Roof Truss Metal Plate Failure 2

[Walder]

Hello All,

During a recent inspection (St. Louis Area for reference) we noticed diagonals separated from top chord at peak/metal plate and are uncertain of why this happened. No recent construction activity or remodels. The resident of 40 years recently noticed the ceiling was sagging/drywall cracking. Let us know your input/opinion/experiences on causes of damage. Thanks!

 

[jayrod12]

Higher moisture potentially causing the teeth to let go? I've seen this on quite a few trusses, sometimes it's overloading, but rarely. UIsualyl it's either the plates weren't pressed properly in the shop, or a high long term humidity has weakened the wood just enough to have the plate teeth let go.

 

[KootK]

1) I used to build design these things once upon a time for what that's worth. Not that I'm a world expert in toothed plate behavior or anything.

2) My take is that the plate separation is product of two factors:

a) Load and;

b) Incorrect plate installation during fabrication.

3) My guess as to the mechanism of failure would be this:

a) The plate with the tooth slots oriented horizontally was installed 90 degrees to what it ought to have been and, more importantly, had very little bite on the web members based on the photos.

b) Because of [3a], the incorrectly oriented plate lost it's grip, making the correctly installed plate on the other side an eccentrically loaded thing.

c) Toothed plates do not like being eccentrically loaded as described in [3b] so the correctly installed plate then gave up the ghost too.

 

[KootK]

Similar to jayrod12's comment, you can see a bit of air gap behind the plate. It's hard to know that's a function of the initial installation or the plate working it's way out over time, however, unless one attempts to measure the depths of the tooth holes. The plates are typically designed to have an 1/8" gap from the outset but, none the less, the gaps don't help matters any.

 

[EdStainless]

Those are small plates for that joint. My old boss would have gone ballistic over that one.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[XR250]

Looks like a roof leak may have occurred in that spot which would support jayrod's hypothesis.

 

[KootK]

Excellent point XR, I'd missed that.

 

[jayrod12]

I think it's a good combination of all of the above. I didn't even catch the mis-oriented plate. That's a great catch KootK.

Also, the amount of purchase of those plates does looks suspect. There's guidelines regarding minimum purchase for truss plates on the members.

 

[hokie66]

Agree with EdStainless that the plates are too small. And they look like the ones which can be bought in a hardware store. Are these trusses home made?

 

[KootK]

A joint like this would mostly likely be a 4x6 plate oriented vertically so it's probably a little small.

I was going to ask about DIY too. The plates look like the real thing to me but they are most definitely cut from sheet stock. The normal ones used in a shop don't have edge cuts across the tooth slots. Sheet stock is regularly used in the shop but it does kinda tilt things a bit in the direction of DIY.

Another thing of note is the combination of:

a) a top chord butt joint gap on the side with the plate oriented incorrectly but not the other side and;

b) the appearance on that side that the teeth on that side either weren't dug in too deep and/or pulled out fairly cleanly.

I wonder if the gap is due to the top chord having an out plane kink at the joint rather than a defect in the squareness of the butt joint member cuts. If that's the case, the plate on the side where the tooth slots are horizontal may never have had much bite into the webs. And that would be a defect much more common of a truss built on site using a portable press than of a factory built truss. Not that weird stuff doesn't happen in the shop too due to rolling handling issues etc.

 

[JLNJ]

So why did the web pull away from the chords? Was there a large bottom chord load which needed the web members to resolve itself?

At the ridge, those web members wouldn't normally be doing a lot of work in a top-chord-loaded roof truss.

 

[KootK]

At the ridge, those web members wouldn't normally be doing a lot of work in a top-chord-loaded roof truss.

I believe that statement to be in error although, I know, it's tempting to imagine that a truss just turns into rafters and a tie with top chord only load. I've tried to come up with a persuasive example below to convince you of this. Noodle on it and let me know if you're unconvinced.

It is of course true that a large bottom chord load will increase the web tension in the members coming to the peak. But the peak plate will be doing some real work in tension whether that's the case or not.

 

[JLNJ]

You are correct, of course, about the load in the web members.

I guess what stands out to me is the magnitude of the separation at the ridge.

If the webs were lost, I would expect significant sag in the roof from the resulting rafter flexure. Maybe it's there and the homeowner didn't notice.

 

[hokie66]

There has been a lot of tornado and severe thunderstorm activity in the St Louis vicinity in recent years. Maybe wind uplift jerked the chords up away from the webs?

 

[jayrod12]

I was honestly just in a house that had the same issue

It had the sag on the outside as well that JLNJ is indicating.

In this case it was not new, you could see the sag on historical google streetviews. And the wood that used to be covered by the plate looked the same as the stuff exposed the entire lifetime.

 

[KootK]

I guess what stands out to me is the magnitude of the separation at the ridge.

I see what you'd mean. I think what we're seeing is the miracle of perfect triangulation or, in this case, the transition to the absence of it. This is what I'd expect the deformation to look like in the present state.

 

[dik]

I've used the criteria... the thickness of a credit card.

from TPIC:for some reason it won't upload image... next message.



Dik

 

[dik]

Try again...

worked...

Dik