for deformation compatibiltiy. I think you would have a hard time arguing this other way through the permit office.

 

[KootK]

I can't speak to jurisdictions where I've not done work but, where I have, it goes utterly unnoticed through the permit office. Middle earth/america. Take a spin through any industrial park east of the rockies and these things are more common than nipples.

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.

 

[KootK]

My gut feel is that the prohibitions get more enforcement when they pertain to the situations that probably spawned them in the first place: crusty infill walls gumming up nice flexy frames and shearing off column tops.

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.

 

[Backcheckrage]

In our office we wouldn't consider the short jog wall as participating because it is surrounded by much longer stiffer walls. I'm surprised your semi-rigid analysis didn't capture that... and more surprised that the soil springs under the walls didn't confirm that as well.

In any case, we'd merely exclude it from overall analysis, OR detail a slip connection so the diaphragm won't pump shear into it.

 

[Once20036]

b) Sort of. On these types of buildings, with essentially 360 shear wall, it's pretty easy to argue that you don't actually needs chords. Or, more accurately, that it was ridiculous to really think of them as participating meaningfully in the first place.

b. We have modeled the diaphragm as a series of "horizontal beams" along the edges, so to speak, with a stiffness both in flexure and shear that attempts to mimic the deflection behavior of a metal deck diaphragm under the proposed loads.

I`m curious about this, but perhaps the question is best suited to its own thread...
Say you have a neat rectangular building, with no unfortunate shear walls.

KootK is indicating that no chord members are necessary, while JAE is accounting for the flexural stiffness. Seems to me that these two statements are at odds with one another.
I believe that the deflection of diaphragms is, for most normal geometries, controlled by shear deflections and not flexural deflections, however, I've never considered a diaphragm with no flexural load path.
When I`m describing diaphragms to the younger generations, I always compare them to a beam. How can we have a beam without any flexural capacity...?

KootK - are there any other diaphragms that you design with no chords or collectors?
JAE - how do you account for chord forces, given that you're counting on the flexural stiffness?

 

[DaveAtkins]

When I run into issues with justifying the absence of proper chords, I look at the diaphragm as two separate three sided diaphragms. Of course, you must have shear walls on three sides of each diaphragm to do this.

DaveAtkins

 

[JAE]

Once20036, there are perimeter steel angles (supporting roof deck on the inside face of the panels that can serve as chords.

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[KootK]

KootK is indicating that no chord members are necessary

No, not really. Although I can see how my previous response suggests that. No matter how the flexibility splits out, I don't think that you can have an equilibrium satisfying diaphragm that doesn't need to have a flexural mechanism. What I'm saying, more precisely, is that the thing that we typically, and often quite reasonably, call the "chord" doesn't actually have to be the chord. In the context of this conversation, the chord could also be:

- The wall panels themselves if the wall panels are interconnected.
- The grade beam / frost wall if the wall panels are not interconnected.

Precast wall panels are pretty stiff in plan, especially if they have in plane shear transfer capacity at the joints. We provide a wee angle at the deck edge because it's clean, and I think that the code may suggest it, but in scenarios like this, I just don't see the angles picking up a lot of axial strain. For thermal considerations, it's actually much nicer to have the angles not be continuous.

This isn't dissimilar to the case with parallel to framing CMU walls where the deck angle often is not continuous and you are, rather, relying on bond beams in the neighborhood of the deck edge to be the designated chord. Of course, in many instances, that bond beam has a bunch of jogs in it to capture deck slope and you're back to theoretically questionable once again. And really it's the CMU wall and foundation doing the work just like with the precast panels. If you ever actually strained your bond beam reinforcing the way that our calcs would suggest, your poor wall would be a cracked up mess.

KootK - are there any other diaphragms that you design with no chords or collectors?

Yes. I don't rigorously implement "chord technology" in the jogging perimeter walls common to residential complexes. I don't have a snappy technical argument for that though. It's just hard work, expensive detailing and, more than anything, I just have next to no faith in light frame wood lateral design methods. That stuff's great fun to talk about and read about in the Terry Malone sense of course. For me, however, I've got that filed under "intellectual self-gratification".

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.

 

[KootK]

Finally found the last thread that I recall dealing with this: Link. In it, CELinOttawa pointed us to a nifty, light duty slip device. Pretty cool. One thing that confuses me about the whole slip / soil spring business is that, in most incarnations that I've seen, the short walls still wind up getting inadvertently, in plane shear connected to their perpendicular neighbors for a variety of reasons, some of which I've mentioned above. Once that happens, it's hard for the diaphragm to deflect without pushing the short wall even with diaphragm slip capability. And at the foundation, the short wall and it's perpendicular neighbors are probably sharing the same continuous grade beam or frost wall. So you're engaging all kids of extra bite there as well.

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.

 

[Once20036]

The grade beam / frost wall if the wall panels are not interconnected.

I`m with you for most of your post. I agree with you and JAE that a perimeter angle (if continuous) could function as a chord, as could properly detailed and connected walls panels.
I could even support masonry bond beams, despite their jigs and jogs. But a frost wall? A concrete wall 20' below the diaphragm can act as the diaphragm flexural mechanism? I think this is where you lose me.

 

[P205]

I second Once20036's comment, I'm curious about the frost wall acting as a chord.

 

[KootK]

But a frost wall? A concrete wall 20' below the diaphragm can act as the diaphragm flexural mechanism? I think this is where you lose me

.

I second Once20036's comment, I'm curious about the frost wall acting as a chord.

Then the show must go on. The concept essentially just takes the deck shear around the bend and utilizes the wall panels as very stiff extensions of the "web" in our beam analogy. Most of us carry truss/beam analogy, WL^2/8 diaphragm model around with us in our heads. I think that an expanded, more nuanced definition of "chord" would be:

Any thing or assemblage of things, located anywhere in space, that sufficiently limits diaphragm strain at the deck edge.

Of course, the more complicated one makes the load path, the more work it takes to demonstrate sufficiently restrained deck strain. For most building morphologies, it's not worth the effort.

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.

 

[JLNJ]

Nice sketch. In long squatty masonry buildings, those shear walls probably suck up that diaphragm shear long before those two #5s in a bond beam can stretch to fully act in tension.

When you understand load path, a whole world opens up to you. I'm trying to emphasize load path to a recent graduate I sit next to, but he seems to prefer to enter data into a program. He asked me what I used to design reinforced masonry - I just held up my pencil.