Originally Posted By: jpope This post was automatically imported from our archived forum.
Trusses are designed and engineered by the manufacturer based on load calculations and roof designs. It appears that this portion of the truss was determined by the engineer to be in tension rather than compression.
I would question this if it were a field modification, but it appears to be an engineered design.
-- Jeff Pope
JPI Home Inspection Service
"At JPI, we'll help you look better"
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Originally Posted By: ppetroska This post was automatically imported from our archived forum.
Thanks for the reply Jeff.
Yes, my foggy recollections of the mechanics courses I took 30 years ago are coming back, so I can agree that the member is probably in tension along it's length. However, isn't there another component force in play along it's width that the engineer may have neglected or underestimated as evidenced by the cracks? That 2x6 seems to be a waste as it is only as strong as the portion covered by the connecting plate, essentially making it the equivalent of a 2x3, right? Sort of the same principle as the strength of a notched joist?
Originally Posted By: jpope This post was automatically imported from our archived forum.
ppetroska wrote:
That 2x6 seems to be a waste as it is only as strong as the portion covered by the connecting plate, essentially making it the equivalent of a 2x3, right? Sort of the same principle as the strength of a notched joist?
Not necessarily. The design of a truss system is much different than that of a beam and joist (rafter and joist) system. Trusses are designed to transfer loads equally throughout the entire truss whereas joists and beams rely (essentially) on "brute force," based on the size of the member.
The plate in your picture is a tension plate. I can only assume (as I am not a PE) that the design required the tensile strength of a 2x6 beyond the plate.
-- Jeff Pope
JPI Home Inspection Service
"At JPI, we'll help you look better"
(661) 212-0738
Originally Posted By: ppetroska This post was automatically imported from our archived forum.
Well Jeff, I’ve thought long and hard about this problem and this is what I’ve decided:
Yes, the 2x6 is in tension so that the 3 inch gap probably isn't that big a concern. However, I think the 2x6 was a mistake given the size of the connecting plate at the bottom. If it had been a solid 2x4 instead, even with the gap, there probably would not have been any cracks. The reason that the 2x6 cracked is because the 1-2 inches of material below the connecting plate was excess material not under tension and the flexing of the member under a load was enough to cause the split where the material under tension meets the material that is not. The only way to get the full benefit of the 2x6 member would have been to use a 6" wide connecting plate.
The end result is that the cracked rafter looks worse than it is as the bulk of it is still doing its job (sort of like foundation cracks). This construction is probably sufficient to do the job. However, because most people wouldn't understand my analysis (if it's correct), and because I believe this particular construction is prone to the type of cracks I found, I think I'm going to continue to recommend analysis by a structural engineer and let them do the explaining.