Cracked Wood Rafter
Cracked Wood Rafter
(OP)
Thank you for consideration.
I viewed a cracked rafter in an attic. Crack is at bottom of 2x8 rafter which spans about 19 feet and spaced about 16 inches on center. The crack does not appear to be fresh. I have been told the crack is due to wind. I am not buying it. The wind speed was about 60 mph and there were about 5 shingle tabs blown from roof. Metal ridge vent did not blow off of roof. I suspect it would prior to wood framing cracking from wind load.
So, why the cracked rafter? Could it be; inadequate lateral bracing?,,,undersized for span? knots/imperfections in wood?..... bundles of shingles laid upon it during construction (but 8:12 roof slope)?....The crack is located on rafter where it meets wall below and where it meets ridge. I have attached photographs, please view them and comment.
thanks.
I viewed a cracked rafter in an attic. Crack is at bottom of 2x8 rafter which spans about 19 feet and spaced about 16 inches on center. The crack does not appear to be fresh. I have been told the crack is due to wind. I am not buying it. The wind speed was about 60 mph and there were about 5 shingle tabs blown from roof. Metal ridge vent did not blow off of roof. I suspect it would prior to wood framing cracking from wind load.
So, why the cracked rafter? Could it be; inadequate lateral bracing?,,,undersized for span? knots/imperfections in wood?..... bundles of shingles laid upon it during construction (but 8:12 roof slope)?....The crack is located on rafter where it meets wall below and where it meets ridge. I have attached photographs, please view them and comment.
thanks.






RE: Cracked Wood Rafter
RE: Cracked Wood Rafter
As for the crack, it has a notch in the end that bears on a 2X plate, and the failure occurred at the notch which is a stress riser. Looks like quite a substantial notch for the depth of rafter too.
As for knots, there is not enough detail in your picture to see any, at least on my computer. Maybe someone else will have better luck.
As for the wind load, I doubt it. More likely a vertical load.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Cracked Wood Rafter
RE: Cracked Wood Rafter
______________
MAP
RE: Cracked Wood Rafter
You're correct, but my comment about the span still stands - just not quite so critical. I would have been much better with 2X10's, or 2X12's cconsidering the end notching.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Cracked Wood Rafter
Oh, I definitely agree. I was about to post something similar to what you wrote except only focusing on the span. One issue at a time is all I can manage; your response was more complete.
RE: Cracked Wood Rafter
DOES ANYONE KNOW OF THE CODE SECTION (IRC) WHERE RAFTER BEARING IS ADDRESSED? I WOULD LIKE TO REFERENCE THE APPROPRIATE CODE SECTION WHEN CONTRIBUTING THE CRACK AT BOTTOM OF RAFTER TO INADEQUATE CONSTRUCTION.
THANKS, ALL.
RE: Cracked Wood Rafter
By the way, no worries, but when you post, don't use all caps. That's considered screaming, and a lot of us here have hearing aids! :)
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Cracked Wood Rafter
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Cracked Wood Rafter
RE: Cracked Wood Rafter
RE: Cracked Wood Rafter
So the rafter, per this section, must start bearing at 1/4 the depth from the inside of the member.
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.
RE: Cracked Wood Rafter
RE: Cracked Wood Rafter
Can you provide me with a copy of the details?
thanks,
jimjxs263
RE: Cracked Wood Rafter
Please find the two details attached, one in this email, and the other in the second. The details are the same in both editions of the AITC.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Cracked Wood Rafter
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Cracked Wood Rafter
thanks for the attachements! They were both the same and started with detail A6. Can you also send detail A5 on p. 7-702? thanks again!
RE: Cracked Wood Rafter
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Cracked Wood Rafter
RE: Cracked Wood Rafter
BA
RE: Cracked Wood Rafter
Wind? Not likely. I would expect more damage to the other rafters too this this much on one. Additionally, the overframing at this area would have something to say about this if if were true. The crack is not very "fresh" looking either. So if you are looking for a recent "NATURAL EVENT" or "ACT OF GOD" this is not it. That is unless something strategically fell on it due to the wind. But not damage the others?
My Official Guess: a bad detail with a improperly placed impact construction load and possibly a bad board to boot. I'm calling it "the convergence theory". The top crack is the most interesting to the bad board part. Almost looks like 1st: damaged, 2nd: water got into it and 3rd: then was left in the sun to bake before installing. The way it is separated along the grain and has a bit of a warp to it... The bottom most likely was damaged in place. Strange location within the roof for this one rafter to be singled out. Maybe someone dropped a sheet of plywood or packages of shingles or stepped on it wrong. Wouldn't take much -the bad detail clearly contributed to its demise.
jmjxs263: what did you report?
______________
MAP
RE: Cracked Wood Rafter
Maybe those smaller cracks are from tension perpendicular to the grain caused by the end condition? Then a better detail would solve the problem. I question why a notch is required. Resist the thrust a different way.
RE: Cracked Wood Rafter
The IRC has huge holes in it as far as rafter framing goes. The prescriptive code does not properly address the collar tie connections. I started a job using prescriptive IRC design and scrapped it when I saw a 10x factor of safety concern.
I won't even touch a rafter framed project unless it is very simple. No one knows how to build it and an architect doesn't understand how to design it. Trusses are the way to go.
In your particular condition, the joist cracked because the bearing condition is causing stress across the grain.
RE: Cracked Wood Rafter
RE: Cracked Wood Rafter
What concerns me is the slippery slope of prescriptive design.
"This (accidentally) worked before, let's do that 100 times"
"If that works for a 10' beam, I bet it would work for an 11' beam"
and so on
The ICC has caught some of these errors and has tried to fix them at the reluctance of builders. For example, basement wall bracing was addressed in IRC09 to change wall bracing from 1/2" bolts at 6' o.c. (not even close to working). I was happy they changed it since it was one of my gripes. After enough prodding by builders, they removed the structurally adequate section from the code. "It just works". Sure it works, but try that on a 15' wall and tell me how that works out for you.
/rant
Be careful. Sometimes the prescriptive stuff doesn't work.