interior truss heel sheathing

[struct_eeyore]

I have a series of wood trusses terminating on the interior of the building which have some very tall heels (13'+)
There is no shear transfer by design here - although some might occur in reality.
I'm leaning towards sheathing this section full height, but am wondering if that might be overkill - in lieu of X-bracing.
(truss profile is hatched in figure below)

 

[KootK]

This is in regard to the nonplanar case, which I think we can agree relies on load paths that we at least in theory avoid such as cross grain bending, moment transfer at the chord/web, etc.

Agreed, that sounds ugly. But then, does sheathing or bracing the end of the truss help that in any meaningful way? I would argue that, if the chord/web non-planarity is a problem at all, it's surely a problem even under just vertical load and unlikely to be ameliorated by the sheathing. I don't see a rotational bracing demand exacerbating that appreciably.

What would be your take on the quote below from the NDS Commentary:

My take would be:

1) I 100% support the notion of providing rotational support at all bearing locations.

2) I feel that the lateral restraint top (roof sheathing) and bottom (wall sheathing) does a pretty kick-ass job of providing said rotational restraint.

3) My hunch is that the "lateral restraint" bit eliminated from the NDS was originally expressed as it was precisely because they were assuming that most members have automatic lateral restraint at the bottom coming from the sheathed walls that support them. Where that is true, top restraint = rotational restraint. But, then, assumptions are dangerous so it's best to avoid them if you're a code writer.

This was unknown to me. Thanks for tabling it.

This would tend to make your spring quite springy

It would but, at the same time, I would argue that springiness is a local, unbraced top chord issue rather than a true member end rotation issue. I'm going to use some hyperbole on this to try to make a point. Please humor me.

Imagine a truss sheathed as shown below. Is there a problem at the end? You be there is. But that problem is the top chord buckling as an unbraced cantilever past the last sheathing faster. I would argue that the truss does not suffer from a true, end rotation issue per NDS. No doubt, the NDS provision exists to prevent a form of lateral torsional buckling in non-truss members. I submit that the failure that we should expect from the condition shown below is not actually lateral torsional buckling. This is a subtle and pedantic difference but a difference none the less.

 

[KootK]

In contrast, I would argue that this condition would represent a true lateral torsional buckling of the truss. I know, we're getting pretty deep into the weeds here.

 

[gte447f]

@KootK, you have just posted an exact replica of my previously proposed column example, which you dismissed as trivial. In your partially sheathed truss, the end vertical is a freestanding column that is pinned at the base and unrestrained at the top. It is inherently unstable, and will scarcely stand on its own, much less support any appreciable weight without toppling over. In this case, you are right to say that the cantilevered top chord will provide a restraining/bracing force, but you are wrong to say that this is not a true end rotation issue per NDS. It most certainly is. This type of rollover/end rotation will happen under vastly lower loading effects and long before any lateral torsional buckling.

 

[ANE91]

In your partially sheathed truss, the end vertical is a freestanding column that is pinned at the base and unrestrained at the top

Sorry to butt in, but doesn’t the unsheathed length of top chord provide restraint via tension to the column, such that it is not freestanding / unrestrained at the top? If you push the peak out-of-plane, then the top chord pulls it back.

 

[gte447f]

Sorry to butt in, but doesn’t the unsheathed length of top chord provide restraint via tension to the column, such that it is not freestanding / unrestrained at the top? If you push the peak out-of-plane, then the top chord pulls it back.

Yes, of course. I acknowledged as much when I said,

you are right to say that the cantilevered top chord will provide a restraining/bracing force

By describing the end vertical as a free standing column (needing to be restrained/braced, in this case by the cantilevered top chord) I was trying to illustrate that the need for this bracing stems from the most basic of stability problems and not just from lateral torsional buckling effects.

 

[KootK]

@KootK, you have just posted an exact replica of my previously proposed column example, which you dismissed as trivial.

I disagree. My example differs from from yours in a very important way.

My objection to your examples was that they described rigid body systems having no element(s) possessing flexibilities that would influence the stability of the system. Barstools and such.

I said that it it defined what I consider to be meaningful structural stability: that where stiffness comes into play such that practical engineering solutions exist.

My example describes a system that does depend on stiffness for stability. Namely, the stiffness of the cantilevered top chord that you described nicely yourself.

...restrained/braced, in this case by the cantilevered top chord...

 

[KootK]

....but you are wrong to say that this is not a true end rotation issue per NDS. It most certainly is.

It is end rotation in sense that the end rotates. But I submit that it is not end rotation of the sort that I feel the NDS provision is concerned with: lateral torsional buckling. Rather, I feel that it's just the local buckling of the end web under compression as shown below.

 

[KootK]

What exactly constitutes LTB in a truss is something that doesn't get much airtime. So we'll have to dial it back to the fundamentals. I'll present it in the context of wide flange beams for simplicity.

I feel that the distinguishing feature of LTB relative to other buckling modes is that the curvatures of the flanges in plan will be different at all locations along the member span between points of rotational support (or free ends).

The truss of my example does not possess this feature.

This, of course, is not something that we are likely to be able to settle by making recourse to Merriam Webster's dictionary.

 

[gte447f]

@KootK, I agree with most everything you continue to present regarding stability and bracing. I just don't agree with your insistence that a meaningful stability situation has to include a flexible brace. Your definition of meaningful stability is made-up, and it's actually a narrower subset of real stability, which is simply and fundamentally the state of being stable (i.e., in static equilibrium) and the degree to which an object or system will tend to remain stable or in equilibrium. The addition of a flexible brace can add to the stability of an object or system, and increasing the rigidity of said brace can further add to the stability.

Humans intuitively understand and recognize stable objects and systems versus less stable objects and systems all the time as a self preservation mechanism. The footstool versus barstool example is an example of this. A lumber 4x4 laying on its side versus a 2x4 or 2x12 laying on its narrow side are examples of this. Most people, even trained engineers, would not say that each of these examples are equally stable. These are valid and legitimate examples of "meaningful structural stability" whether a flexible brace is included or not.

You want to define stability as involving a flexibility component, but if you think of that flexibility component instead as just, resistance to movement, then your definition of stability will broaden from a narrower subset of stability to the full set of real stability situations.

 

[KootK]

Your definition of meaningful stability is made-up, and it's actually a narrower subset...

My definition of meaningful stability is also mirrored in every textbook on structural stability that I've ever read. Every. Single. One. It's not "just me".

All of those references spend 99% of their time on problems that include flexibility because that's what matters for the overwhelming majority of real world structural stability problems.

Where references spend any time at all on problems with no aspect of flexibility, those are usually just kindergarten level problems intended to introduce basic concepts to hapless newbies.

So, yes, I have attempted to limit the discussion to a particular subset of all possible stability problems. That subset being the useful ones. The ones that might actually provide meaningful insight on practical problems in structural engineering.

Got a copy of AISC Design Guide 28 on Structural Stability? Take a spin through there, count the number of examples that they deal with that don't involve flexibility and let us know what you find.

Don't have that, try the practitioner's stability bible, Stability Criteria for Metal structures. What percentage of the material there covers problems where flexibility plays no part?

Got neither of those? Count up the non-flexibility problems covered in this freebie by Timoshenko and compare that to the total: Theory of Elastic Stability. Why do you think that "Elastic" is included in the title of Theory Theory of Elastic Stability? It's the flexible bracing. That's what's elastic.

 

[gte447f]

Feisty indeed! Perhaps more than feisty, but let's move on.

My definition of meaningful stability is also mirrored in every textbook on structural stability that I've ever read. Every. Single. One. It's not "just me".

I have already posted a quote from Salmon's and Wang's "Introductory Structural Analysis" that defines the broader meaning of stability as a basic concept of equilibrium. Once again, your definition of "meaningful" stability is not a real. You continue to focus on "elastic" stability, a narrower subset of stability problems. Ignoring static stability will be at your own peril. I am sure none of the accomplished structural engineering authors you have mentioned would ever do so.

All of those references spend 99% of their time on problems that include flexibility because that's what matters for the overwhelming majority of real world structural stability problems.

What matters in addressing any engineering problem is to address fundamentals first and foremost. Ignoring fundamentals to get into the weeds with more "meaningful" problems will again be at your own peril.

Where references spend any time at all on problems with no aspect of flexibility, those are usually just kindergarten level problems intended to introduce basic concepts to hapless newbies.

Thank you for the thinly veiled insult. I hope you feel better after landing that one.

So, yes, I have attempted to limit the discussion to a particular subset of all possible stability problems. That subset being the useful ones. The ones that might actually provide meaningful insight on practical problems in structural engineering.

Again, I say to overlook or dismiss engineering fundamentals is never going to be practical or wise.

Got a copy of AISC Design Guide 28 on Structural Stability? Take a spin through there, count the number of examples that they deal with that don't involve flexibility and let us know what you find.

Don't have that, try the practitioner's stability bible, Stability Criteria for Metal structures. What percentage of the material there covers problems where flexibility plays no part?

Got neither of those? Count up the non-flexibility problems covered in this freebie by Timoshenko and compare that to the total: Theory of Elastic Stability.

Wow! Talk about a nerd pissing contest. You are challenging my library of engineering reference textbooks, and putting yours on the table to show off. Well done, but you should know that AISC Design Guide 28 is available to AISC members for free, so I think I can manage at least that one. Thanks.

Why do you think that "Elastic" is included in the title of Theory[sic] Theory of Elastic Stability?

Seems like you should be asking yourself this question. Obviously, it is as I've been saying. It's to specify that the focus is on a subset of stability problems, namely elastic stability.

Look, I don't think this is an either or discussion. A "kindergarten" discussion for "hapless newbies" or an intellectual discussion for geniuses. I think this is a discussion of a real world, practical, structural engineering problem that should be approached like all engineering problems, starting at a fundamental level. It is not elementary or "kindergarten" to address and take care of fundamentals. It is good sense, practical, and usually necessary.

Take the concept of joist blocking at points of bearing. Why do we need it? You say for lateral torsional buckling restraint, and you seem to say that since we have a diaphragm attached to the top of the joists, we actually don't need joist blocking at all. Yet, the code says we do. Why? Those dumb code writers.

I say, from a practical standpoint, first and foremost we need joist blocking to ensure static equilibrium (stability) of the joist against rollover. Other considerations, like lateral torsional buckling, or shear transfer out of a diaphragm, or whatever, follow suit in due time under various loading conditions. To me, there should be a logical sequence of engineering analysis and design that progresses from the fundamentals, and coincidentally, such a sequence often mirrors the service conditions of the structure and the loading sequences it will be expected to experience, including during construction/erection.

I say, try building a 2x12 joist floor without blocking at bearing locations, and you will quickly discover the very real and most fundamental reason for restraint against end rotation or overturning long before you have the diaphragm attached or load the joists to lateral torsional buckling failure. Restraint against end rotation is necessary to ensure basic, fundamental, static equilibrium (stability) of the joists. It would not be wise to handwave away this or any other practical, fundamental, engineering principle because one considers it too simple to be considered worthy of the greatest of minds.

 

[RWW0002]

I have to admit I have not followed along with much precision, but I find myself less interested in comparing theoretical stability models as I do comparing performance of similar framing/conditions. I think it is the 2x10 blocking analogy that got this started, and the code-related requirement for blocking in that situation (along with a bit of uncertainty about exactly why that blocking is required).

I still think it might be useful to come up with other similar conditions, where we either require some sort of blocking or do not. This is why I proposed the deep prefab wood truss with interior bearing example earlier. It is a case where, to my knowledge we historically have not had blocking or lateral restraint, other than erection bracing, (unless for load transfer to shear wall below), and a case where the demand on some sort of chord bracing might be considered higher than the condition being discussed here.

Are there any other meaningful examples of deep wood framed members that either do require some sort of "rollover" or torsional bracing @ bearing, or do not that come to mind? Extending this to other materials might be helpful as well. I just finished the AISC steel seminar webcast this week and bearing stability at steel beam bearings is fresh on the mind..