Wood Truss Buckling in Existing Building
Wood Truss Buckling in Existing Building
(OP)
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.
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.






RE: Wood Truss Buckling in Existing Building
RE: Wood Truss Buckling in Existing Building
1) I would say yes you could theoretically push the trusses back into place, however the feasibility of that being at the top of a second floor wall may be questionable. It will likely take a good chunk of force to push them back in place.
2) Whether or not number 1 is possible, you should be adding bracing, especially if there is no rigid ceiling. Take a look at BCSI publication titled "Guide to Good Practice for Handling, Installing, Restraining & Bracing of Metal Plate Connected Wood Trusses". This document has everything you need regarding bracing. Please note (everyone not just OP), temporary bracing of the trusses during erection is the responsibility of the contractor, PERMANENT BRACING is the responsibility of the EOR to specify on the drawings.
3) This is a judgement call. I don't think you'll get any guidance one way or the other on this item.
RE: Wood Truss Buckling in Existing Building
For the wall to move, the roof diaphragm has displaced some with it, or the wall is not anchored well to the roof diaphragm. So I still would investigate connections to MWFRS if you haven't already. Pulling them back into place might be an option, but I would think about pulling from the wall. Hard to say without seeing the entire roof framing system.
Also, look at a clear load path in the bottom chord bracing all the way to roof diaphragm. Could it possibly be from the two trusses with the rafter overhead? Are their top chords not braced at all? That with a combination of poor bottom chord lateral brace anchorage could be the possible cause. As those couple trusses tend to buckle and rotate, if the btm chord bracing anchorage is inadequate, the entire truss system could begin to rotate as you've shown. From the drawing, something appears unstable and a load path is missing somewhere for resisting out-of-plane and lateral forces (maybe???)
RE: Wood Truss Buckling in Existing Building
I guess my question about the wall leaning, is what is too much? I don’t recall right off hand tolerances for walls being plum.
I’m having a hard time with evacuating and going back and forth. I will probably have them evacuate as my gut instinct is usually right.
I have the BCSI document so will be digging into that again to update my own standard details!
RE: Wood Truss Buckling in Existing Building
There are erection and manufacturing tolerances on the trusses, even with the bow/rack/shift/whatever it is are you in tolerance?
The bottom chord needs some kind of bracing, for uplift is nothing else.
RE: Wood Truss Buckling in Existing Building
Upon inspection, each truss had (2)Simpson H2.5A's from truss to double top plate. I didn't find any that looked to be bent or stressed in any way, so I'm not necessarily too worried about the out-of-plane loading. The big concern is the truss bracing, which is the likely culprit.
RE: Wood Truss Buckling in Existing Building
Ive never run into this situation and this might be a crazy idea for a fix if you determine the wall is leaning too much....Cut the joists that are bracing the wall at the top (shoring required), push the wall back to plumb (not as easy as it sounds), repair cut joists, brace trusses properly. This is obviously a shot in the dark without seeing anything but maybe an option.
RE: Wood Truss Buckling in Existing Building
What else would cause the diaphragm to deflect at the top, which is what has caused the wall to rotate outwards, if it's not the trusses racking/rolling? The side walls are in great shape with no signs of distress. There are parts of the acoustical ceiling as well that appear to have "crushed" at the partition walls from apparent movement.
RE: Wood Truss Buckling in Existing Building
Dik
RE: Wood Truss Buckling in Existing Building
RE: Wood Truss Buckling in Existing Building
If my hypothesis is correct:
- Conventional truss bracing may not help you here. Typically, after erection, that bracing is about restraining individual members from buckling. It does little to address whole truss, rigid body buckling unless specifically designed to do so. And I'm not even sure how you'd do that. With nothing restraining the bottom chord, bracing the top chords to the bottom chords alone isn't likely to restrain the racking. It would all come back to the heel condition.
- I really don't know how you'd re-plumb such a large roof without at least resetting the original trusses somehow. I'm sure that the trusses themselves are fine once they've been righted. One approach might be to assess the diaphragm demands considering the out of plumbness as an initial imperfection. If the heel to wall connection can be improved such that things check out considering that initial imperfection, perhaps you can leave the out of plumbness as is.
- I would't be inclined to evacuate the facility unless there's another snow storm on the way. Chances are, the structure just moved to where it needed to go in order to develop the resistance that it needed. This just a judgment call of course. I expect that you'll get a diversity of opinion on this.
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.
RE: Wood Truss Buckling in Existing Building
RE: Wood Truss Buckling in Existing Building
It seems to me that anything causing the bottom chords to kick out laterally would generate a force that would eventually be transferred back up to the diaphragm without producing a net force on the diaphragm or the walls.
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.
RE: Wood Truss Buckling in Existing Building
RE: Wood Truss Buckling in Existing Building
RE: Wood Truss Buckling in Existing Building
RE: Wood Truss Buckling in Existing Building
Not sure. It's interesting. I need to do some destructive investigation at this point of the roof to plate connections and also see if I can find what the diaphragm nailing is.
RE: Wood Truss Buckling in Existing Building
In this case, you're claiming that the BC are displaced, not that the trusses are rotating due to diaphragm movement, right? If so:
1) What would be the mechanism for the BC displacement given that it's a tension member under snow? Tension chord buckling is all that I can think of and that's pretty exotic. It also wouldn't aggregate much force in any bottom chord bracing.
2) Any force developed in the bottom chord bracing would make its way out to the truss heels at the end of the building and back up into the diaphragm wouldn't it? If so, that's a net force of zero on both the roof and the walls.
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.
RE: Wood Truss Buckling in Existing Building
Dik
RE: Wood Truss Buckling in Existing Building
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: Eng-Tips.com Forum Policies
RE: Wood Truss Buckling in Existing Building
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.
RE: Wood Truss Buckling in Existing Building
RE: Wood Truss Buckling in Existing Building
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
RE: Wood Truss Buckling in Existing Building
RE: Wood Truss Buckling in Existing Building
Dik
RE: Wood Truss Buckling in Existing Building
RE: Wood Truss Buckling in Existing Building
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.
RE: Wood Truss Buckling in Existing Building
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!
RE: Wood Truss Buckling in Existing Building
RE: Wood Truss Buckling in Existing Building
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.
RE: Wood Truss Buckling in Existing Building
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.
RE: Wood Truss Buckling in Existing Building
BA