Wood Truss Buckling in Existing Building

[CBSE]

Have an investigative project. A 2-story wood structure that the owner called and said the second story wall shifted outward from the building. I spent about 2 hours looking at different structural aspects and then decided to look at the trusses a little further after not finding any thing structurally out of order or looking stressed (other than the obvious fact that the wall had rotated outward). What appears to have happened is most likely during the heavy snow we had last year, the bottom chords of these trusses buckled under the weight and pushed the wall out. The owner is just now noticing the movement due to a light fixture cover falling down.

The wall that has rotated outward is leaning about 3/8" in 48", so it's a pretty good sized deflection. The total height of the studs is 10ft. The trusses span 54ft, there is a single rat run down the middle of the trusses that runs nearly perfectly perpendicular. Has anyone seen this type of failure before? Questions:

1) Is it possible to "pull" the trusses back into place?
2) If it's not possible, is the best solution to add bracing to the trusses to keep them from buckling more? Or replace the trusses all together?
3) It doesn't appear that the tenants are in imminent danger at this time because the roof is not loaded, other than dead load, however, it seems if there was a sizable wind event, the suction force on that end of the building could potentially exacerbate the situation. Evacuating the end of the building that has the issue may be in order until it is fixed?

Attached is a section of the trusses.

 

[JAE]

For the question of residential tolerances - look here:

https://builderbooks.com/media/flip...tractor/r/c/rcpg_4th_ed-contractor_final4.pdf

This is the NAHB Residential Performance Guidelines book.
A portion of it that deals with wall out-of-plumb limits:

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[CBSE]

I tend to agree that the bottom chord buckling may be very odd thing. The bottom chords of the trusses displacing outward is the only thing that really makes sense to me right now, or some form of rotation of the trusses to make the diaphragm shift out in the middle. In the section I drew in the original post, there are a few trusses that are actually quite shallower and the overframing goes over them. There is about a 4x8 sheet of plywood attaching them together, but no where for any load to go to speak of. The compression side of these trusses may have buckled and started the rotation.

I haven't seen any signs of distortion in the roof diaphragm at any of the hips. The exterior building is all brick and stucco. There aren't any cracks in it, other than at the typical control joints.

The owners are claiming that the movement just happened in the past week. The acoustical ceiling tile connection to the wall has pulled away from the wall as well, but only in the northern most wall. I checked the local weather stations for exceeding large wind loads and I check the seismic activity. There hasn't been anything major for a long time. Last year was a record snow year, so it's really the most logical design event I can think of at this point. For all I know, the wall could have been like this for years.

I checked the first story walls and floor for plumbness. The first story walls are straight as an arrow. The floor system is level, aside from some dips here and there from long term deflections.

 

[CBSE]

JAE: thanks for the reference. It appears the wall is most likely within the tolerance limits of the residential guidelines at least!

 

[dik]

Is there any sign of movement at the wall to ceiling interface or the adjacent wall to wall? For recent movement, there should be some distress? Any windows? any movement at windows? are they binding?

Maybe the movement was only noticed in the last week? and it could have been constructed that way. I've often done building reviews where a new crack has developed, only to find 'spider webs' and dirt within... and often looking for one thing, another item is noticed...

Dik

 

[CBSE]

I looked at the return walls and there is nothing. No new cracks, walls are plumb (in and out of plane). I just don't know, I'm baffled in a sense. The wall that moved exhibits the classic smiley face that we all see in text books showing diaphragm deflection. If the entire building racked, there would be lots of cracks in the drywall and exterior stucco, especially with this much deflection.

 

[dik]

Is there a chance it could have been built out of plumb? or, anything to indicate that it wasn't?

Dik

 

[CBSE]

I would have said yes originally, however, the edge track for the acoustical ceiling tile is visibly pulled away from the wall that rotated outward. There are also areas where the acoustical tile has been scrunched at interior non-bearing wall areas/partition walls. It just seams like it has had to have been like this for quite some time.

 

[KootK]

In the section I drew in the original post, there are a few trusses that are actually quite shallower and the overframing goes over them. There is about a 4x8 sheet of plywood attaching them together, but no where for any load to go to speak of. The compression side of these trusses may have buckled and started the rotation.

This makes me nervous. Are the tops of the rotated trusses attached to the diaphragm at the top? As a former truss guy, I can tell you that it's a common problem for contractors to omit full truss sheathing beneath the overbuild to brace the tops of the primary trusses. And it's rare for truss designers to design truss top chord bracing to deal with that.

I don't like that the snow storm was last year but the movement is reported as having occurred last week. That's making me feel less confident about my "no evac" recommendation.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[CBSE]

The main trusses are attached to the roof diaphragm. These shallower trusses are not attached to the roof diaphragm, they just have some sheathing on top with some 2x4's coming down from the overbuild.

My camera's memory card is corrupted and I'm unable to retrieve my photos from the site, so I'm trying to get their maintenance staff to take more pictures for me. I will be going back to the site later this week anyways.

I already wrote a memorandum giving the recommendation to evacuate this particular wing of the building until they can get a contractor in to place more bracing. It may disrupt their operations, but all I have to go by is this movement just happened, and better safe than the alternative. I didn't sleep much last night thinking about the decision of not recommending this on my initial site visit, so I will certainly sleep better tonight!

 

[hokie66]

Just one of the downfalls of depending on a diaphragm in the plane of the roof, rather than the ceiling where it should be.

 

[dhengr]

CBSE:
If your hip roof section is correct, the roof is not framed the way the plan shows. There is a sizeable 3 member girder truss 3 truss spaces out under the hip roof. How much does it deflect under the heavy snow loads? Then you have 54' long main trusses, and what is their deflection under the heavy snow load, and thus the ridge deflection. Then you have continuous hip roof rafters, with potential substantial thrust outwards at their exterior wall bearings. Finally, you have a 2x6 continuous rat run on the bot. chords back into the bldg. The hip rafter thrust, and the ridge settlement would easily move the top of that left ext. wall out 3/8" and the 2x6 would likely try to pull the truss bot. chords along with it.

 

[CBSE]

dhengr:

The truss plan doesn't match what was done...I should have clarified that. The girder truss sits back 10ft from the edge of the wall. The truss manufacturer ran the hip tails wild over the shorter trusses and the contractor installed vertical 2x members from the tails down to the shorter trusses in the section view. I imagine there is quite a bit of deflection.

To exacerbate the issue further, there are hangers in less than half of the trusses that frame into the girder truss...left that nugget out as well.

It's just a bad scenario all around.

 

[BAretired]

From geometry, a 2" deflection of the girder truss results in an outward deflection of 1" at the top of the wall at midpoint. This probably occurred earlier under heavy snow load. It likely occurred at the other gable end as well but that wall came back into position. The difference between the two, as mentioned by dhengr is the rat run which likely prevented one wall from returning to its original position when the snow was gone.

BA