Scissor truss failure 3

[Sam165]

Hi fellow structural engineers. I was recently out on an investigation of a one-story wood frame structure located in Oklahoma built in the 1970’s. The ceiling gypsum board was showing major straight-line cracks across the seams of the panels and diagonal cracking at air conditioning penetrations.

The roof is framed with scissor trusses that span approximately 45-feet. I climbed into the attic and noted that the vertical members directly under the peak of the truss are completely separated from the metal plate. This happened on the majority of the trusses above this structure. One truss also had a weird looking vertical that seemed to be spliced. The supporting exterior walls were not visibly displaced laterally. No cracking in the exterior brick veneer to indicate differential movement.

I’m trying to figure out what caused this failure to occur. Was it bad manufacturing practices when these plates were installed in the 70’s? Moisture/temperature variations in the wood causing the metal plate teeth to pull out? One truss vertical being modified at an unknown time that failed causing a “domino” effect to the adjacent trusses?

Weather data does not seem to indicate any high-wind events occurring at this location recently, and no wind damage to shingles or other components of structure/property was observed.

Thanks,
Sam

 

[Ron]

This is likely a result of several things. Improperly placed truss plates, mishandling during initial construction, then all exacerbated by thermal cycling. It only takes about 1/16 of an inch of penetration loss in the truss plate points to reduce the capacity by 25 percent.

I hope you have shoring under it at this point!

 

[Ron247]

Are those factory-built trusses or site-built? They look site-built to me. I know in the 70s, a lot us carpenters thought we could build our own cheaper than buying them. I also know most of us had no idea how high the connection forces were on a truss system.

 

[BAretired]

I agree with both Ron and Ron247. Could be moisture, but more likely bad workmanship of site-built trusses. The roof needs to be shored because the structure is unsafe as it stands. A good snowfall would probably bring it down.

Then, it needs to be thoroughly examined to see if it is worth repairing.

BA

 

[1503-44]

They look hamered into place. I designed trusses at Clary Corperation and Gang Nail in Dallas for 4yrs when going to university there, actually about the same time as those were made. The truss members were automatically cut to exact spec to exactly fit the geometry of each joint and plates were carefully laid out on each joint before being uniformly pressed into the joints with a hydraulic press. No way will a hammer work. As you pound on one point, the surrounding plate area lifts up and disengages with the wood fibers. Our plates had teeth punched down at right angles to the plates, row by row, thus not making excessively large holes in any one place. Those look like a star pattern punched around resulting circular holes. I can't see that pattern ever developing the capacity of the other method. The longer the teeth, the bigger the hole becomes. Plus each tooth cutting the grain at different angles to the next. Lucky they lasted as long as they did.

 

[XR250]

They look like plates that use common nails. Could be site built. I see these a lot in houses from the 60's and 70's.
I doubt any real design was ever performed other than "looks good to me". I am not surprised that they are failing.

 

[LittleInch]

Really needs a dimensioned drawing/sketch to figure that picture out.

What is supposed to be connected to what?

Looks like a parrallogram to me!

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[oldestguy]

How about changes in dimensions due to temperature effects ad the expansion-shrinkage involved? I've seen several structures with this, but then usually most lift at noon and less at night.

 

[BAretired]

What is supposed to be connected to what?

Looks like a parrallogram to me!

And you're right, remove the vertical and you have a parallelogram, which is unstable.

BA

 

[msquared48]

How long did it take you to get out of the attic?

Mike McCann, PE, SE (WA, HI)

 

[hokie66]

And what additional load did you leave behind?

 

[3DDave]

It reminds me of the Sigourney Weaver scene in the machine room in the movie Galaxy Quest.

Read her lips.

https://www.youtube.com/watch?v=gqRdT8m1Suo