Wood truss gusset plate failure example

[PATF1engineer]

Following are a couple of pictures that demonstrate some of the problems that lightweight wood trusses with steel gusset plates can present. In the first photo (quality is not too good, sorry) the plates are pulling away from the member. This truss was subjected to heat but only to minimal direct fire impingment. The second photo shows a section where there was some direct fire on the members (I am guessing about 1/4" depth char) and the plates popped off. It shows the depth of the char very well. Both of these plates were in the top chord of the trusses at a splice between two pieces of wood which is common construction practice. They were not at a panel point where the diagonal members come together. Obviously, if these plates fail there is going to be a loss of stability of the members, even when the top chord is in compression like these trusses. This is another good example of why we need to be careful with this type of construction.

[BLSboy]

Great timing of this post!
One of our FF/EMTs and I where just talking about trusses this morning

[Rescue2947]

Thats why when I build I also reinforce the joints with 2x4 or what ever the size truss. Nice post it brings back the dangers in how fast these trusses can come down.

[5pts384]

Just bought a set of trusses they had nails in each plate, don't know if it would help in a fire but he is trying.

[DaSharkie]

How hard can it be to actually put a few nails into the wood insead of these cheap @$$ money savers. Can't cost but a buck more per truss for crying out loud.

[DonSmithnotTMD]

Did anybody see the technique from (I think) NJ? They weren't using gussett plates at all, rather something like the dovetail that used to be used on wooden boxes. The problem is it's still lightweight wood, so there's not a lot to the joint. Naturally, i can't remember where I saw that.

[Yankee750]

How hard can it be to actually put a few nails into the wood insead of these cheap @$$ money savers. Can't cost but a buck more per truss for crying out loud.

It is not the materials that would be the big cost. It would require a significant change in the way they manufacture the trusses and thus create an added labor cost for refitting their assembly line and the extra work to add the nails. Unfortunately until someone is killed directly because of this and their family sues the truss manufacturer, there probably would not be change.

On a little bit of a different note, I was doing an electric job a few weeks ago in a taxpayer type building and the roof trusses had wood top and bottom flanges with diagonal metal pipe for the web of the truss. It is in a neighboring district and I would not be very hot on the idea of going inside a fire there.

[MemphisE34a]

The problem is it's still lightweight wood, so there's not a lot to the joint.

Thats not the problem, thats a good thing. In most construction that uses gusset plates, the wood will burn up before it fails and falls. Large open spans are the only thing you really need to worry about, but I see or hear of chiefs not letting people go into houses where the attic is involved, because the roof may fall.

I call BS. Home and apartment roofs burn up. They very rarely completely fall down due to fire.

Quit coming up with excuses and go inside and put the fire out.

[FWDbuff]

Thats why when I build I also reinforce the joints with 2x4 or what ever the size truss. Nice post it brings back the dangers in how fast these trusses can come down.

Is that approved by a registered design professional (IE Structural Engineer or Architect?) If not you could be opening yourself up to a massive liability. Check the cut sheet that comes with each truss- they are signed and sealed by an engineer, and it specifically states on them, that any modification thereof will void any warranties and/or performance guidelines.

[DonSmithnotTMD]

Thats not the problem, thats a good thing. In most construction that uses gusset plates, the wood will burn up before it fails and falls.

No kiddin? It looked cheesy to me, but I've never built anything big. Anyway, that's why I asked -- thanks. Doesn't look like anybody around here uses that method anyway.

oh yea, I found this -- http://www.cdc.gov/niosh/docs/2005-132/

[PATF1engineer]

Not looking to start a fight, but....

Thats not the problem, thats a good thing. In most construction that uses gusset plates, the wood will burn up before it fails and falls. Large open spans are the only thing you really need to worry about, but I see or hear of chiefs not letting people go into houses where the attic is involved, because the roof may fall.

I call BS. Home and apartment roofs burn up. They very rarely completely fall down due to fire.

Quit coming up with excuses and go inside and put the fire out.

Not quite sure what you are getting at here. I don't think that exercising some discretion and conducting operations in a smart manner based on the type of construction and the fire conditions falls under the area of making excuses. I would never advocate letting a building burn because of the type of construction. But you better believe I am going to factor in that information before I put myself or my people at undue risk. If, in my and the other officers judgement, we can do that operation relatively safely, you better believe we are going to do it. However, going home to my family, and seeing to it that the people working with me do the same, is a hell of a lot more important then trying to prove how mush of a hero I can be by doing something blatenly unsafe.

There is more then enough information out there on issues involving lightweight construction, including some that should hit pretty close to home for you.

http://www.interfire.org/res_file/pdf/Tr-069.pdf

I would never advocate cowardace or excuse making on the fire ground. But ignoring potential safety issues related to building construction is stupid.

[MemphisE34a]

Maybe you need to re-read my post. I stated that in houses and apartments roofs will burn up and away before a complete system fails. Even if a section were to fall, you have the interior walls that will prevent a complete pancake style collapse.

You have proven my point even better than I could have. You apply a lesson you have learned, in this case a roof collapse in a church (wide open spaces) here in my hometown that killed 2 firefighters over 15 years ago when the entire roof system did fail and pancake style collapse due to the lack of interior walls to all situations with truss roofs and gusset plates.

Your speech about going home and being safety susie are well taken, I have 2 1/2 kids myself, but I say again:

Lightweight construction in dwellings and apartments with load bearing and non-load bearing interior walls and a lack of wide open spaces will burn up and away before they fall down.

And just for the record, I am not trying to pick a fight either.

[PATF1engineer]

I am more worried about going through it then it falling on me. I agree that a total collapse may be unlikely. But being aware of this situation is going to make me think a little harder about getting on the roof without a roof ladder or working off an aerial to do ventilation. Or even changing tactics to avoid the roof altogether. If that makes me a safety susie then so be it.

Look, the fire service can bitch about these things all they want, but they are not going away. Being aware of this type of construction and other types of lightweight construction and using smart tactics are the only way to deal with them.

And I am not a chief, just a glorified peon.

[DeputyChiefGonzo]

Is that approved by a registered design professional (IE Structural Engineer or Architect?) If not you could be opening yourself up to a massive liability. Check the cut sheet that comes with each truss- they are signed and sealed by an engineer, and it specifically states on them, that any modification thereof will void any warranties and/or performance guidelines.

The cut sheet may be "signed and sealed" by a professional engineer who designed the truss, but you can be damn sure he wasn't in the factory at the time it was built.

He wasn't there when it the trusses with his name and seal on the cut sheets were loaded onto a flatbed truck.

He wasn't there when the trusses were unloaded at the lumber yard, reloaded onto another flatbed and delivered to the job site.

The truss may be damaged at any time... from the moment it leaves the factory to the moment it is nailed in as part of the lightweight roof support system. All we can do is hope that the local building inspector checks out everthing dealing with lightweight construction when they do the rough carpentry inspection phase of building approvals prior to occupancy.

[Rescue101]

If it's raining plates,I'm moving to another area.If you guys wanna believe lightweights burn "up and out"have at it.Under the remains isn't my idea of a good time.Of course the snow load up here can have quite an effect on the gravity side of the issue.And it isn't just the roof you got to watch on the lightweights,it's what's holding up you feet;or knees if you're doing it right.As Elmer would say:Be veeeewwary careful around these. T.C.