swsengineer
Structural
- Jul 3, 2008
- 29
Just wondering if anyone had some ideas that I haven't thought of.
I have a relatively new (3 years old) glulam truss that spans 75' over a Firehouse. The roof experienced an above average snow load in December and now the truss is deflected 3.5" at centerline and is very noticeable from the outside in the metal roofing/tectum panels above it. It seems the deflection occurred from the snow but for some reason the truss has not responded and come back to it's normal dead load only deflection. Dead load only deflection should only be about 1" I and the original engineer agree with this. The truss members do not appear to have split or been damaged in any way. It was suggested that reaming of bolt holes to facilitate erection is to blame, but their are lots of bolts and even with several web members removed in the truss model the dead load deflection only changes slightly. The deflection by everyones account was not present before the snow load and previous although less snow loads in the past have not presented a problem.
Another thought was the 3/8" side plates may have been put on too tight with the nuts being too tight instead of a snug fit. Friction in the plates or some kind of clamping forces is not letting the truss members rotate back into place.
Possibly something in the tectum roof panels shifted and something is preventing the truss from coming back.
The truss is on the hip end of a roof with a flat bottom chord, partial sloped top chord, then flat top chord for the center 3/4 of the span give or take.
I didn't engineer the truss but I don't see any issues with the design.
3.5" over 75' isn't too bad but the problem is this truss is 8' away from a cmu wall so 0" deflection in the roof 8' feet away make the 3.5" deflection jump out and be very noticeable.
I realize I'm not giving enough info to present the full picture. I'll be glad to give more info to specific questions. Really I just wanted to see if anyone had some good ideas on what might be happening.
Thanks,
I have a relatively new (3 years old) glulam truss that spans 75' over a Firehouse. The roof experienced an above average snow load in December and now the truss is deflected 3.5" at centerline and is very noticeable from the outside in the metal roofing/tectum panels above it. It seems the deflection occurred from the snow but for some reason the truss has not responded and come back to it's normal dead load only deflection. Dead load only deflection should only be about 1" I and the original engineer agree with this. The truss members do not appear to have split or been damaged in any way. It was suggested that reaming of bolt holes to facilitate erection is to blame, but their are lots of bolts and even with several web members removed in the truss model the dead load deflection only changes slightly. The deflection by everyones account was not present before the snow load and previous although less snow loads in the past have not presented a problem.
Another thought was the 3/8" side plates may have been put on too tight with the nuts being too tight instead of a snug fit. Friction in the plates or some kind of clamping forces is not letting the truss members rotate back into place.
Possibly something in the tectum roof panels shifted and something is preventing the truss from coming back.
The truss is on the hip end of a roof with a flat bottom chord, partial sloped top chord, then flat top chord for the center 3/4 of the span give or take.
I didn't engineer the truss but I don't see any issues with the design.
3.5" over 75' isn't too bad but the problem is this truss is 8' away from a cmu wall so 0" deflection in the roof 8' feet away make the 3.5" deflection jump out and be very noticeable.
I realize I'm not giving enough info to present the full picture. I'll be glad to give more info to specific questions. Really I just wanted to see if anyone had some good ideas on what might be happening.
Thanks,