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Flexible Diaphragm & 3 Shear Walls
2

Flexible Diaphragm & 3 Shear Walls

Flexible Diaphragm & 3 Shear Walls

(OP)
I've gotten into some discussions with coworkers recently about whether or not it's appropriate to use only 3 shear walls to stabilize a single story building with a flexible diaphragm.
To create an arbitrary example - say you have a 100' x 100' building with a shear wall or lateral frame on the west, north, and east walls.

When the wind blows north/south, I think we can all agree there aren't any issues.

When the wind blows east/west... for a rigid diaphram nobody in the office sees any issues. The north shear wall would take the load and the north/south would act as a pair to eliminate the eccentricity, but is the same true for a flexible diaphragm?

Typically a flexible diaphragm is analyzed as a simply supported beam, and a simply supported beam isn't stable if there's only one support.

A reasonable counter argument is that flexibility only has to do with the difference in stiffness between the supports and the diaphragm. A diaphragm will still have enough stiffness to utilize the east/west walls as torsional restraint.

The first question is simply how other people treat this condition and whether or not it's typical.
The second question is how to quantify that the deck is stiff enough to utilize the torsional restraint.

Thanks in advance!

RE: Flexible Diaphragm & 3 Shear Walls

This is, I believe, the type of structure that existed in those apartment buildings that fell down in Northridge.

You can pull the full shear to one side and then design the transverse frames to resist the moment created by the asymmetry. But, you will get very large deflections at the side of the building without a lateral frame. Is this technically unstable? No, but it doesn't have a lick of redundancy either.

If it were me, I would put a series of moment frames on the one end instead of just gravity posts. You're not taking up much more space than the gravity columns, but you are vastly improving the safety / redundancy of the structure.

Just my $0.02.... What do I know about what designers are really doing though? I'm mostly a computer / FEM jockey at this stage of my career.

RE: Flexible Diaphragm & 3 Shear Walls

I avoid that type of structure and provide a load path on all sides. A channel shape is a terrible choice for resisting torsion.

RE: Flexible Diaphragm & 3 Shear Walls

Yup, these are a seismic disaster... And Hokie's comment is dead on; In fact the channel shape created by these three-walled monstrosities would only be torsionnally stable were the load applied out and away from the "middle" wall (check out the location of the Shear Centre for channels).

RE: Flexible Diaphragm & 3 Shear Walls

In Guide to the Design of Diaphragms, Chords, and Collectors (http://shop.iccsafe.org/guide-to-the-design-of-dia...), there is an example problem with a 3 braced frames and a bare metal deck diaphragm. The text strongly recommends against this arrangement.

I don't have the book in front of me and I read it several years ago. I think they treated the diaphragm as a cantilever and calculated the chord forces which is then transmitted into the perpendicular frames.

RE: Flexible Diaphragm & 3 Shear Walls

@CELinOttowa

They are stable, but not efficient - just as a channel is stable even though it does not get loaded thru it's shear center.
The APA's diaphragm guide has an design example. I design these quite frequently for garages. Takes some additional detailing, however.
We have little seismic load in our area.

RE: Flexible Diaphragm & 3 Shear Walls

(OP)
I`m not familiar with the collapse in Northridge, but I'd like to read more about it. When & where was it?

To reiterate - we're only discussing flexible diaphragms here. Does anybody see any issue with this system for rigid diaphragms?

I`ll look into the guides listed above - Thanks.

RE: Flexible Diaphragm & 3 Shear Walls

Hard to say without the dimensions. The diaphragm may not be truly flexible or rigid, but somewhere in between. If you want to use flexible diaphragm assumptions then no of course it isn't stable. If you want to utilize rigid behavior than design/analyze it as such. You could design it for both conditions and pick the worst case. I don't really understand the point of the question.

RE: Flexible Diaphragm & 3 Shear Walls

This configuration is used often for large warehouse/data centers using tilt-wall panels around the perimeter where there is an expansion joint. This creates very stiff lateral elements on 3 sides and none at the expansion joint. The diaphragm and the lateral elements perpendicular to the load direction must be designed for all of the torsional load cases.

RE: Flexible Diaphragm & 3 Shear Walls

steellion,
That may be true, but not on my watch. I think anyone who designs that type structure is kidding himself.

RE: Flexible Diaphragm & 3 Shear Walls

@Hokie

A channel is not efficient for torsion in the traditional sense, but what you have is a very short channel (relatively speaking) with cover plate on top.
Take a piece of channel with similar proportions as a warehouse, weld a cover plate on the top and anchor all corners to a base. It becomes extremely stiff.

RE: Flexible Diaphragm & 3 Shear Walls

EE,
I'm still not buying it with a flexible diaphragm, especially for a large structure in plan with expansion joints. The load path is not reliable enough to suit me.

RE: Flexible Diaphragm & 3 Shear Walls

(OP)
Of course a real life diaphragm is neither 100% flexible nor 100% rigid, but somewhere in between. For a 4+ lateral frame system I'd agree wholeheartedly with designing for both rigid and flexible and taking the worst case, however, that's not an option here. As truly flexible, it's not stable. As truly rigid, it's fine - but I`m not sure that a rigid assumption is valid here.

I agree that a small steel channel with a cap plate would be suitable in torsion, however, this approximates a rigid diaphragm. What about a channel without the cover plate? Someone mentioned large deflections for the opposite walls under the torsional load case - I assume this would be out-of-plane deformations?

As discussed above, I've seen this scenario come up for large warehouses with an expansion joint, sometimes with tilt up walls sometimes without, and it also comes up with new construction adjacent to an existing building.

The point of the question? To learn a little something and better understand how buildings behave.

RE: Flexible Diaphragm & 3 Shear Walls

Well, a couple of the above posts mentioned it, but this can't be idealized as either flexible or rigid per ASCE 7-10 section 12.3. You have to take into account the actual diaphragm stiffness. This would then be classified as an extreme torsional irregularity per Table 12.3-1 and you then have to run a modal response spectrum analysis on a 3D model. You also have to increase forces and apply several other provisions.

It's not prohibited, and it happens (think an aircraft hangar with an open front), but it's not ideal.

RE: Flexible Diaphragm & 3 Shear Walls

To add to what Gumpmaster and maybe some others said, it depends on the relationship between the diaphragm stiffness and the shear wall (or frames) stiffness. This is what determines if it is rigid, semi-rigid, or flexible. This is true for all structural systems. For a lot of common systems, say a one story retail building with CMU/concrete walls, three walled building with an open storefront with no shear wall at the front, and steel deck diaphragm, the problem is the deflection of the diaphragm will almost always exceed that of the shear wall story drift (for common one story configurations), making it a flexible diaphragm. Of course, it depends a lot on the geometry of the overall building, lengths of the shear walls, openings in the shear walls, stiffness of shear walls, etc.

I have never been comfortable modeling steel deck as anything other than flexible and then using braced or moment frames along the front of the building, or adding enough CMU wall along the front to make it work.

It has been a while, but I think the Steel Deck Institute Design Guide has three-sided diaphragm examples.

This link is Canadian and deals specifically with wood-frame, the concepts are well explained and illustrated.
http://www.fpinnovations.ca/ResearchProgram/Advanc...

RE: Flexible Diaphragm & 3 Shear Walls

I really meant all columns. From an analysis standpoint, I think it is possible that these secondary shears / moments will get almost entirely sucked into the lateral columns.

But, I think that's a case where I'd be concerned enough about the gravity only columns, that I'd take a really close look at the analysis results for them. If the analysis did not show them getting any decent moment then I would design them to resist the a moment equal to their axial force times the diaphragm displacement at that location. I would also want to look at the diaphragm to column connection and make sure that it could adequately handle the displacement and shear that would be produced by this. Beefing up the column does little good if the middle of the diaphragm doesn't have sufficient connectors to ensure that these leaner columns stay connected.

Not sure that these "leaner" columns will ever really see this moment. But, this system has so little redundancy that I wouldn't feel comfortable about it if I didn't do that.

Even so, this is a totally theoretical discussion for me. I'm not putting my stamp on any drawings these days. Even if I were, I would not use this system in a high or even moderate seismic zone.

RE: Flexible Diaphragm & 3 Shear Walls

Gumpmaster,
All the hangars I have seen have a portal frame across the front.

RE: Flexible Diaphragm & 3 Shear Walls

Portal frames on hangars seems to be the norm for warm climats, while massive flat moment resisting frames or end-wall openings are most common in cold climats. Either way, these are not examples of three walls with a diaphragm.

RE: Flexible Diaphragm & 3 Shear Walls

True, but the portal frames at the front of the hangars are significantly less stiff than the other three sides. I've seen several helicopter hangars that were 12" CMU on three sides with steel portal frams in the front. When you look at the relative rigidities the portal frames might as well not be there and you still have an extreme torsional irregularity. Different in the specifics, but the same effect.

RE: Flexible Diaphragm & 3 Shear Walls

Gumpmaster- In your example, assuming the roof system is steel deck, I think you would most likely have to analyze it as a flexible diaphragm, because the diaphragm deflection will greatly exceed the CMU shear wall story drift. Then with a simple flexible diaphragm the front portal frames will resist half the lateral load when the lateral load is parallel to the front. But you would have to make sure the portal frame story drift is less than half (I think) of your diaphragm deflection.

Take that same hangar example, add some concrete to the deck to make the diaphragm rigid, then I agree that the CMU walls will attract all the shear and the portal frames will become useless.

Found this article about diaphragms which cites the IBC:
http://www.gostructural.com/magazine-article-gostr...

RE: Flexible Diaphragm & 3 Shear Walls

Except you can't idealize it as flexible with moment frames (ASCE 7-10 12.3.1.1). You have to consider the actual diaphragm stiffness.

RE: Flexible Diaphragm & 3 Shear Walls

For what it's worth, the ASCE trial problem for this month is a building with masonry shear walls on three sides and a storefront on the fourth wall - no moment frame.

RE: Flexible Diaphragm & 3 Shear Walls

(OP)
Where do you access these trial problems?
I just checked out the AISC website and didn't see anything.

RE: Flexible Diaphragm & 3 Shear Walls

@Hokie93 - it looks like the question is still up on the website. Did they ever publish any findings or common responses that you are aware of?

RE: Flexible Diaphragm & 3 Shear Walls

I don't much like the three sided lateral systems either. However, I think that their portrayal in this thread is unduly harsh. To that end, I'll be the devil's advocate for a spell:

1) The ASCE test for rigid versus flexible behaviour is really only germane to an investigation of load distribution to vertical SFRS. It isn't relevant to a load distribution for a three sided building where the load is being applied parallel to the open side and it is obvious where the direct shear goes. That test shouldn't be used as a litmus test for whether or not a three sided building diaphragm can be employed.

2) The argument that the shear centre of a channel is outboard of the channel section only applies to the purely flexural component of shear wall deformation and, then, only if the three shear walls are detailed to act compositely (likely true in CMU, less so in wood frame). In these kind of buildings, often shear deformation dominates. And for pure shear deformation, the shear centre coincides nicely with the shear wall opposite the open side.

3) As Gumpmaster implied, for many real world scenarios involving three sided buildings, the three sides would be so stiff relative to any open side moment frame that there may not be any point to the moment frame. If the three sides were CMU, for example, you'd probably have to utterly destroy the CMU before the moment frame would really kick in. Obviously, much depends on the aspect ratio of the three sided diaphragm.

4) A three sided diaphragm, in any material, is absolutely a rigid diaphragm when considering load directed parallel to the open side. This ties into point #1: questions of diaphragm rigidity are only pertinent when considering load distribution to the vertical shear force resisting system. One of the few nice features of a three sided diaphragm is that load distribution is pretty straight forward.

In answer to the two questions posed by the OP:

1) The first question is simply how other people treat this condition and whether or not it's typical. It is common of a certain class of building, often retail or warehouse. I handle it in a few ways. Firstly, I ensure that I haven't violated any diaphragm aspect ratio limitations that are code mandated for the material being used. Secondly, I'll use discrete diaphragm bracing if I'm not comfortable with the flexibility of the diaphragm. Lastly, for seismic loads, I'll use an old trick of professor Paulay's. I'll make sure that if my direct shear wall yields (rare), the two perpendicular "flange" walls remain elastic.

2) The second question is how to quantify that the deck is stiff enough to utilize the torsional restraint. There is no binary "rule" for this. Calculate drift at the open side considering vertical LFRS flexibility and diaphragm flexibility. I the drift satisfies code limits and is compatible with the deformation capabilities of the cladding, lean-on columns etc, you're done.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Flexible Diaphragm & 3 Shear Walls

KootK; Say what you like, but I have seen many of these building on the ground following EQs.

Perhaps, and I only say perhaps, this system is fine for non-seismic areas. That said, it would take a great deal of significant detailing, site verification above the norm, and is very unlikely ever to make me (or many other of your peers as shown by this thread) happy with the design.

Codes are a legal minimum, and particularly in the case of high seismic zones, should not be used as a target for buildings outside of the "norm". Without meaning to offend, I will pass on some advice I received from my first mentor upon graduating: Sometimes when everyone is saying "no", it isn't a challenge, but a lesson.

RE: Flexible Diaphragm & 3 Shear Walls

There is some further relevant info here:

http://www.eng-tips.com/viewthread.cfm?qid=356606

Good points by Kootk. Although I'm not sure if I completely understand this:

Quote (Koot)

Lastly, for seismic loads, I'll use an old trick of professor Paulay's. I'll make sure that if my direct shear wall yields (rare), the two perpendicular "flange" walls remain elastic.

Maybe I'm confused by what you mean when you say the wall has 'yielded'.




EIT
www.HowToEngineer.com

RE: Flexible Diaphragm & 3 Shear Walls

The comments about the portal frame (or the moment resisting frame if you prefer) across the open end being useless are, in my opinion, incorrect. In ultimate strength design, the most important thing is to have defined load paths. If you had a portal framed building, then introduced stiff walls on one end and two sides, does that make the building less strong? No, it doesn't.

I agree with CEL...there are inherent problems with depending on only three sides, and I won't do it.

RE: Flexible Diaphragm & 3 Shear Walls

@RFreund: my strategy is a loose interpretation of the one employed in this paper: Link. In short, if the "web" wall taking the direct shear develops a plastic hinge to cap the seismic demand, I ensure that the two "flange" walls stay comfortably elastic. That way, there's a competent mechanism for resisting torsion throughout the entire load history. This is more likely to be a concern if the web wall is short or if it's a steel brace instead of a wall. The usual configuration that I deal with is masonry or wood walls where the the wall opposite the open side is relatively long compared to the end walls. In that scenario, everything remains elastic.

@CEL: no offense taken. I generally take a lot of heat for being excessively conservative so it's fun for me to be on the other side of the fence for once. If you re-read the first sentence of my last post, however, you will see that I mostly agree with your concern.

@ Everybody: to add some more food for thought:

1) I just got back from a three week tour of Japan and southeast asia. The dominant building form by far in that region consists of permutations of the building shown in the attached photo. 90% of the building stock is multi-story concrete buildings with concrete shear walls on three sides. While there certainly are buildings like this that have collapsed during earthquakes, the overwhelming majority of them seem to have remained standing. And we're talking about tens of millions of them located around the pacific rim from what I saw. In particular, most of the three sided buildings in Kyoto have remained in tact.

2) In my north american experience, most three sided buildings are elongated rectangles where the "web" walls are relatively long compared to the "flange" walls. This "C" shaped system is fundamentally not much different from an "I" shaped system which is quite common in mid rise hotel construction. This is particularly true when one considers the impact of code minimum eccentricity. Do we have the same beef with "I" shaped LFRS configurations? I've done several of these using wood and CMU/precast systems with long "spines" down the corridor.

3) In NZ, there is research interest in looking at buildings braced by a single central core. The question becomes this: when the core develops a plastic flexural hinge due to direct shear, what then is the remaining mechanism for resisting torsion? Great question, particularly if you've considered your elevator banks to be a pseudo-closed sections.

4) Is a cantilever really anything more than half of an equivalent simple span? Just sayin'.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Flexible Diaphragm & 3 Shear Walls

@Hokie:

1) My argument is not that adding a portal frame weakens a three sided building. It doesn't. My argument is that adding a portal frame doesn't strengthen a three sided building if the two systems never get a chance to act concurrently due to stiffness/deformation incompatibility. I agree that there is some improvement in redundancy however. This, particularly, given that the usual scenario will be the "web" wall going plastic rather than losing its load carrying capacity altogether.

2) Under seismic loading, most of our buildings become something akin to three sided buildings at some point in their loading histories. Even in a building with shear walls on all four sides, the two walls taking direct shear will not yield at the same time. After the first wall yields, any subsequent load will be resisted by what is effectively a three sided lateral system. Of course, torsional inertial of the diaphragm comes into play with this as well. Have fun quantifying that. This ties back in with the strategy of Paulay's that I mentioned above.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Flexible Diaphragm & 3 Shear Walls

Good to know, and I appreciate the clarification.

Re your photo: These are popular in NZ as well, though usually older designs (again similar to the attached) but narrow and long with lots of good connection to multiple floor slabs is not where this becomes a problem.

In discussing a three sided box design, I have been thinking more the car dealership, grocery store, or strip mall type application. Effectively my concerns occur with square or flag-proportion rectangular where the long side is the open wall, not a tube without a cap (though I am still not a fan). In the NZ examples.the front wall is typically detailed as a concrete frame.

Please note that NZ and Japan both tend to use reinforced concrete floors, making the diaphragms here close to ideally rigid. That helps, and along with the proportions, helps a lot.

RE: Flexible Diaphragm & 3 Shear Walls

We don't typically allow 3 sided buildings (per hokie66's view) but in some cases we do - these are typically light framed buildings such as one-story garages in apartment complexes where there is a long series of overhead doors. In those cases, the diaphragm aspect ratio is very large such that the diaphragm deflection, and thus the potential for second order effects, is small.

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RE: Flexible Diaphragm & 3 Shear Walls

Here I'll have to disagree CEL. All other things being equal, I think that a rectangular building is better off with one of the long sides being open than it is with one of the short sides being open. Less eccentricity that way. Also, I still contend that diaphragm stiffness is not a particularly relevant parameter for a three sided building. Stiffness only comes into play for the load distribution to the VLFRS and, to a much less important extent, for P-delta effects. In a three sided system there is little redundancy so the load distribution is apparent. Diaphragm strength matters but that's another story and is generally resolvable.

My best hope for the SE asia building type is that the concrete floors and walls will form kind of a shitty moment frame. I think that was your point in your second sentence. Unfortunately, I got to see a few of these under construction while I was there. The slab / to wall rebar detailing leaves much to be desired. While in Hanoi, I spent a few days at a local structural engineering office. They tell me that these buildings are usually "designed" by the contractor rather than by an engineer. The examples that I saw seemed to have way to much bottom steel and not nearly enough top steel / corner bars at the slab to wall connection.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Flexible Diaphragm & 3 Shear Walls

If the shear system is shaped like an I beam, I have no problem with it. Not so if it is in the basic shape pf a U. Never would do it either.
.

Mike McCann, PE, SE (WA)


RE: Flexible Diaphragm & 3 Shear Walls

I agree, Mike. The I shaped resisting system is fundamentally different than the C shaped system, and I don't understand why KootK thinks otherwise.

RE: Flexible Diaphragm & 3 Shear Walls

Maybe KookK is crayyyyyyyyyyyyyz. It's entirely possible. KootK hasn't slept in days, since returning from Asia, due to the worst case of jet lag imaginable. I wish I had a scanner at home. Let's see what I can do with just prose.

Firstly, imagine a channel and a wide flange beam, both with flanges of equal dimensions that will be held constant. Next, extend the overall depth of both sections towards infinity and note the effect on the shear center of the channel. It moves towards the center of the web. So, taken to the limit of this mental experiment, the torsional behavior of the the two sections is the same. That's why I feel that, for the normal north american case of the open wall length >> than the side wall length, the torsional behavior of a "C" shaped shear wall configuration is not so bad compared to that of an "I" configuration. And we generally seem to be quite comfortable with the "I" shaped layout for some reason.

The previous paragraph assumes that flexural deformation predominates and that the wall segments act compositely. For the usual case of a three sided low rise building, that's unlikely -- shear deformation will predominate. Moreover, if the construction is wood, the walls will not normally be assumed to act compositely. And, of course, if the wall segments simply don't touch, they again won't act compositely. All of this means that, for a low rise building, the assumption of independently acting shear walls is more reasonable than the assumption of composite action. And the shear center eccentricity for a group of walls in a non-composite "C" layout is less than it is for the same group of walls viewed as flexurally composite. So, again, this adds more credibility to the notion that "C" configurations aren't that much worse than "I" configurations.

For sport, imagine a hotel that is 60' x 300' in plan with non-composite shear walls. Give it 60' shear walls at the short ends and a single 250' shear wall on one side of the corridor, say 5' off center. Assume that accidental torsional eccentricity amounts to 5'. Call this the "I" configuration. The "C" configuration would have the 300' wall moved all the way to one side of the plan. The direct shear on the 300' wall will be the same in for each case (V). For the "I" configuration, the end wall shear would be [V x 10' / 300' = V/30]. For the "C" configuration, the end wall shear would be [V x 35' / 300' = V/8.6]. Yeah, it's three times as much as the "I" configuration. However, it's still a relatively small and manageable number.

I misspoke somewhat in my previous post. For a three sided building, diaphragm stiffness does matter. It matters very much for drift prediction and overall P-delta stability. It does not matter, however, for determining the load distribution to the VLFRS. Nor does the ratio of diaphragm to VLFRS deflection matter in the ASCE "rigid diaphragm" classification sense.

In several threads, I've seen this analogy whereby it's proposed that a cap plate welded to the end of the cantilevered channel would significantly improve that channel's torsional properties. Is that really the case? I would think that a cap plate would do next to nothing to improve warping torsion response just like web stiffeners in a torsionally loaded wide flange do next do nothing to improve warping torsion response. In both cases, you just create discrete points where the individual plate elements cannot move relative to one another.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Flexible Diaphragm & 3 Shear Walls

I still don't think you are technically wrong, KootK, but that it is simply unwise and poor engineering.

Judgement still counts for something in this profession. I judge a three sided system to be a poor, crappy, trouble prone system.

Also, about the long buildings in Asia, I forgot to mention that the often have many interior load bearing walls accross the short dimensions. They aren't really the three sided boxes we are all wringing our hands about.

RE: Flexible Diaphragm & 3 Shear Walls

I don't think KootK is crazy. I designed a three sided industrial facility in SDC B several years ago. I wasn't thrilled about it (and still am not), but I followed all of the load paths, allowed for accidental torsion, provided for diaphragm deflection at the open side, etc.

So...I guess I agree with both sides of the argument here. It is best to avoid the configuration (mainly because a four sided diaphragm has more redundancy), but it can be done.

DaveAtkins

RE: Flexible Diaphragm & 3 Shear Walls

Thinking about the moment frame idea to close the open side of the "C", I have a serious problem... as does Hokie...

The wood diaphragm deflection is not only dependent on the diaphragm stiffness, but also the variable transverse rotational deflection of the end walls, adding to that deflection. This total deflection is not directly calculable and is inherently the problem. Without the actual deflection, you cannot model, or equate, the stiffesses of the diaphragm and frame to equate the deflections.

I just do not trust the "C" scenario, and never will. It just feels unstable, inherently unstable.

Mike McCann, PE, SE (WA)


RE: Flexible Diaphragm & 3 Shear Walls

@ DaveAtkins: Thanks for the non-crazy vote. Your last paragraph summarizes my opinions on the matter rather succinctly.

@ MS^2: Can the displacement component that you mentioned not simply be calculated, or at least bounded, by treating it as a rigid body movement of the diaphragm? That's how I've been tackling it.

@ CEL: the SE Asia buildings that I observed directly were mostly frontage buildings. As such, they had open floor plans at the ground floor level to accommodate commercial retail space and/or indoor scooter parking.

I believe that judgement and intuition reign supreme in structural engineering and that calculation should only serve to supplement and inform judgement. I would never encourage anyone to set aside their personal intuition solely based on a technical debate like the one that we're having here. If the argument shifts your needle a bit one way or the other, great. If not, so be it.

It's interesting to note that our collective wisdom regarding torsionally regular buildings will likely turn out to be incorrect. The Christchurch event exposed a bunch of textbook, torsionally regular buildings that experienced the kind of damage one would expect from torsionally sensitive structures.

Examples include low rise buildings with similarly proportioned moment frames on all four sides that wound up having a disproportionate amount of inelastic damage concentrated at one corner. Academia is focusing research attention on this area with an eye towards structural reliability theory. The thinking seems to be that random variation in otherwise regular structures is sufficient to trigger a torsionally significant response.

That mysterious thing that we call"judgement" is in a state of constant evolution, as it should be.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Flexible Diaphragm & 3 Shear Walls

The "surprise" found in the ChCh EQs has been no surprise at all... Several of Canterbury University's professors have been talking about "true" response modeling for years. I even bought Priestley's "Displacement Based Design" text. Impressive but, to be frank, not practical. Nor does it matter; I paraphrase the eternal words of Wooten: Not knowing doesn't matter if you are still able to produce a reliable structure. In some ways the theory most widely applied in New Zealand (Capacity Engineering) is a school of thought based on not trusting our ability to calculate the real effect of an Earthquake on a structure. You should also read John Scarry's Open Letter to the Structural Engineers of New Zealand. It is a very interesting read, and well worth every practicing Structural Engineer's time.

While it is interesting to see where the "surprises" came from, and the knowing will be of value to every Structural Engineer, this is where the trolley should stop. Not one of the *cough* difficulties *cough* in ChCh was in any way fatal without also involving major issues missed during the (often omitted) site reviews. I for one think that trying to yet again further refine the code would be a mistake. We are already at a level of complexity that is causing errors; Many young engineers are expected to produce results with little supervision, leveraged to death. If we try to make things any more complex, when the existent rules are already proving to be very capable of producing safe and reliable structures, is dumb.

Here I become the nay-sayer that I always oppose when it comes to the ASD versus LSM/LSD debate... As has been stated before in this thread, a robust system with redundancy and secondary load paths is what makes for good Structural Engineering.

P.S. I am confident what will be found in the research being undertaken at

RE: Flexible Diaphragm & 3 Shear Walls

(continues)

Canterbury Uni, et al, will be that the otherwise minor variations normal to a construction site which cause the variation from ideal/model to real world/actual are responsible for changes in stiffness and strength that will prove all but impossible to eliminate. What I mean is that the random variations, ie: bar spacing, specific height, formwork discrepancies, mech/elec openings, cold joints, material strength variations, etc, are going to mean that there is a practical limit to the accuracy of our modelling.

Anyone who expects a structure to behave exactly as modeled is a fool. Practical considerations must be allowed to drive the final solution: Safe and sound, and economically non-crippling, is frankly good enough.

RE: Flexible Diaphragm & 3 Shear Walls

I don't see this linked here, a 1999 discussion on the issue:
http://www.seaint.org/wood.htm

In my book, it is generally unacceptable to design with a truly 3-sided, c-shape any longer. Too many bad things happen under lateral loads when this layout is used. Implementing some kind of 4th side reduces the risk, even if it is very short, stiff wall segments or a moment frame.

We have also had success by stiffening the diaphragm (such as by adding diagonal ties) to create a stiff lateral load path.

As far as distribution based on diaphragm and wall stiffness for 4-sided systems, I like Dr. Richard Klingner's (U Texas, retired) approach:
compute distribution based on 1) rigid (ratio of wall length) AND 2) flexible (proximity/equidistance) and then design based on the higher value for each wall. For example, if you have two lines of wall, one twice the length of the other, the flexible scenario puts half on each wall, while the rigid scenario puts twice as much into the longer wall (2/3 & 1/3). Thus, you design the long wall to take 2/3 of the force and the short wall to take 1/2 the force (the greater of the two scenarios for each wall.) This approach could easily be used for a "C plus moment frame", even though doing so by stiffness would result in about 100% of lateral for the wall and 50% of lateral into the frame (infinitely stiff wall compared to a moment frame.)

RE: Flexible Diaphragm & 3 Shear Walls

>>>Here I become the nay-sayer that I always oppose when it comes to the ASD versus LSM/LSD debate... As has been stated before in this thread, a robust system with redundancy and secondary load paths is what makes for good Structural Engineering.<<<

Sorry, can't have it both ways.pokebigsmile

(Just funning with you.wink)

RE: Flexible Diaphragm & 3 Shear Walls

I must have a copy of that Scarry Letter. Do you have a copy that you can post CEL? I did some serious googling but was only able to find links to documents discussing the letter.

In my opinion, structural reliability theory is of great practical value. Few things exemplify dispassionate rationality better than statistics, at least until they wind up in the hands of some politician.

One of the best features of structural reliability theory is that the results generally do not lead to a bunch of extra code complexity. Rather, they lead to a simple factor being adjusted one way or another. That is my expectation for the fallout from Christchurch. Currently, torsionally regular buildings are heavily favoured by seismic codes. The data suggest that a reduction of that favouritism may be in order.

I agree completely when it comes to avoiding additional complexity in our design work. I find the following examples particularly salient:

1) There is code committee talk of eliminating the need for accidental eccentricity when non-linear RSA/Time History analysis methods are used.

2) In the US, some buildings are required to have dual lateral systems, usually shear walls in combination with moment frames, UNLESS performance based design methods are employed.

In both of these instances, safety margins are being relaxed in exchange for employing a more complex analysis method and, ostensibly, "knowing more". This, while I find that I often have to correct even licensed junior engineers with regard to which direction their reinforcing hooks should be pointed.

It has long been accepted dogma in our profession that results can be "skinnied" when more sophisticated analyses are performed. Modern analysis methods have become so complex, however, that I almost feel that it's time for na additional safety factor to be instituted to cover the increased odds of error when wildly complicated methods are used.

phi_material; gamma_load; phi_method too frickin' complicated for humans.


The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Flexible Diaphragm & 3 Shear Walls

>>>Modern analysis methods have become so complex, however, that I almost feel that it's time for na additional safety factor to be instituted to cover the increased odds of error when wildly complicated methods are used.<<<

Assuming that's tongue-in-cheek then wow, that was a good one!thumbsup

RE: Flexible Diaphragm & 3 Shear Walls

@KB4894: To the best of my knowledge, ASCE has not published a solution to the "3-Sided Diaphragm" Trial Design Problem mentioned above. I just checked the ASCE website and did not see a solution. If I recall correctly, for certain problems in the past, it has taken 12 months or longer for a solution to be posted.

RE: Flexible Diaphragm & 3 Shear Walls

KootK: I did not know that the current "vogue" in the code committees was a tendency toward relaxing some of the duplications. Nice to hear, to be frank.

As for the Scarry letter, I do indeed have copy which I could post. I will not, however, do so.

The letter was and remains an open letter to STRUCTURAL ENGINEERS. John Scarry is a personal friend, and a very sensible man. He knew when he penned the letter that it would shock and possibly unnecessarily frighten the layman, or be used to embarrass the profession. He is wise enough to know that unintended consequences are the main outcome of every act and omission. As such it was never made publicly available and is *not* available anywhere on the web. Hence your dry search despite earnest googling.

I will distribute a copy of the Open Letter to any member who's concentration is listed as "Structural" or "Civil". Email me on m (no space here) quinn <at> celottawa (dot) ca.

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