Reuse of MPC Trusses subjected to lateral deflection?

[jaytee01]

I am looking at a structure that is about 5 to 6 years old. The roof framing consists of 60 ft scissors trusses with piggyback trusses on top. The lateral bracing was insufficient and many of the trusses have buckled laterally. (See attached photo) The trusses were spaced 16 on centers and some of the lateral displacement is 12 inches or so. The sheathing and apparent redundancies have prevented the roof from failing catastophically, but the building is unusable as is, of course.

A preliminary close look at the trusses does not show any obvious splits or cracks in the wood and the metal plates all seem to still have sufficient engagement of the teeth. The insurance company wants to just remove the load, straighten and rebrace the trusses properly and reuse the framing as is. I'm not entirely comfortable with that idea, but I don't see any physical defects, aside from the lateral displacement of the top chord relative to the bottom chord. Would this long-term displaced buckle cause a permanent set in the wood truss that would make it unusable or can it be straightened and reused without any loss of original strength? The insurance companies engineer recommends reuse but the contractor is skeptical.

 

[JAE]

You might find that depending on how long the trusses have been bowed sideways like that - the wood may be more or less permanently deformed. You might try looking in the TPI spec to see if there are set tolerances for out-of-plane sweep of trusses. I suppose you could straighten them sufficiently with enough braces and cross members though.

As far as the connections with the nail-plates, if each one is very closely inspected and there is no (zero tolerance) separation between wood and plate then I would feel it safe to assume that the truss connection is adequate. If there is any degree of gap, I would reject the truss.

 

[woodman88]

For MPC wood trusses the critical is the in-place plumb of the trusses which is recommended to be less than D/50 and/or a maximum bow of 2” in 33' sections of the truss.
For a truss with a 60' span just the handling of the trusses would have had more than 12” lateral deflection over the length of the truss occurring. The deflection shown in your picture appears to have the deflection occurring on just a portion of the span rather than the full length. The lumber should be okay and the plates, not at splices also, as the continuous chord member will minimize any effect on the plate holding capability.
The chord splices may have problems as the non-continuous chord member may have acted as a hinge causing the plate to buckled on one side which could also cause the other side tension forces to the plate to exceed the plate allowable holding. In your picture the top chord splices, the plates with blocks attached to them, appear not to have these problems. But I would require a close look at each splice to check them.
One big problem here is that this condition may void the truss warranty so are you going to be held as being partly responsible for any problems with the trusses down the road? Now if the insurance company engineer seals (with a seal for the state the trusses are located) the repair drawing/letter for this condition, I would be willing to let them be the responsible party. But I would still want them to state that their inspection of the trusses showed no structural truss deflects.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.