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Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion
9

Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

(OP)

Hi all, I wanted to inquire about how some of you perform your iterative design process for concrete shear wall crack factors and also discuss my current methodology. I am hopeful that I may be able to create a more efficient process and try to hone down on what may be considered more of a standard “industry practice.” Please note that for this discussion I am discussing the stiffness factors for concrete walls used for design under factored load combinations (not serviceability) as per ACI 318-14 sec 6.6.3.1.1a. Also note that seismic does not control therefore this discussion will strictly pertain to wind loading and linear, pre-yield behavior of the structure.

Scenario: 20 story concrete structure with (3) shear wall “groups” that make up a “shear core.” Each shear wall group consist of (4) walls that form a rectangular shape.

Iteration 1: My first iteration is performed with all walls uncracked using .70Ig -> plot results -> All walls that are ‘cracked’ have their respective cracked factors modified to .35Ig to represent Iteration 1 results (This is done at a per level, per wall basis).
Iteration 2: Using the updated wall crack factors from Iteration 1, I perform a 2nd iteration of the model with crack factors of .35Ig on walls that cracked in Iteration 1, and .70Ig for uncracked walls -> plot results -> Walls whose status match their crack factor remain as is (cracked stays cracked, uncracked stays uncracked, remaining walls have their factors adjusted to match iteration 2 results.
Iteration 3 & beyond: Continue to adjust factors as discussed in iteration 2 until all values converge
At this point I should clarify that the ultimate goal is to have all cracked factors for each individual wall segment at each floor converge with the input factor when checked for all load combinations. This means that if a wall is shown to crack in any of the checked load combinations, then this wall shall have an input of .35Ig and vice versa if the wall is shown to remain uncracked when checking all load combinations, this wall will have a .70Ig.

The issue that I come upon is that I end up chasing my tail when limiting myself to only .35/.70Ig. As you change the stiffness of individual wall groups, you are modifying that load path and subsequently distributing more load to adjacent wall panels. By limiting myself to .35/.70 it seems as if I am trying to say everything is either “white or black” and ultimately consecutive iterations are mere inverses of the previous results. It would appear that in order to accurately reflect conditions, I would need to find “effective” moment of inertia’s for each respective walls segment, but this would be a very time consuming process when one is forced to do this by ‘hand.’ This need for an effective stiffness becomes apparent when one sees the results constantly inversing between 2 sets of walls flashing between “cracked and uncracked” when in reality the wall groups share this load and are somewhere in between.

I have heard from a colleague that "once a wall cracks it is cracked." This is an obvious statement, however it is a more complex issue when we as the engineers are telling the programs which walls are cracked/uncracked and subsequently manipulating where the load is to go. I am under the belief that in a perfect world I would create effective moments of inertia for each individual wall segment (per floor), however I am curious to see how far other engineers/companies take this design approach to get a stiffness model that accurately represents the intent.

I am also curious to find if I am overthinking this and if the general engineering community uses a cracked factor of 0.5Ig for all entities as allowed in ACI 318-14 Section 6.6.3.1.2 😊.


RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

As you have noticed already, using the iteration method is not very practical, we did the old ways - envelop the results from all cases.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

What analysis program are you using? This seems ripe for a non-linear type of analysis if your program can handle it.

Not sure how I would handle setting up the elements, but I am sure we could figure something out if you are using ETABs or SAP.

S&T

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

(OP)
I am using RAM Structural system. I believe that this software is commonly used in our industry and was hoping to get input on how others typically approach the 'iterative' portion of their design with similar software. Having (2) models, one with cracked walls and the second model with uncracked walls, and designing for both extremes (envelope solution) is certainly one approach, however I have a hard time believing that most engineers currently use this method.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Quote (OP)

however I have a hard time believing that most engineers currently use this method.
Why do you say that? To me that's much simpler than running the model multiple times iteratively over and over. Only to, at times, find out that the majority (if not all) of the walls do end up in a cracked state.

I'd rather build one model (either cracked or uncracked), then save a second model and adjust the stiffness accordingly to the other option. Then I've only got to run two analyses. Grab the maximum results from each run and design each element only once.

That being said, I practice in a non-seismic location and therefore don't need to account for hinging, energy dissipation or anything like that.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

(OP)

Quote (jayrod12)

Why do you say that?

I honestly do not have much to base my opinion off of besides more recent structural literature that discusses iterative approaches such as this one:

https://www.structuremag.org/?p=10924

With discussions in our community regarding 'iterative' processes along with the development of structural analysis software, one can only guess if other engineers have created their own processes to replicate the iterative procedure or if most are sticking with the old school envelope procedure. It would be nice to receive input and see if other engineers on this forum also have stuck with the envelope solution.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Polskadan -

Thank you for posting. I believe your question is a very good one that not very many engineers are putting much thought into. I'll have a slightly different perspective coming from more of a software background (I've worked for both RISA and CSI in my career).

My thoughts on this are the following:
1) First we need to acknowledge and understand that we are only estimating an elastic stiffness for these walls. The goal is to come up with a reasonable stiffness for our analysis and our design. We know 100% that these walls do NOT behave elastically.

2) 0.70Ig isn't really an "uncracked" wall. I know that's what the code says. But, I think of it more as a "normal" level of cracking for a compression member. You'll have temp / shrinkage cracks and such. But, you won't have the major flexural cracks that result from the flexural tension stresses exceeding the axial compression stresses.

3) I agree that the "black / white" designation of "cracked/uncracked" is problematic. Such a dramatic stiffness change can cause iteration / convergence issues. What we would really like to see is a "transitional" stiffness between 0.70 and 0.35 times Igross. Right?

4) Table 6.6.3.1.1(b) does a okay job of this.... transitioning between 0.875*Ig and 0.35*Ig. So, you could use that formula to do the transition manually. Though, ideally, you would have a program do this (or something like it) for you automatically.

5) To some extent, I think we can lose track of the goal. We're not looking for a perfect analysis. Any elastic analysis is going to be imperfect. We just want to be reasonable with the stiffness we're using. In that respect, I would probably advocate for a simpler formula than given in 6.6.3.1.1b if I were doing this my self something like adjustment factor = 0.70*I*(Pu/A - M/S) where A and S are based on gross properties. And, you limit this to a maximum of 0.70 and a minimum of 0.35.

Again, I'm not trying to be perfectly accurate. I'm just trying to come up with something halfway reasonable. No matter what, I would probably stop after two iterations.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Quote (OP)

I believe that this software is commonly used in our industry and was hoping to get input on how others typically approach the 'iterative' portion of their design with similar software. Having (2) models, one with cracked walls and the second model with uncracked walls, and designing for both extremes (envelope solution) is certainly one approach, however I have a hard time believing that most engineers currently use this method.

For serious high rise buildings in competitive high-rise markets, you'd better believe that engineers are are iterating to get favorable results rather than just bracketing the the extreme solutions. That, in combination with all of the extra complexity that comes along with introduction composite shear wall coupling beams etc which have their own issues with respect to accurate modelling. This will have an enormous impact on the building wind design for occupant comfort and high dollar ULS items like hold down, tension pile demands. And then there's seismic...

Most everybody acknowledges that all of this stuff is pretty serious BS with respect to accuracy and predictive value. In my mind, what really need to happen with this is for some jurisdictional body to set out clear guidelines for what the modelling rules will be going forward for everybody. The key is to set a level playing field for this so that it is not so heavily dependent on designer/firm judgment. That system winds up just being a race to the bottom design wise.

To this day, I don't believe that there are even clear guidelines establishing what slab widths one should use for an equivalent frame, slab-frame lateral system where those can be used. And I know for a fact that there are designers out there using all of the slab width for this purpose. And slab torsion too! Why not? Better answers, happier clients, no consequences. Where nobody minds the store, it gets jacked. No big surprise there.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

I've a little different take on this - when I feel the necessity to perform iteration, the goal is to identify the weakness that may not be uncovered through simply envelop the extremes. Do we really want the code to mandate the design/analysis approaches that everybody shall follow and be doing the same, I doubt. If it is going to be the trend, for an extreme, the code body no longer needs to issue/update codes, but to write programs instead. Please note the difference between code suggest/recommend methods, and the codified solutions. I welcome the former, but don't wish to see the latter. Sorry, don't mean to dispute, just let my feeling out.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

KootK, I'm going to disagree with you. That's seems like it kills innovation. Why should a firm be penalised if they can demonstrate they are pushing the boundaries of structural engineering, knowledge, research and application of codes/design guides. This is all assuming they are doing this innovative work based on sound engineering principles and the latest research.

I've had a look into this very problem several times over the past few years and it's not straightforward. Applying general stiffness modifiers, smearing them across various walls and iterating the whole process is like setting the FEA mesh size to 10 metres. Rarely worth the effort for the accuracy it spits out.

Shall we talk about non-linear layered shells with x% vertical and horizontal reinforcement now?

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

4

Quote (Trenno)

That's seems like it kills innovation.

I can only speak from the perspective of my own personal experience on this. To date, I've worked at three big high-rise firms and have been involved in peer reviews with four others, including yours. From what I've seen, "innovation" at the skyscraper firms mostly means:

1) Digesting the same, very shallow pool of literature and resources available to us all.

2) Interpreting #1 in the boldest, most liberal manner conceivable.

3) Stuffing #2 into ETABS, PERFORM 3D, or whatever the latest fad is and aggressively marketing the results as boundary pushing innovation that only the insiders are capable of.

4) Sometimes pairing these NASA level analyses with fundamental detailing errors that any decent, 3rd year EIT would be ashamed of.

As far as I can tell, the only difference between a senior engineer at Thornton Tomasetti and the analogous guy at Mike's Beams-R-Us is that the TT guy has the confidence that comes from knowing that, even though he's scared witless about the design assumptions that he's making on his 90 story building, the guy in the cube next door made the same assumptions on an 88 story building last year and lived to tell the story.

The one exception that I'll allow for, and I'm sure that there are others of which I am unaware, is Magnusson Klemencic Associates. Via their partnership with the Charles Pankow Foundation and the west coast universities, they really do seem to be pushing the body of knowledge forward in a meaningful, non-BS way. And I'd not begrudge them the creative use of the technologies that they themselves have helped to develop.

Quote (Trenno)

Why should a firm be penalised...

I don't see it as anyone being "penalized". Rather, I see it as the leveling of the playing field and the cessation of reward in exchange for excessive risk taking. This question gets asked here constantly. And nobody ever has a definitive answer to offer up. If the answers on this are buried so deeply that nobody here knows of them, and you've got to be a member of the Priory of Sion to gain access, should anybody really be making use of such information? It's civil engineering for Pete's sake. It's kind of meant to be public domain stuff. It's not like we're developing secret space lasers etc.

Quote (Trenno)

Shall we talk about non-linear layered shells with x% vertical and horizontal reinforcement now?
.

By all means, please enlighten us and tell us how to do this the right way. I, for one, would absolutely love to finally know.

Let me guess... it's proprietary and would compromise your firm's competitive advantage?

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

KootK -

Tell us what you really think. Don't sugar coat it this time! LOL

In all seriousness, some quick comments (some of which agree with your post, some of which do not):

Quote:

4) Sometimes pairing these NASA level analyses with fundamental detailing errors that any decent, 3rd year EIT would be ashamed of.
This (IMHO) is an excellent point. One that everyone should pay attention to. What good is a 5% reduction in force from your analysis if you screw up the construction documents with bad detailing?! To me this is what separates people like me (i.e. the FEM jockeys of the world) from the really good engineers. The really good engineers know how to put a set of drawings together and and sniff out bad detailing like a blood hound.

Quote:

Stuffing #2 into ETABS, PERFORM 3D, or whatever the latest fad is and aggressively marketing the results as boundary pushing innovation that only the insiders are capable of.
Just to be clear, if you're talking about Perform3D then you're talking about doing a non-linear analysis. This is very different than the subject of this thread.... which deals with assumptions about the LINEAR stiffness used for shear wall analysis. Hence, it may be an unfair comment / criticism.... at least for this thread.

Caveat: I work for the company that writes ETABS and Perform3D.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

I am also curious about how to set nonlinear material (concrete) properties into the program. Any commercial program package does that? Otherwise, seems it will loop back to the dilemma of 0.7Ig, or 0.35Ig.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

To attempt to put my money where my mouth is, here's one of the big innovations from a NY highrise firm. Apparently, these magical, STM, punching shear preventers have been installed at the base of some Manhattan Skyscrapers. All the FEM'in in the world won't be worth squat when the columns just slide through the hole in the middle. What do I know though? I'm just some internet nutball lacking a back stage pass to Club Skyscraper





RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Quote (JP)

Just to be clear, if you're talking about Perform3D then you're talking about doing a non-linear analysis.

Oh, I know. My beef is not with the software or the software vendors. They simply supply that for which there is burgeoning demand.

Quote (JP)

This is very different than the subject of this thread.... which deals with assumptions about the LINEAR stiffness used for shear wall analysis. Hence, it may be an unfair comment / criticism.... at least for this thread.

Pffft... fair schmair.

1) So I've moved the needle in this thread a little. OP will live.

2) I really take OP's fundamental question to be "How the hell are we all, as a community, designing our cracked core walls?". In this context, all methodologies are on the table for discussion and the overarching truth of things is more important than the particulars.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Quote (retired13)

Otherwise, seems it will loop back to the dilemma of 0.7Ig, or 0.35Ig.

That's exactly right. I can't find it now but somebody in my LinkedIn network is a purveyor of an ETABs post processor that will analyze your ETABS shear wall stresses, crack your wall elements accordingly one by one, and then feed those values back into the ETABS model as adjusted input. All fine and dandy if you don't mind ignoring the effect of reinforcement in your concrete.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Quote (retired13)

I am also curious about how to set nonlinear material (concrete) properties into the program. Any commercial program package does that? Otherwise, seems it will loop back to the dilemma of 0.7Ig, or 0.35Ig.

In order to get the juices flowing here... I'll take the first stab of how I would do it in ETABs/SAP.

Set nonlinear material stress-strain plot that is somewhat binary in nature. Up to a certain stress level you have a certain E (representing the 0.7Ig region) after that, cut your stress-strain slope in half (representing 0.35Ig region). Images shown below, i have not done the research on what these values are, just shown for example





After that, setup a layered shell in ETABs with non-linear nature in the S11 and S22 direction.



Note, I have no idea if this is the correct way to handle this, but those are my thoughts. I can run some test models for the group and post results if interested.

I am sure Josh might have some good pointers for us as he works for the company.





S&T

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Stress strain curve for the concrete? I need time to digest, thanks anyway :)

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Actually now that I thought about it a little more, maybe the stress strain curve would look something more like this
(sorry i flipped stress and strain on the diagram, you should get the picture though)




S&T

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Quote (JP)

Caveat: I work for the company that writes ETABS and Perform3D.

Now I can't resist and am curious to know what you think of this...

I've always attributed the meteoric rise of CSI primarily to the innovation that is ETABS. In particular:

1) Early anticipation of, and catering to, the structural engineering community's desperate wish for the ability to handle gravity & lateral design in a single, 3D model and;

2) Not explicitly telling designers how building lateral design ought to be undertaken while simultaneously providing convenient tools for designers to do what their instincts will lead them to want to do naturally.

3) Staying the hell away from light frame construction where the value proposition is much lower while the programming is probably actually trickier.

Marketing & strategy genius it is/was.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

I couldn't connect stress-strain curve with this problem. Rather, IMO, the program needs to be able to compare the resulting tensile stress with concrete rupture stress. If the former exceeds the latter, then it shall calculate the revised moment of inertia using next specified depth (say "x inches" negative increment). The program shall stop when the moment of inertia has hitting the goal, say 0.7Ig. When compare tensile stress, it shall also keep an eye on compressive stress, and so on so forth... But this would still be considered "linear analysis", wouldn't it be?! So, no good.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

i maybe in left field here... but I think the 0.7*Ig and 0.35*Ig is a convenient remnant of past analysis techniques for frame type members. For a frame element (stick) it is very easy to access Ig and modify as required.

For shell elements (how i would guesstimate 95% of concrete walls are modeled nowadays)... the concept of Ig becomes a little more fuzzy right ? The elements stitched together inherently make up Ig of a wall. A lot of literature suggests modifying E of concrete to account for the 0.35Ig of 0.7Ig. This inherently is inputting a stress strain curve whether we realize it or not in my opinion.

I think the technique that i have outlined above captures the heart of the 0.35Ig and 0.7Ig requirements for cracked and uncracked walls.
By inputting the explicit stress strain curve into the shell element, ETABs or sap should be able to check whether or not the shell element has cracked and then change the modulus of elasticity accordingly (hopefully in the end somewhat accurately (dripping with sarcasm here) capturing the softening effect of a cracked concrete wall).

Maybe a better stress strain curve would be this:



Quote (retired13)

But this would still be considered "linear analysis", wouldn't it be?! So, no good.
I've come to think of non-linear analysis as synonymous with iterative. I think the process you have outlined above retired is a non-linear analysis at the heart of it.



S&T

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

KootK, you seem to have ignored my statement "This is all assuming they are doing this innovative work based on sound engineering principles and the latest research."

Horses for courses... buildings that sit in the outlying 1% probably call for the NASA level analysis. Shall we be picking the next pencil-scraper out of a generic structural engineering cookbook? Only 3 outriggers allowed at the 30th, 60th and 90th levels!

I can sympathize with you though, there are many simple things we as a community can't agree on. Look at that steel beam LTB thread from a while back... Perhaps a cookbook is an easy way out?

On the topic of non-linear shells, I'm not privy to those in-house secrets - so would like to explore the topic myself. This thread seems like a good place to discuss such things.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Quote (KootK)

I've always attributed the meteoric rise of CSI primarily to the innovation that is ETABS. In particular:

1) Early anticipation of, and catering to, the structural engineering community's desperate wish for the ability to handle gravity & lateral design in a single, 3D model and;

2) Not explicitly telling designers how building lateral design ought to be undertaken while simultaneously providing convenient tools for designers to do what their instincts will lead them to want to do naturally.

3) Staying the hell away from light frame construction where the value proposition is much lower while the programming is probably actually trickier.

Keep in mind that I've worked for CSi for less than 2 years. I worked for RISA a lot longer (16 years). So, my insight into CSi isn't as thorough as you would think.

1) I think you're partially correct with item #1. Though I won't point specifically to ETABS. I'll point to SAP80 / SAP90. I think building up an early market share in the industry was very important. STAAD was likely in a similar position. The very early players in a market.

This is my opinion (heavily influenced by my time at RISA): "Momentum" is very important within the structural software industry. What I mean is that it takes time to learn a new software. It's partially learning the interface. But, it's also understanding the peculiarities of the program. How to de-bug the model. How to dig deeper into the results to see what you did right or wrong. Once an engineer (or company) is truly experienced and comfortable with a software (or software company) it is difficult for them to switch.

2) I think your point is essentially true, especially with ETABS. Give the engineering community the software tools they're asking for and which they need. I will amend this slightly.... I think it is also really import to show the engineering community that you respect them and their business. I remember a time (late 90's BEFORE I worked for RISA) where STAAD had real quality control issues. They were releasing features that just didn't work. It felt (to me at the time) like they were concerned about have a list of features that they could show to engineers on a sales call. But, they didn't care whether those features actually worked or not. Who knows what the real cause of their issues was, but to me it felt like a lack of "respect" for their customers.

I'm not aware of the folks at CSi ever doing that. Sure the program has bugs (all programs do). But, my main point is that the company has always been run by TECHNICAL people who are very much concerned about the accuracy and reliability of their program.... People who love and respect the structural engineering profession.

3) Yes, CSi has very much stayed away from light frame wood buildings. My guess is that the reason is they preferred to focus on the higher end / more academic forms of analysis. They have a lot of connections to the folks over at Berkeley (Graham Powell, Ed Wilson, etc). And, that influences what the company is interested in doing. In fact, the get a real head start on the rest of the industry on any of the "new fangled" analysis methods coming from those University researchers.

Contrast that to wood construction. You really need to hang out with engineers who specialize in wood construction to understand the intricacies of that field. And, once you go down the road of wood design, you quickly figure out it's a lot more work to fit it into an analysis program than other materials.
Therefore, avoiding wood (as I see it) is likely related to CSi leaning into its strengths (more advanced analysis) and letting the other companies (Enercalc, etc) focus on the areas they perceived to have lower profitability.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Quote (Trenno)

KootK, you seem to have ignored my statement "This is all assuming they are doing this innovative work based on sound engineering principles and the latest research."

I didn't ignore your statement at all Trenno. In fact, I'd sued it for source material:

1) As I mentioned, by and large, we all have access to the same research and literature pool.

2) I think that it's dangerous and erroneous to imply that only the folks working at the mega skyscraper firms possess a knowledge of sound engineering principles.

In the past 24 months, I've been involved in peer review assignments for two NY buildings involving WSP and Thornton Tomasetti. In both cases, I've had a look at the wizards behind those curtains and magic being performed has been this:

3) Use ETABs, just like everybody else.

4) Use the latest research papers on coupling beams etc, just like everybody else.

5) Make thumb in the air guesses about effective wall stiffness based on the research and the ACI recommendations, just like everybody else.

6) Be very, very aggressive with #5, not always like everybody else.

And it is the same at the big Canadian firms for which I've worked. If your group is doing something special that is not what I've described then I guess that my comments do not apply to your team and their work. That said, based on my personal experience of this, your team would then be the only one that I'm aware of that is doing something significantly different from that this.

Quote (Trenno)

Horses for courses... buildings that sit in the outlying 1% probably call for the NASA level analysis. Shall we be picking the next pencil-scraper out of a generic structural engineering cookbook? Only 3 outriggers allowed at the 30th, 60th and 90th levels!

Now this is you misinterpreting my statements. I do not bemoan the aggressive design methods that firms doing this kind of work are using -- I use them myself. And I have no problem with the results or the use of advanced modelling to get them -- I do the same. If we collectively decide that we want these kinds of aggressive designs, as a profession and as a society, that's just peachy by me. What I do have a beef with is the implication that some groups are entitled to the use of such aggressive methods because they are using top secret, advanced research and computational methods that the rest of us are somehow ignorant of. When I've peaked behind that particular curtain, that simply is not what I have seen.

If we're all going to be out there doing the WAG thing on very important, high dollar design elements, then I think that the standards ought to step into a degree to standardize the practice, at least until the state of knowledge is such that we can calculate effective wall stiffness accurately based on established engineering principles. For now, let's innovate based on things like clever schematic design layout rather than our ability to be brave enough to say that a stiffness factor might 0.55 instead of 0.35.

Quote (Trenno)

On the topic of non-linear shells, I'm not privy to those in-house secrets - so would like to explore the topic myself.

So is it then the case that these methods are being used by your firm to design shear wall system?

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

One of the best examples that I know of for a ubiquitous, questionable, aggressive, iterative design approach is the way in which the design of coupling beams is often handled. It goes like this:

1) Decide to couple your shear walls because there's no competitive way to stiffen your lateral system enough without them.

2) Assume an aggressively high stiffness for you coupling beams to give you good drift control.

3) Find that your coupling beams now attract way more shear than you could ever possibly deal with.

4) Start selectively cracking your coupling beams to a larger degree to remove the problems from the hot spot locations.

5) Justify #4 with some sketch arguments like:

a) at high loads, they probably do crack more, don't they?

b) I only need the stiffness for SLS, not ULS.

And again, I'm fine with all this so long as we all know what each other is up to and agree that it is the best path forward. It's the backroom BS that I think is unfair.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Here's a personal pet peave that maybe someone can help me with.

Most shear walls have regularly spaced, horizontal construction joints at the floor levels. Are those not flexural cracks present from the outset? Or are they somehow different because they are discretely spaced? I'd actually consider full depth cracks in a 20' long wall at 10' oc vertically to be somewhat distributed flexural cracking.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

KootK,

Don't quite understand your question. How "construction joints" are discretely spaced, or you meant the cracks are?

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Quote (retired13)

Don't quite understand your question. How "construction joints" are discretely spaced, or you meant the cracks are?

What I mean is that construction joins are flexural cracks in that they represent planes of weakness incapable of transmitting tensile stress perpendicular to the joints from the outset.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Some testing with layered shells and frame members... set up below:

Concrete shearwall plan view and elevation (100kips applied at the top of the core):




ETABs model with Stick element and shelled wall:


Stress strain curve for non-linear shell (finally took a stab at calc cracking stress at 5*root f`c for 4000psi concrete):


I applied a 0.7 and 0.35 factor to the stick element for Ig

Deflection results:

Stick element moves 1.45in (0.7Ig) 2.86in (0.35Ig), nonlinear shell element moves 1.82in at the top.

Stress results:

Tension maxes out at around 575psi, compression maxes out at around 880psi






S&T

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Interesting discussion would like to see how this progresses.

Doing an adjustment on Ig is all fine and dandy but that still ends up applying over the height on an entire story's worth of concrete, I think given the budget and schedule that allowed for the type of analysis we are talking about you'd want software that you could put in a moment-curvature relationship for the wall panel which lets you capture the interaction with the reinforcement. You'd need to do some front end development on panel designs and then as stiffness's and forces change you swap in a new panel design and M-C relationship and re-run or the program does that as part of it's non-linear run.

This is the software area where I think some of the AI stuff could dip it's toe into our realm, when your talking about concrete it's hard to automate this stuff because you can't design the reinforcement until you know the forces and you can't get an accurate estimate of stiffness without the reinforcement, so stuff like our slab software has you already say hey I want you to use #6 bars in design because otherwise it would have to do section designs for every bar size then have criteria to determine how to select a result from those designs.

One thing that seems lost in all of this is these are technically deep beams and the contribution of the shear deformation is significant, how does the amount of flexural tension impact the "shear area", I haven't found much research on this topic which I tried looking into as part of my work on deriving general Timoshenko beam formulas. Most calcs I see on this just use the shear area for a solid rectangle if the fibers are separating under tension not sure this should really be the case.

My Personal Open Source Structural Applications:
https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:
https://github.com/open-struct-engineer

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

I will work to post some results with capturing the reinforcing effects as well since that seems to be of interest. Maybe Trenno can give us some pointers on how to handle this.

The example I posted above I think could be used as a "poor mans" non-linear analysis that attempts to capture the 0.35-0.7 effects that ACI spits out.

Quote (Celt83)

Most calcs I see on this just use the shear area for a solid rectangle if the fibers are separating under tension not sure this should really be the case.

I've thought about this as well but arrived at feeling OK with utilize the full solid rectangle area. Mind explaining why you feel that this wouldn't be the case?

S&T

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

I deleted my previous comment for incorrect strain diagram. Is the stress-strain diagram below non-linear?

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

With rebar in the non-linear shells (which I have no idea how to setup correctly)... I get 1.35 inches of deflection at the top of my example.



Concrete Stress strain curve (note that it will drop off any appreciable load carrying capacity in tension):

Steel Stress Strain curve:


Nonlinear shell setup (#6 @ 12"oc ea way ea face):





S&T

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

celt is correct. The tension flange will be lifted up, and the stress redistributed on the un-cracked section.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Retired13:

I'm not sold on it either way as in theory for shear the tension steel should still be holding things together so you get the aggregate interlock mechanism across the shear plane, is that interlock enough to justify still using the gross shear area, not sure but nobody has done any different than that and buildings aren't moving crazy amounts.

My Personal Open Source Structural Applications:
https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:
https://github.com/open-struct-engineer

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Trenno and I discussed the shear flexibility some in this 2016 thread: Link. That included the clip below. This doesn't really provide answers but, rather, just more fuel for the uncertainty fire.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Sticks and Triangles -

My thoughts about modeling the non-linearity of shear walls in a program like Perform3D (or the version of ETABS that does this):

1) I'm more familiar with the Perform3D version of this. The concepts should be the same. Though the terminology and way it is described might be slightly different.

2) I have no practical experience working on projects like this. My experience comes from seminars, teaching myself the CSi programs and such. So, that is a pretty strong caveat about my level of knowledge.

3) For a performance based analysis, you'd want to define what type of Hysteresis model you want to use for the shear wall. This should be based on research on the type of element you are modeling. To me, this is more clearly defined for moment connections than for shear walls. But, the concept still exists for shear walls. Something like the following:
Pivot Hysteresis model (which involves a pinching in of the curve due to different loading vs unloading stiffness. It starts off with an uncracked stiffness, then cracks and reduces it's stiffness. Unloading follows a stiffness closer to the uncracked stiffness.
There are many other hysteresis models that one could use. What best matches up with FEMA / ASCE recommendations and or test data, I just do not know. Below shows soemthing akin to the "pinched" curve that results from a Pivot Hysteresis model.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

i did a quick pushover test for the layered shell (one directional push only).


Sorta of looks like the first hysteresis image posted by Josh.

Josh, is there a way to set this up to go back and forth like a hysteresis model in ETABs?
I would be interested in seeing what type of loop pops out.

S&T

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

This would be an interesting video to watch, if you have 7 minutes. Link - "Design Stress -Strain Curve for Concrete"

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

celt,

Quote:

I'm not sold on it either way as in theory for shear the tension steel should still be holding things together

Wasn't that your position - " If you have a non-zero crack width in my mind you have lost cross section continuity"? Correct, but I wouldn't call it non-zero crack, rather a fine frack. But a crack is a crack, there is no concrete tensile stress can across, nor shear. The steel is design to connect the parts together, but concrete shear resistance is lost, at least theoretically, because how can we measure/assure the interlock exist between the cracked parts?

Note the crack here is flexural crack. For other type of cracks (shrinkage, temperature), the shear is resisted by shear friction through gravity force and interlock mechanism, or by steel.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Yeah poor word choice on my part there. Without some more solid research on the topic I'm more not convinced it is a critical item for design right now, but that may change in the future.

I may try and dig up that PCA Design Handbook referenced in Kootk's image and see if they have some cited references to look into more.

My Personal Open Source Structural Applications:
https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:
https://github.com/open-struct-engineer

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

KootK, I'm actually not too sure about the layered shells, but after some research it seems to be the way to go about all of this. As I'm sure you know, even within the same office you can get large variance about how engineers go about a problem. That can be both a good and bad thing. I think we're actually on the same team here, just coming at the argument from different angles. There are so many many things there isn't a black & white answer for.

Are you advocating for codes to become more like textbooks? Chapter on core wall design? Transfer slab design? Where does "Engineering Judgement" come into all of this?

Thanks Sticks&Triangles for starting some tests. Once I get some more free time I'll investigate further too. Perhaps we can start an Eng-Tips test building for this thread, with all the bells and whistles - 60st tower, coupled shear walls, perimeter columns, maybe an ourtigger somewhere? Best design gets a purple star.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Quote:

Josh, is there a way to set this up to go back and forth like a hysteresis model in ETABs?

First, I'm want to clearly state that this is beyond my area of expertise. I haven't done this type of non-linear analysis before. So, I have little more insight into how exactly to set up this model than you do. However, I am pro at navigating the CSi documentation, website and wiki / knowledge base. wink

Take a look at the ETABS Watch and Learn video below about performance based design. To me, this is the best place to start.
https://www.youtube.com/watch?v=v75u62GQReY

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Quote (Trenno)

KootK, I'm actually not too sure about the layered shells, but after some research it seems to be the way to go about all of this.

Is the layered shells business what your firm is doing for shear wall analysis? Surely you must know that much about their operations. I think that it would be of great value to finally know of at least one data point out there in the big old world represented by a firm that is actually walking the walk with the "smartest guys in the room" advanced analysis routine. It would be inspiring and I might have to apply for a job there just to get backstage access to the secret sauce.

Quote (Trenno)

Where does "Engineering Judgement" come into all of this?

It doesn't. There's no such thing as engineering judgment in the absence of:

1) Rigorous testing of a sufficient similitude OR;

2) Established design procedures (these usually have to be corroborated via testing as well).

If those things existed, this thread would not.

Quote (Trenno)

As I'm sure you know, even within the same office you can get large variance about how engineers go about a problem.

This is different. One guy arbitrarily using 0.55 Icr where another uses 0.35 Icr is more nefarious than a simple variation in problem solving technique. Rather, it's applying more or less the same technique with more aggressively chosen input parameters.

Quote (Trenno)

Are you advocating for codes to become more like textbooks?

I'm not. I don't see how adding a couple of prescriptive provisions to a code already full of hundreds of prescriptive provisions would somehow turn it into a loathsome textbook. The codes already prescribe how we should calculate wind loads, concrete shear capacity, and gobs of other stuff that could also have been left to engineering judgement. What's the big deal if they also prescribe shear wall stiffness parameters until the state of knowledge advances to the point where we're able to work them out for ourselves using accepted analysis methods?

Quote (Trenno)

Chapter on core wall design? Transfer slab design?o all of this?

Yes, absolutely. Add raft slabs for good measure too. Not necessarily chapters but definitely provisions and, where warranted, sub-sections. ACI already has some provisions particular to core walls.

The root question here really seems to be how we ought to delineate between things that should be codified and things that should not. My opinion is that codes should be kept as as sparse as possible and only include items where the state of the art is such that:

1) There is confusion within the profession about what should be done and;

2) Different, reasonable engineers are routinely coming up with very different answers.

If shear wall stiffness estimation doesn't fit the bill, I don't know what does.

You seem to come from the libertarian school of thought that views our codes as enemies trying to encroach on our freedoms and creativity. I don't see things that way at all. When codes are structured properly, as they mostly are, I view them as "friendly" assets allowing me to make good decisions on "grey area" topics without having to worry that my competition is going to undercut me merely by having the balls to do so via aggressive analysis choices.






RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Quote (KootK)

1) There is confusion within the profession about what should be done and;

Don't you feel code have confused us in many areas/ways :) I prefer to see them make things clearer in the commentaries, and make pointed references for learning and finding solutions. Just my 2 cents.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Quote (retired13)

Don't you feel code have confused us in many areas/ways :)

Not at all. In the overwhelming majority of cases, I feel that the code has been a help to our profession rather than an hindrance. Consider two recent, salient examples:

1) Anchorage of steel components to concrete (Appendix D) and;

2) Full continuity steel moment connections under seismic (Nothridge).

In both cases, we were designing these elements in accordance with first principles and poorly applied "engineering judgement". And in both cases, testing and real world performance showed that to be woefully inadequate. So now we have code provisions for these thing to protect us from ourselves and to ensure a uniform standard of practice across the profession. And, as always, there was initially a cadre of engineers out there that felt that the new provisions were just so much freedom limiting code bloat.

Yeah, codes are sometimes imperfectly written. As in all things, however, it is unwise to allow the perfect to be the enemy of the good. Keep the baby, even if it means keeping the bath water.

Quote (retired13)

I prefer to see them make things clearer in the commentaries.

I'm a big fan of extensive commentaries as well but it must be recognized that, whenever a definitive recommendation is made in commentary, it eventually becomes defacto prescriptive or for real prescriptive. Appendix D was, after all, non-mandatory commentary for a good long time.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Quote (Trenno)

KootK, I'm actually not too sure about the layered shells, but after some research it seems to be the way to go about all of this.

Dang Trenno, I also thought you were going to drop some sweet knowledge on us as to how your firm handled this. I thought when you mentioned horizontal and vertical reinforcing % at the beginning you were doing the layered shell business yourself.

Quote (Trenno)

Perhaps we can start an Eng-Tips test building for this thread, with all the bells and whistles - 60st tower, coupled shear walls, perimeter columns, maybe an ourtigger somewhere? Best design gets a purple star.

I'm down, I think we would want to create a respository on github or something of the like where we could swap models back and forth. I think we could do some great stuff for the engineering community to demystify some of this stuff and bring the big boys down off of their pedestals. I agree with KootK, in that there is very little difference between Beams_R_US and Skyscrappers_R_US... with experience breeds confidence. In the end we all do the best that we can and lean on that fact that over the past 100 years, our designs more than likely won't fall down.

Quote (JP)

3) For a performance based analysis, you'd want to define what type of Hysteresis model you want to use for the shear wall.

I get a little lost here, if we were are using layered shells, doesn't the Hysteresis model of the wall just "inherently" pop out from the stress strain type curves we define for the wall? I have done a little bit performance based analysis at my work, but only with ASCE 41 pre-defined hinges and picking a moment curveature relationship makes a lot of senses for hinges... but not so much for layered shells?

S&T

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Quote (SnT)

I also thought you were going to drop some sweet knowledge on us as to how your firm handled this.

I can provide a data point of interest although it's not going to be anywhere near as intellectually satisfying as what you/we are hoping for.

In 2015 I was working with a skyscraper legend who cut his teeth doing a bunch of the high-rises at Canary Wharf in London. Same guy went on to do a bunch of tall stuff in Tornonto, NY, Vegas, and Calgary. So, naturally, I asked THE question.

His answer was that he divided up all of the compound shear wall cores into individual straight wall segments and left it at that. After I guffawed pretentiously, I pointed out the myriad of ways in which that practice would be inaccurate, inefficient and, occasionally, unconservative. Skyscraper guru defended his method as follows:

1) to date, concern over overly conservative shear walls does not appear to have cost the firm work.

2) this method is simple and efficient to apply and can easily be taught to junior/intermediate staff with easy quality control.

3) this method will be safe in most instances with respect to ULS concerns.

4) the current state of knowledge with respect to concrete shear wall behavior doesn't justify the use of anything more sophisticated.

His opinions, not mine. As you can imagine, I was simultaneously terribly disappointed and impressed by this guy's very pragmatic thoughts on the matter. Older dude. Who knows, maybe he's been forced into fancier analytics since 2015 in a nod to market pressure. Some ANSYS, Grasshopper, AI hybrid or something.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Quote (KootK)

His answer was that he divided up all of the compound shear wall cores into individual straight wall segments and left it at that.

Wow, never would have thought big skyscrapers would calc out without taking into account flanges and crazy amounts of coupling. I would have thought deflection would get out of control quickly...

S&T

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Quote (S&T)

I would have thought deflection would get out of control quickly...
Agreed, but I bet that's why there was this caveat

Quote (KootK's Skyscraper Guru)

3) this method will be safe in most instances with respect to ULS concerns.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Quote (KootK)

Yes, absolutely. Add raft slabs for good measure too. Not necessarily chapters but definitely provisions and, where warranted, sub-sections. ACI already has some provisions particular to core walls.

Would be nice if ACI would produce design guides similar to AISC for these items. I don't think the code is the right place for it, but I agree that there is a substantial amount of unnecessary ambiguity involved in a number of the provisions. The closest thing we have is CRSI and the old PCA notes - but even those seemed to fall short of tackling the nuances you are discussing.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Quote (sticksandtriangles)

Wow, never would have thought big skyscrapers would calc out without taking into account flanges and crazy amounts of coupling. I would have thought deflection would get out of control quickly...

Correct me if I am wrong KootK, but I believe in this process the analysis considers the core's total stiffness, but design each face of the core for the load it experiences. I.E. in liue of considering the moment capacity of a full box section, this process would only consider the capacity of the tension flange of the core by itself not taking into account any benefit of the "web" flange reinforcement for resisting moment demands.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Quote (SnT)

Wow, never would have thought big skyscrapers would calc out without taking into account flanges and crazy amounts of coupling. I would have thought deflection would get out of control quickly...

Sorta depends where you're at with cracking and neutral axis depth for SLS. If you're cracked with a shallow neutral axis depth, your compression flange adds little to flexural stiffness and your tension flange adds less.

An interesting corollary is whether or not one should consider the SLS event to have occurred fresh or after the occurrence of some version of a ULS event that would exacerbate cracking for the next SLS. As far as I can tell, everybody's going with fresh.

Quote (jayrod12)

Agreed, but I bet that's why there was this caveat

Yup, good eye. Some liberties taken with SLS.

Quote (EZBuilding)

I don't think the code is the right place for it...

Quote (EZBuilding)

...but even those seemed to fall short of tackling the nuances you are discussing.

I'm not sure that I've accurately conveyed how simple my proposal would be. Something like.

Element; %Ig Seismic ULS; %Ig Wind ULS; %Ig Wind SLS

Wall; 35; 50; 75
Coupling Beam; 15; 35; 50

Do this or provide some damn convincing reasons for why you didn't.
<-- already exists a general statement in ACI/CSA.

That's it. I didn't put much thought into the particular values and, frankly, I don't even care all that much. I just want some guidance and a level playing field.

Quote (EZBuilding)

Would be nice if ACI would produce design guides similar to AISC for these items.

Hell yeah it would be nice. It's actually been a longstanding dream of mine to create such a guide myself. The trick is that I'd get lambasted six ways from Sunday on every aspect from engineers disagreeing with me. I figure it would have to be sort of a "survey of common practice and pertinent issues" rather than "design your walls like this jackasses.". You know, in the interests of diplomacy and book sales. Wouldn't want to screw my publisher in the insanely unlikely event that I actually found one.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

I used to tear my hair out wondering at the particular form of zones like this. How much bar goes in which wall? How do you design the stirrups for longitudinal shear to be able to properly engage the bars in the other wall? What about bi-axial bending of the core? What about closed shaft torsion? Arghh... the complexity!!

Turns out most of these will just be overlapping linear wall designs and nothing a lick fancier than that. To a degree, I believe that even a compound wall, ETABS design will be handled tin a similar fashion.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

KootK - which one are you referencing the overlapping linear wall designs pre or post analysis?

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Quote (EZbuilding)

Correct me if I am wrong KootK, but I believe in this process the analysis considers the core's total stiffness, but design each face of the core for the load it experiences.

Nope, modeled with boxes literally broken up into line segments with no vertical shear connection between them. I know... that's what makes it so shocking.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Quote (EZBuilding)

KootK - which one are you referencing the overlapping linear wall designs pre or post analysis?

Post.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

I would wager 50% of the companies in my area of practice consider as such. The other 50% lean towards considering entire cores in their design - but often without considerations of the topics you raised.

I have once considered the longitudinal shear between cores of a shear wall before in a more complex core configuration. We were able to justify it as acting as a composite group at the time.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Quote (EZbuilding)

Correct me if I am wrong KootK, but I believe in this process the analysis considers the core's total stiffness, but design each face of the core for the load it experiences.

Unless I'm mistaken, I believe that you've described what I feel is one of the bizarre idiosyncrasies of the ETABS design process. Flexural capacity will be allocated as you've described above which:

1) Does provide enough aggregate flexural capacity.

2) Is not really how anybody would reinforce a compound shape, flexural member by hand and;

3) Results in more reinforcement further from the extreme tension face which would compromise stiffness to a degree.

As somebody said previously, this is just the best that we can do for now so we roll with it.

Quote (KootK)

3) Stuffing #2 into ETABS, PERFORM 3D, or whatever the latest fad is and aggressively marketing the results as boundary pushing innovation that only the insiders are capable of.

I realize now that it's been sounding like I have some kind of beef with the CSI products. I don't, at all. My comments would apply equally to any FEM package and are more about the particular users of the software rather than the particular software package. This is almost a disadvantage of CSI's incredible success. In my brain, ETABS = BIM Structural like KLEENEX = the stuff I wipe my nose with.



RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Quote (EZbuilding)

I have once considered the longitudinal shear between cores of a shear wall before in a more complex core configuration. We were able to justify it as acting as a composite group at the time.

1) My experience has been the same. Usually the column-ish ties that you'll provide to the zones will be more than enough to get this job done and, if anything, I suspect that this is one of the better legitimate reasons to divide zones up into overlapping sections. Otherwise, the layout shown below would be just fine and, perhaps, more constructible.

2) One thing that makes things worse for this is that, unlike with composite steel design, a concrete design has to consider the peak longitudinal shear stresses at any point rather than laying claim to an averaging of the demand over the interface. This comes up often when somebody wants to put a vertical construction joint into a shear wall.

3) For small zones, I sometimes worry about shear friction on a plane take just outside of the zones. You always have enough horizontal bar to handle shear friction there but you don't always have full development on those bars within the zones. I don't sweat this too much though since I'm not all that sold on the whole [shear fricition = fy development] thing anyhow. You know, that and the fact that nobody in the history of mankind has every heard of such a failure taking place.





RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Quote (JP)

What good is a 5% reduction in force from your analysis if you screw up the construction documents with bad detailing?!

The most egreggious detailing fail that I see routinely in FEM shear wall design is shown below. It's like our brains turn off shortly after our computers are turned on. This one's super easy to spot too. If I see that that somebody has terminated shear wall zone bar within the first couple of stories, I know that I'm dealing with the uninitiated.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Just so I am clear on this longitudinal shear concept here... A few clarify images/comments:

For steel to make a composite section you need to ensure shear flow between the connected flanges and webs... is this the same for concrete core walls?


S&T

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Almost. As I see it, it's your horizontal bars and zone ties acting as your SF reinforcing.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

KootK, that makes senses on a shear friction check... my mind went to the vertical rebar type check similar to our longitudinal welds in plate girder.

The method I had in my mind would be difficult to figure out how many pieces of rebar are effective in resisting shear flow across a theoretical plane.

S&T

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Quote (KootK)

As I see it, it's your horizontal bars and zone ties acting as your SF reinforcing.

Thank you! I was about to write something similar. Then, I started 2nd guessing myself. In the low rise buildings that I've worked on, we usually designed the walls as if they were separate. Added in some end zones ties maybe. But, pretended the walls didn't connect with each other except at floor levels. Therefore, I figured it best to let someone else with more experience reply.... So, thank you for clearing that up!

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

You're most welcome Josh. Until EZbuilding building mentioned his longitudinal shear check, I'd been operating under the assumption that I might be the only engineer on the planet to have thunk this particular thought. And I'd encourage everybody to table this issue gently with their superiors and colleagues lest they be tagged with the career killing "over thinker / analysis paralysis" tag. I should probably get a T-shirt to that effect.

Actually, somewhere in Paulay and Preiestly's tome, I'm pretty sure that they look at a similar issue for the corners of basement walls when the basement is used as a monstrous, stabilizing box for overturning purposes. There, they conclude that vertical bar is required in the joint because the joint may not be long enough for an "around the corner" transfer of strut compression as is the case with taller walls. This is sort of heading in the direction that sticksandtriangles was heading I suspect.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

From the IStructE Manual for the design of concrete building structures to Eurocode 2:

5.6.4.2 Intersecting walls
Where the composite action of intersecting walls to form a core is assumed the interface shear should be checked in accordance with EC2, Clause 6.2.4 Shear between web and flanges.

Found on Page 91

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Nice... thanks for reporting back on that Trenno. I feel like less of a load path weenie now.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

So what's the consensus here?

Should we pursue an automated linear elastic approach that iterates the stiffness of walls based on codified equations for Ieff?
OR
Should we pursue a non-linear approach with layered shells?

Would like to hear Agent666's opinion.

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

2
What a thread, what a read! The amount of salt being poured out is quite incredible. I salute and thank you all for your opinions, mostly because they to large extent validate my own. Let me join in as I have watched designers average longitudinal stress across the entire height of a building core and I can no longer stay silent. My personal nemesis are stress concentrations around core openings. When modelled properly the fully-fixed coupling beams distribute flexural stresses very unevenly to the inevitably short wall sections around lift openings. This results in very high design loads at such locations and I am tired of being yada-yada'd away and watching some of my more agreeable colleagues take over.

Regarding "to iterate or not to iterate" dilemma - I think that no amount of fudging of an inherently linear FEM model can yield a physically accurate result for a concrete structure. Representative? Yes. Accurate? No. Besides when approaching an iterative problem it is important to first establish that the iterative approach converges; then it is important to establish that the iterative approach converges to a correct solution. I think that the following two things are major hurdles:

1. Design loads that typically get applied to a structural model are somewhat arbitrary approximations at best; they are backed by good research and decades of experience, however they are not perfect. I think that starting with arbitrary loads, then iterating Ig values and expecting accurate results is just not a sound line of design reasoning.

2. Once/if one does begin iterating Ig values across an entire structure I am just not convinced that one will not end up in an endless loop chasing their tail. The OP has already stumbled onto this fact and I think that no matter how refined the adopted Ig scale is one will likely end up in a maze with no end.

I think that a more viable solution is the one suggested by @Kootk - a set of building-wide Ig values for walls/columns/beams. Is such approach accurate? Hell no, such approach is doomed from the onset (see point above regarding nature of design loading). Is such approach representative? I hope that it is because in the essence it is the state of current code guidance and it is what I use in my FEM models.

Lower Ig values are certainly more conservative for SLS states and thus safer to adopt. They assume concrete cracking and it is important to bear in mind that concrete cracking is a cornerstone of concrete design. The only issue I have with lower Ig values is that they yield lower seismic loads...which is a broader debate but then again if we assume in ULS design that concrete is generally allowed to crack then we have to carry this assumption through into analysis.

Physically accurate modelling begins with design loads and lies in the land of performance based analysis and true non-linear FEA modelling. This land is a dark place and my opinion is that we are only just starting to chip away at it. My only worry is that the incentives may not be there as the focus is being shifted away from material savings and towards building program savings (once again, this is just my perspective).

RE: Concrete Shear Wall Stiffness Adjustment Factor – Iteration Process Discussion

Great post, Captain Slow.

Your 1) point has always urked me. We can calculate numerous factors with big fancy equations with endless analysis. On the other side of the Demand v Capacity equation, we just say "oh this building is X type, so it takes Y load."

I'm sure there is some sort of quote out there that says something along the lines of: "it's not so much about whether you need that amount of rebar, it's more about whether you need the beam in the first place."

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