Scissor Truss Failure
Scissor Truss Failure
(OP)
Hello everyone, I'm currently doing some forensic analysis of a scissor truss that has failed. The truss was built in 2008. I've duplicated the truss manufacturer's truss model in RISA 3D and have applied all of the same loads, and the vertical reactions are similar enough that I'm reasonably confident they're similar for the purpose of analysis comparison.
During my modeling I encountered something unsettling - The manufacturer modeled the end conditions as a pin and a roller. I tried this in my analysis and the lateral deflection was ~1.5 in (theirs was 0.83 in). Shouldn't the end condition be pinned-pinned, as the ends of the truss are being nailed to the wall?
During my modeling I encountered something unsettling - The manufacturer modeled the end conditions as a pin and a roller. I tried this in my analysis and the lateral deflection was ~1.5 in (theirs was 0.83 in). Shouldn't the end condition be pinned-pinned, as the ends of the truss are being nailed to the wall?






RE: Scissor Truss Failure
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: Scissor Truss Failure
The most realistic way to model the condition might be to set the scissor trusses on columns (height = wall height). The base of the column may have to be fixed with a weak spring of some sort to make the model stable, but it should give you the best estimate of actual truss/wall behavior
I generally try to force the truss supplier to stiffen the truss such that the lateral deflection at the walls is limited to around H/200.
RE: Scissor Truss Failure
With their deflection being .83" and yours being 1.5" are you sure that theirs is 0.83" on each side? After all .83"x2=1.66" which is almost 1.5"
RE: Scissor Truss Failure
RE: Scissor Truss Failure
Mike McCann, PE, SE (WA)
RE: Scissor Truss Failure
Their lateral displacement may be 0.67" Dead + 0.83" Live = 1.5" Total. The manufacturer might not report the dead load displacement because it's permanent and only relevant from a force-stiffness standpoint.