The subject of web braces was brought up in another thread a couple of days ago. I got to thinking that there might be some value in discussing what exactly CLBs are, why they’re needed, etc. I know that many of you are familiar with CLBs. (Continuous Lateral Braces) But I suspect that others may not be.
For starters – What exactly is a CLB? Basically it’s a brace that’s required on a truss web that’s in compression.
To explain why they’re sometimes necessary – Imagine taking a yardstick and putting one end on the floor so it’s in a vertical position. If you put one hand on top of the yardstick and pushed down, the yardstick would buckle with a fairly small amount of pressure.
Now imagine taking your other hand and restraining the yardstick halfway up from the floor. If you pressed down now, it would take a heck of a lot more pressure to make the yardstick buckle. In effect, you’ve added a CLB to the yardstick.
The web of a truss is sort of like a yardstick. It can withstand a certain amount of compression without any bracing. The amount of compression a web can withstand depends partly on its size, species and grade. But the biggest factor is the length of the web.
In some cases, webs can actually require 2 rows of bracing rather than just one. You’ll typically see this in very tall trusses. (Like 12’ tall or more) Thinking back to the yardstick again, imagine how much more pressure you could put on it if it had 2 braces on it rather than just one.
It’s worth noting that the CLBs have to be anchored to something solid at some point. If you use a long board to tie a bunch of similar webs together, they can all buckle in the same direction. Anchoring the CLB is typically supposed to be done with a diagonal brace to a rigid point such as the top chord.
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More in the next post. (I don’t want this to get broken up by Prospero)
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Once in a while the question comes up – Are CLBs really necessary, or is this just something else the truss industry has dreamed up in order to cover their butts?
Like many questions, this one is not all that simple. In many cases, I think the CLBs are just barely necessary and it’s no big deal if they’re left off. In other cases they can make the difference between the truss working and completely failing.
I once spoke to an engineer who had been hired to defend a truss manufacturer in a lawsuit over some large pole barn trusses that had collapsed. The bottom chord splice plate had broken when there was a moderate snow load on the trusses. The building owner was claiming that the trusses were defective.
In the trusses in the building, one of the webs required 2 CLBs on it that were never installed. When the snow loaded up the roof, the webs buckled badly. That put a lot of twisting/bending stress on the bottom chord, which caused the splice to break. Once the bottom chord came apart the trusses all collapsed.
Obviously in that case, the CLBs were needed – They weren’t called out to cover anyone’s butts.
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I’ve never personally investigated a truss failure due to missing CLBs. But I have seen some older buildings where CLBs had been left off. In most cases webs that required only one CLB were in reasonable shape. But anywhere that TWO rows of bracing had been left off, the webs were buckling badly.
Does that mean that it’s O.K to ignore webs that only require one row of CLBs? I wouldn’t recommend it. There may only be on big snow in the life of the building where the trusses are maxed out and CLBs are really needed. But all it takes is one.
One other thing to consider is that if there are ever any problems with the job, they’ll use every little detail they can think of to trash you. If you’ve deliberately left off CLBs it will sure look bad if you wind up in court.
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I'll attach a couple of things in the next post...
I attached 2 things to this post. First is a picture of a trus called "CLB Example".You'll notice that there are 2 different webs in this truss that require CLBs. The 4th web from the left is about 11' long and has about 150# of compression. So it needs one CLB.The 6th web is also about 11' long, but has about 1,500# of compression. So it requires 2 rows of CLBs. The 2nd web from the left is about 7' long and has 550# of compression. Due to the fact that it's shorter than the other webs it doesn't need a CLB..I also attached a PDF ile from the WTCA called "Bracing Dissimilar Webs". It has a lot more info about CLBs than I would want to try to type her. It also has some alternatives to CLBs in situations where you don't have several identical trusses next to each other..Ther's more to CLBs if anyone has any questions about it. But I don't want to go on and on about it if no one is interested...
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Interesting. Looks like you are bracing the diagonal members, but not the vertical ones.
And is the need for a brace a function of the slenderness ratio of the member? The 7 ft 550# member would seem (to an outsider) to have more potential for bending than the one with only 150#."Put your creed in your deed." Emerson
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"Looks like you are bracing the diagonal members, but not the vertical ones."
Yup. Most of the vertical webs are in tension. Webs in tension don't generally require braces. (Although there are exceptions)
"is the need for a brace a function of the slenderness ratio of the member? The 7 ft 550# member would seem (to an outsider) to have more potential for bending than the one with only 150#."
It's more like a column buckling formula. I've seen the formula, but don't have it handy.
Have you ever done any beam design? If you have, you know that when you double a span there's 4 times the bending stress on a beam.
The same is true with webs in compression. When you double the length of a web, it will only handle 1/4 of the compression without a brace.
Let me know if that doesn't makes sense, and I'll take another crack at explaining it.
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OK, Ron, so what do I do when I get trusses that don't call out CLBs?
I just set sevral last fall that weren't taged or shown on the truss spec. sheet. Simple common truss 4/12 pitch, 36' span, that look just like figure 1 of the technical notes.
I can easily add them if needed, but my concern is that my supplier did not indicate that I needed them. Kind of shakes my faith in them.
BTW great thread. As always, the more I learn about trusses, the less I really knew in the first place.
Dave
When you get a delivery, do they send you drawings of each truss? If they do, and CLBs that are required would be noted on the individual truss drawings. A call to the truss company would also likely answer your question. They could tell you if they normally tag CLB locations or not. In general, I wouldn't expect a 36' truss at 4/12 to require any CLBs. If any were required, it would be on the first web out from the center. But it would only be roughly 6' long, so I doubt it would.
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I dug out the drawings.
You are right. They were there. I just missed them. Web #3 and #5 at midpoint.
Glad the ceiling isn't in yet. I'll put them in ths weekend.
Thanks,
Dave
What kind of loading are you working with?I wouldn't have expected a truss like that to need CLBs...
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Thanks for this thread, Boss. It is always good to learn real stuff, whether I think I might need it or not.
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Nice thread, thank you for taking the time to create it and the newer one you started today that led me to this one.
You wrote:
"you know that when you double a span there's 4 times the bending stress on a beam"
Are you sure about that? I thought bending stress was = (M*C)/I where M = bending moment, C = distance to neutral axis and I = area moment of Inertia. If the beam length doubles and there are no other changes, the only thing being affected in that equation would be the Moment which would double, not quadruple.
I would agree that doubling the length of a column would have a squared (2 squared = 4) effect .
The formula I was taught to design beams with is:(Span in feet squared multiplied by the loading in PLF) over 8. The result is the bending moment in foot pounds..I've never seen the formula that you used.
STRESSED is just DESSERTS spelled backward.
OK I see the difference now that I see your formula.
My equation gives the bending stress given the moment and other factors. If you have a span and a load mid span (as opposed to a uniformly distributed load across the entire beam) you have a certain moment and associated bending stress. If you double the span and keep all else the same-including the load ("no other changes")then the moment and stress would only double, not quadruple.
Your equation is specifically for a simply supported beam uniformly loaded across its entire length. If you double your span and keep the same uniform load - now across a doubled length, you've essentally doubled the total load also. Doubled load, doubled span is a quadrupled moment (and bending stress).
So for the loading scenario of a uniformly distributed load across the entire simply supported span, doubling the length and keeping the same load rate across the now doubled length would quadruple the moment and bending stress, no disagreement there. That scenario I'm sure is common in buildings also. Doubling the length would not quadruple the moment and bending stress for all beam loading scenarios though.
Thanks for replying and the formula.
Tim
Boss,
Well I already learned something.
I have never known that the clb should be anchored to a solid anchor
point.
Most often I have done that , but on occasion we see a situation where just a few trusses need a clb , say in the middle of a roof layout. I have always just tied the trusses together and not thought another thing about it.
Inspectors never said a word either. Nor was there any info in the truss packet and detailed instructions telling us to do that.
Edited 3/19/2007 2:54 pm by dovetail97128
" I have never known that the CLB should be anchored to a solid anchor point.........Nor was there any info in the truss packet and detailed instructions telling us to do that."
Interesting point. I just looked over the HIB 91 sheet to see what it said, and it doesn't mention it. But that document is really geared towards erection bracing.
I'll have to look into that...
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It should also be pointed out that the simplest of truss designs often need CLBs for another very important reason. Keeping your walls straight!
Lets say you had a 20' by 20' garage with a 4/12 trussed roof with gable ends. The walls running perpendicular to the trusses will obviously be kept straight by each truss, but what about the walls under the gable end truss. You must install the ClBs and brace them off with 45 deg braces up to the roof plane before removing the temp braces on those walls.
If you were to remove your temp braces without having the proper truss bracing installed, not only would those gable end walls not be straight, but they would be flimsy until you put up drywall on the lid. The dyrwall would take some of the flimsyness away, but one gust of wind on the gable end would likely give you some good nail pops on the finished lid.
"It should also be pointed out that the simplest of truss designs often need CLBs for another very important reason. Keeping your walls straight!"
What you're referring to is overall building bracing, and is unrelated to web bracing. (CLBs)
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Good thread Boss, I use trusses on every house I frame and like to know why certain things are required.
As for gable walls, I take two 2x4's and nail them diagonally from the center of the gable to the outside walls. One in each direction. THis is on top of the bottom chord.
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Okay, thanks for the lesson. I have a question for ya...
Can the CLB be run in a diagonal pattern in the area tagged or must be in a straight line? I was taught that by installing the bracing in a running "w" pattern gives it more strength. Hope that my hunmble question makes sense...
Aaron
"Can the CLB be run in a diagonal pattern in the area tagged or must be in a straight line?"
The CLB is supposed to be in the center of the web. So running the braces in a "W" pattern would not work.
Remeber that with a CLB you're only bracing one web, not the overall building.
Have a look at the following picture. I copied it from the WTCA document BCSI 1-03.
View Image
In this picture the CLBs are shown as green. You'll note that they run in a straight line.
You'll also notice the diagonal braces shown in red. These diagonals keep the CLBs from allowing all the webs to buckle together in one direction.
Does that make sense? If not I'll try again.
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Good info, thanks Boss.
Joe H