Rafter without fly brace? 22

[fourpm]

I am designing rafters to AS4100 and wondering what if I don't use fly brace. I understand that with fly brace it will give you full restraint. But if I don't use fly brace, will the purlin above be considered as lateral restraint for rafter under uplift? If so. can I take the purlin spacing as segment and the only factor that changes without fly brace is kt?
I have the same question when it comes the continuous steel floor beam design where Z/C floor joints sit on top of the beam. What segment should I take for the beam near the support? Can I take the floor joists spacing as segment with lateral restraint? Can anyone give me some examples? I have read some manuals but the examples they have are simply supported beams only. Thank you.

 

[Tomfh]

6) The AS4100 business about the "absence of any restraint" seems to lead designers to examining a case where all restraints are removed as a means of establishing the LTB buckling shape for a case when all of the physical restraints are present. How much sense does that make? None. And, in spite of the way that the provision is written, I'm sure that it's not what anyone actually intended.

What is intended then?

 

[KootK]

My gut feel is that it's next to impossible to get a multi-span continuous beam to LTB buckle if it has full restraint at the supports and closely spaced lateral restraint anywhere on either flange.

Initially you said laterally bracing the top flange provides negligible increase in capacity unless you also provide twist restraint, hence your advice for OP to take 8m as the buckling length. Now you say such lateral bracing makes it next to impossible for the beam to LTB buckle?

Unfortunately, this has become highly nuanced. What I said was this:

1) The buckling length for calculation purposes should be taken as 8m, the length between points of twist retraint.

2) In terms of theory and standard calculation procedures:

a) Normal code procedures usually do not take account of constrained axis LTB and, thus, offer no calculation benefit associated with the top flange lateral restraints.

b) If one desired additional capacity at the expense of additional complexity, one could consider constrained axis buckling to get more capacity as a result of the top flange lateral restraints.

3) In terms of real world behavior for the system at hand, I am indeed skeptical that LTB poses a real threat. That said, the name of the engineering game is what we can prove, not what we suspect.

I see all this as being internally consistent.

 

[KootK]

What is intended then?

It's always dangerous to attempt to read minds but my guess is as shown below. I also speculate that this was introduced as a way to deal with the usual confusion at cantilevers. And then, as always, came the unintended consequences...

a) The critical flange at any cross-section segment is the flange which in the absence of any restraint at that section would deflect the farther during buckling given the lateral and torsional restraints present. OR;

b) The critical flange at any cross-section segment is the flange which, in the absence of any additional restraint at that section contemplated by the designer would deflect the farther during buckling. given the lateral and torsional restraints present.

 

[Tomfh]

3) In terms of real world behavior for the system at hand, I am indeed skeptical that LTB poses a real threat. That said, the name of the engineering game is what we can prove, not what we suspect.

It’s easy enough to prove with a finite element buckling analysis. Or a real world test.

It's always dangerous to attempt to read minds but my guess is as shown below

I don’t agree with second guessing the code like this and arguing that by black they really mean white. I think that by “compression flange” they simply mean the flange in compression, and that by “absence of restraint” they mean just that, as opposed to the opposite - the presence of the restraints.

 

[KootK]

Black vs white seems an extreme characterization. I'd say that it's the same fundamental intents articulated with greater precision.

 

[human909]

Thanks for your reply KootK. That answers my question and I think we are largely on the same page.

b) The critical flange at any cross-section segment is the flange which, in the absence of any additional restraint at that section contemplated by the designer would deflect the farther during buckling. given the lateral and torsional restraints present.

KootK for AS4100 revision!

That would be a big change in the general approach of AS4100 but it makes much more sense.

Modest changes but enormous implications. It's minor but we also shouldn't be speaking in terms of "cross sections". Rather, we should be speaking in terms of "segments" because LTB is a linear phenomenon rather than a point phenomenon.

Very true and very odd that this terminology is present.

6) The AS4100 business about the "absence of any restraint" seems to lead designers to examining a case where all restraints are removed as a means of establishing the LTB buckling shape for a case when all of the physical restraints are present. How much sense does that make? None. And, in spite of the way that the provision is written, I'm sure that it's not what anyone actually intended.

Wow. I was going to disagree with you and agree with Tomfh. But now I'm second guessing myself. Your intepretation seems to turns a fair bit of the interpretation of AS4100 on its head IMO. But like you argue, it makes more sense from real engineering perspective. I'm going to have to dwell on this one.

 

[Tomfh]

Very true and very odd that this terminology is present.

Either that or the code simply means what it says.

 

[human909]

Either that the code simply means what it says.

But what it says isn't entirely clear. As has been exhibitted in this thread. But you could be right here. Determining the force requirements for the critical flange restraints requires defining a critical flange at a cross section not a segment.

Sorry that I'm sitting on the fence a bit here. (When I've been in doubt over buckling, I jump over to buckling anaylsis software to help check my concerns.)

 

[KootK]

Determining the force requirements for the critical flange restraints requires defining a critical flange at a cross section not a segment.

I disagree with that as a defense of the original verbiage. You could have just as easily said:

Determining the force requirements for the critical flange restraints requires defining a critical flange at a cross section within the segment being braced.

In fact, I would argue that this would be more rational given that any meaningful determination of a brace force has to give consideration to the length of the thing (segment) being braced. Even the ridonkulously simplified 2.5% business is really something similar to:

[anticipated misalignment over segment length]/[segment length] = 0.025

Sorry that I'm sitting on the fence a bit here.

Jump the fence until your pants split and apologize for nothing. The largest impediment to productive discourse is folks holding their tongues until they're 107% confident in their opinions. I say jump in the pool at 60%, act like you own the place at 85%.

This is an awesome discussion...

Right?? It's like the Thrilla in Manilla of 2019 LTB discussions. And it is that because you've got some bold people here willing to put their egos at risk to defend their beliefs vigorously. Welcome to the conversation.

 

[human909]

The largest impediment to productive discourse is folks holding their tongues until they're 107% confident in their opinions. I say jump in the pool at 60%, act like you own the place at 85%.

I definately agree with you on this one. I've learn by vigorous debate and exploration. I make claims based on my gut and then dig up the evidence to prove it! But I'm going to have to give this more pondering than I currently have time for before I get off this uncomfortable fence.

Don't worry. I'll remember this post of yours for the future and I'll endevour to point out your mistakes when I'm only 60% sure.

 

[WinelandV]

Right?? It's like the Thrilla in Manilla of 2019 LTB discussions.

Maybe for 2019, but this is no "Concrete Retaining Wall - Opening Corner R/F" or "Concrete Shear Friction Black-Box/Magic Voodoo". ;-)

 

[hokie66]

Good one, winelandv. Maybe after the NTSB report on the Florida bridge, we should have another brawl about the elusive magic of shear-friction.

 

[KootK]

Maybe after the NTSB report on the Florida bridge, we should have another brawl about the elusive magic of shear-friction.

Can you give me a brief rundown on show shear friction is related that? Sounds like something I might be interested in.

 

[WinelandV]

KootK,

You don't want to read through 13 threads of over 200 posts? Curious.

But I kid. Long story short - Design was using shear-friction to pass horizontal component of truss web member through a cold joint. At least, that's my understanding. But I'll be honest, I've only been skimming the numbers/analysis posts for the last 3 months. hokie66 can hopefully describe it better.

 

[Agent666]

Just heading back to this thread and catching up after a break and missing like 100 replies it seems, not all the way through it yet but need to point out that the following interpretation is actually incorrect.

1) "segment" = the bottom flange between the beam support points where there is full rotational restraint.

This is not how a segment is defined in the code. A segment is just the part of a member (member is the entire member consisting of several segments potentially), defined as follows between any type of restraint be it L/F/P/U. In a positive flexure region with L restraints the segment is between the two L restraints.

I'll get through the remainder of the thread at some point...

 

[KootK]

...need to point out that the following interpretation is actually incorrect.

Prove it. I believe that it is in fact your interpretation that is in error. And I've supplied a good deal of material above to substantiate that claim.

Moreover, after all that has transpired here, is it really appropriate for us to be taking the verbiage of any of this AS4100 section as gospel? I feel that we're already a couple of light years past having concluded that this bit of AS4100 is flawed in a number of respects. To borrow from your blog: engineers vs sheep.

I'll get through the remainder of the thread at some point...

At the least, I'd recommend reading the Yura article if you haven't already.

 

[Agent666]

Don't have time right now before work. But something someone above said set off a eureka moment in my brain & made it perfectly clear how the 5.5.1 clause and subsequent clause is to be interpreted with respect to the cross section (as opposed to segment vs member) nomenclature it uses, no need to change it to suit your interpretations. It's perfectly clear as it's written if you think about how its taught, documented in literature from this part of the world, and interpreted/used etc. This is the intent of the authors, because some of them are still writing and teaching it (like Charles Clifton). I'll post back after work when I have more time, hopefully you can hold your excitement till then.

I don't disagree with many of the other theories put forward, but this is about the exact interpretation and hence intent of these clauses.

 

[KootK]

I'll post back after work when I have more time, hopefully you can hold your excitement till then.

No rush. My excitement will hold for months.

 

[Tomfh]

Kootk, you really should read AS4100 before you explain it to everyone.

 

[KootK]

Kootk, you really should read AS4100 before you explain it to everyone.

1) I have an older version of AS4100 and have read that.

2) I have some steel texts based on newer version of AS4100 and have read those.

3) I asked you and Agent666 to share the recent versions of the AS4100 so that I could participate more meaningfully in this discussion. So, if I'm missing information, that's on you.

4) If your version of AS4100 has some magic bullet clause that I haven't seen yet, and would refute my arguments, produce it. Something tells me that you already would have if it existed.

5) I think that it's poor form to deny my right to participate in this discussion just because:

b) I'm not writing from Canberra and;

a) You're unable to settle this debate as you'd like by way of logical argument.

6) It kinda feels as though we're heading back into this territory again, as shown below: Link.

7) If you don't value my input here, then stop engaging with me. I'm happy to continue the conversation with only those who would wish to continue it with me.