Wood truss web bracing 6

[RontheRedneck]

This subject came up in another thread. I figured it was worth discussing in a separate thread.

I'm talking about CLBs - Continuous lateral braces on truss webs. (Not erection bracing or purlins) Since the majority of you are engineers I assume you know what they are and why they're used.

In another thread someone asked who polices things in our area (rural Illinois) to make sure they get put on. The answer is no one does. We occasionally ship trusses to a municipality that has building inspectors. But for the most part people can do whatever they want.

If you're wondering what we do to notify framers - We send a packet out with every load of trusses. In that packet is a full set of truss drawings. Every CLB location is shown on the drawings.

In the plant they put a tag on webs that require CLBs. Not on every last truss, but at least on each package of similar trusses.

So what happens if the CLBs are not installed? In my experience, mostly nothing. With one significant exception.

In most cases with residential trusses, you'll have webs that require one CLB. In some jobs with really large or tall trusses you might have webs that require 2 CLBs.

I have been out on a lot of jobs and in a lot of attics. I have never seen any problems caused as a result of CLBs not being installed on residential jobs.

However commercial buildings are a different story.

When we get into really span trusses that are tall, webs requiring 2 CLBs are not uncommon.

Here's a picture of a building I was in several years back. The diagonal webs needed 2 CLBs. where the webs buckled so far that some of them broke, and some pulled out of the plates at the ends of the webs.

I've seen buildings go down as a result of CLBs being installed wrong. Two winters back one of our customers had a large livestock building go down. The trusses had webs that required 2 CLBs. They put all of the CLBs on the webs, and even put them in the correct location on the webs.

But they did not brace the runs of bracing off to anything. So when the webs started buckling, the rows of bracing made sure that all of the similar webs buckled together. That increased the bending moment in the BC to the point that a splice plate snapped.

Thankfully the building was unoccupied at the time and no one was hurt.


As a result of the above experience, I do everything I can to keep from sending trusses out that require 2 rows of CLBs on any webs. I push other truss designers to do the same.

If we have a commercial job where that's not possible, I typically write something to the contractor. Or occasionally write a letter that I send along with sealed drawings. Something that will hopefully get some attention. And something they can't claim they never got.

Does it do any good? Maybe sometimes. Certainly not all the time.

The company who lost the trusses a couple of winters back pays more attention now. Others - Not so much. In general men don't like to be told what to do. So it's difficult to get stuff across to them in a way they'll accept.

 

[Aesur]

@RontheRedneck great post, very informative!

One thing you said however I am curious about.

But they did not brace the runs of bracing off to anything.

Was this not on the truss drawings, was this missed, or was this something that should have been provided by others?

 

[bones206]

Thanks for taking the time to highlight this issue

 

[phamENG]

Aesur - per TPI 1, that's the building designer's job, not the truss manufacturer. I've seen web bracing shown on Calc sheets, but never a diagram on site beyond construction bracing. I'll have to start pushing harder for that!

I've seen trusses fail from not having them in a couple houses, but nothing catastrophic. Worst was a church. 200' x 80' sanctuary with unbraced piggyback trusses. The worst were buckled about a foot out of plane.

 

[Aesur]

per TPI 1, that's the building designer's job, not the truss manufacturer.

And that is the issue, the EOR does not know what bracing the truss manufacturer will require, what forces the braces will have, where it will be and therefore cannot provide said bracing without potentially redesigning after shop drawings are issued for the trusses. I see many projects as of late that don't even give a truss layout for the roof and let the truss manufacturer figure it out completely and only provide a couple keynotes, add a column at girder trusses, etc.. (not a fan of this at all, but I was told they were tired of the truss guys not working with them before they had a signed contract on the job and therefore the layout on the contract drawings was not optimal for the manufacturer so the manufacturer changed the layout and refused to budge). Luckily I have a few great connections locally that are willing to look at layouts and give feedback.

I now see why some people say the truss manufacturer shall provide trusses that require no CLB's. I may follow suit soon.

 

[human909]

I now see why some people say the truss manufacturer shall provide trusses that require no CLB's. I may follow suit soon.

Given the typical workflow on the design side and the work flow on the construction side of things this seems to make plenty of sense. Unless there is good and clear communication between the engineer, the truss manufacturer and building contractor then it just seems like a mistake waiting to happen. (And by the comments above it happens quite regularly and most of the time goes unnoticed.)

It seems it seems like false economy in most cases to design a truss that requires CLBs.

Here is a particularly severe example:

**FYI. I am a novice engineer in this field. So take my comments as you will.

 

[RontheRedneck]

Aesur asked: "Was this not on the truss drawings, was this missed, or was this something that should have been provided by others?"

With each load of trusses we send out a copy of BSCI-B3:

https://hancocklumber.cdn.brillianthosting.app/uploads/2022/03/b3.pdf

Which usually goes straight in the trash and nobody looks at it.


phamENG brought up piggyback bracing. That's a different but similar subject. I'll try to write about that when I have some more pics to go along with it.


Aesur and human909 brought up the idea that trusses should be designed with no CLBs required.

In some cases it's possible to increase the size and/or grade of lumber in order to eliminate CLBs. But that makes the trusses more expensive. It's a good way to price yourself out of a job.

Sometime you can alter the web pattern of trusses to eliminate or reduce CLBs. But eliminating all CLBs is impossible. Even if you go with he highest grade of 2X12 that exists, the 2X12 is only 1 1/2" thick.

Wind loads also cause more CLBs than back when we used to design them without wind loads. Like on a flat truss, you could put the diagonals in tension and there were no CLBs. With wind uplift cases they now require CLBs no matter which way you run the webs.

 

[RWW0002]

Ron - Thanks for starting a new thread for this one. I probably should have done that in the other thread instead of tacking on to that discussion..

For what it is worth I have definitely witnessed some web buckling issues in trusses in the field, but I will agree that trusses without the necessary bracing sometimes fall in the "I have no idea how this thing has stood here this long" category..

So what happens if the CLBs are not installed? In my experience, mostly nothing.

It is worth noting that many trusses have not really seen their full design load. I see truss failures and issues after big snow events around here since it is the first time many of the trusses have ever seen anywhere near their design loads. I'm sure you see similar in Illinois.

While we may not notice a ton if issues in the field, I still think it is important that someone polices this. Buckling failures can be nasty and sudden (2 things we like to avoid in the potential failures realm)

I agree that policing the bracing is specifically not your job (as the truss designer). In areas with an EOR and that require stamped truss designs (at least for commercial jobs with more chance of long-span trusses) I think this falls squarely on the shoulders of the EOR and inspectors. It is a pain to police/enforce but it comes with the territory.

I typically write something to the contractor. Or occasionally write a letter that I send along with sealed drawings. Something that will hopefully get some attention. And something they can't claim they never got.
Does it do any good? Maybe sometimes. Certainly not all the time.
The company who lost the trusses a couple of winters back pays more attention now. Others - Not so much. In general men don't like to be told what to do. So it's difficult to get stuff across to them in a way they'll accept.

Good for you for going above and beyond to try to point out the requirements, but you shouldn't have to. I know having engineers involved in projects is sometimes seen as a thorn in everyone's side, but it is just this role that makes it necessary. Trusses are a great product and over time have been engineered down to something that just barely works (in a good way - little waste) but only as long as the assumptions hold true. Adequate bracing is a big assumption to these systems and it needs to be verified in the field. There is a reason that trusses are the #1 failure that I have witnessed over my career (I bet many can guess the industry for the #2 failure..) Someone, either an inspector or engineer, needs to be verifying that the required bracing is in place in the field.

The other "issue" with truss erection is that there is little barriers for entry into the large commercial market from the moderate residential market. You get plenty of contractors that see trusses and just think - yea I've done trusses before - and erect the same way they do on residential (paying little attention to the truss drawings or bracing requirements). Once you get taller and longer the bracing issues really matter. This is why IBC and TPI have special requirements for trusses over 60'..

 

[bones206]

I had a project recently where the truss supplier included a typical detail for reinforcing web members as a substitute for CLR. I like that because it's relatively easy to install the reinforcing and doesn't require coordination of the CLR brace lines.

 

[kipfoot]

I've used this decades old wood truss council of america detail, referred to it on the building section, and still had to point it out the site. I think that if someone were to get killed during construction or during the life of the building, it will likely be due to a lack of web bracing. I've had no success in convincing truss manufacturers to align the webs of different adjacent trusses so that the bracing could be run continuously.

 

[XR250]

I don't do many big roofs with trusses as most things around here are stick framed.
My field experience is that most trusses braced according to TPI requirements perform OK
If I am asked to provide bracing details, I either put a note that the truss manufacturer should design the trusses so they do not require bracing or I just simply have them double up the webs in the field. CLB rarely gets installed properly and then gets kicked off to a really flimsy gable end.
Piggy back trusses are another story.

 

[BAretired]

It has been a few years since I have been involved with continuous lateral bracing of wood trusses, but I can recall one project which was a major headache. I made many inspection trips to the site before the bracing was properly executed.

I've had no success in convincing truss manufacturers to align the webs of different adjacent trusses so that the bracing could be run continuously.

I had the same problem.

A small rectangle was shown on each shop drawing where a CLB was required, whether or not they aligned with the adjacent trusses. Bracing was supplied by the contractor, not by the truss supplier, so a saving in the cost of trusses did not necessarily translate into a saving overall. In the case of misalignment, a T-brace was used, running the full length of the member to be braced. The contractor complained (justifiably) that he had not taken that cost into account when bidding.

 

[StrEng007]

And that is the issue, the EOR does not know what bracing the truss manufacturer will require, what forces the braces will have

From my understanding, the truss design drawings will not tell you the amount of force that is required in the continuous lateral bracing. Depending on the jurisdiction you practice in, the requirements for this stuff varies.

Take the 2021 IRC for example,
Section R802.10.1, "...Truss design drawings shall include, at a minimum, the following information"
Section R802.10.1.11, "Maximum axial compression forces in the truss members to enable the building designer to design the size, connections and anchorage of the permanent continuous lateral bracing. Forces shall be shown on the truss design drawing or on supplement documents."

The truss designer will indicate which members (web or bottom chord plane) require the continuous lateral bracing. My thought is, when in doubt you should design the bracing force to accommodate the maximal axial capacity of a short/stout 2x4 or 2x6 stud.

Those rules change in some jurisdictions with high wind requirements (as Ron mentioned). Per some state codes, the design & detailing for all bracing is put on the truss designer.

 

[RontheRedneck]

A couple of things to add -

Aligning webs in step-down hip trusses is a good idea, and it's something I try to do. It's not something that's widely practiced in the industry. And you can't align all of the webs.

Also - As usual - No one wants to pay extra for something like that.


As far as designing the angled braces and the connections - Minimum nailing is called out near the top left of the BSCI document I linked to. Here's a screen cap:

 

[dik]

These are critical, and even knowing where the locations are, may not be sufficient. I was involved in a investigation of a total building collapse in Bancroft, Ontario a few decades back.

All the lateral bracing was improperly installed and tension members were braced and compression members were unbraced. Superimposed failure load was approximately 5 psf, based on checking members using Euler buckling. This was the approximate actual live loading based on samples. I made up a 6" plastic pipe 'cookie cutter' to take actual samples of the snow depth. Regular calculations showed failure had already occurred. The collapse occurred as an employee left the building. The total building collapsed, as he stepped out. The added suction of him opening the door may have been sufficient precipitate the failure.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[BAretired]

Aligning webs in step-down hip trusses is a good idea, and it's something I try to do. It's not something that's widely practiced in the industry. And you can't align all of the webs.

Also - As usual - No one wants to pay extra for something like that.

It is too bad it is not standard practice in the industry. If you can't align them all, so be it. At least you took advantage of a common sense solution when possible.

The contractor is paying extra when webs don't align...adding T-braces to the misaligned web.

 

[RontheRedneck]

Since the subject of aligning webs in step-down hips was discussed, I thought I'd post some drawings from a recent job.

There are 2 different sets from different parts of the same job. In both cases the drawings are sequenced from a common truss through a series of step-downs.

I'm not sure how well flipping through the drawings will work online. You might have to download the drawings and flip through them in a PDF reader. I have never tried this before, so I'm not sure.

The 2nd set shows the changes in bracing better. You'll see that some of the webs kept CLBs while others had to have T-braces.

Anyway - If you clock on these links they'll take you to the PD for series 1 and 2.

https://app.box.com/s/zewvw1q83ddabahyyv9b0dbojrt19db9

https://app.box.com/s/uem58cr5vp5eyko8e0nr3hrux0fa2kla

There's no specific point to this. Just thought some of you would be interested in seeing an example of these situations.