Cracked Wood Rafter 5

[PSUengineer1]

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.

 

[Screwman1]

To me that lower crack looks really old. the grain is also really bad looking on it. the top crack looks to be the result of 'shake' in the board (a grain delamination from wind bending in the tree state or improper drying). The seat cut is not right, there is very little support for any kind of load on that rafter. It needs more bearing surface to rest on. Looking at the pictures; I don't think wind had anything to do with this crack, it's been there for years. It's a combination of back wood and bad carpentry.

 

[msquared48]

Well, a 19 foot span for a 2X6 could be a bit much (more likely than not, is), depending on the snow load and roof slope.

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

 

[Archie264]

msquared, I think they're 2x8's. Still a long span, though. jimjxs, do you get much snow your way?

 

[focuseng]

Could have just been installed that way. Some carpenters do that sort of thing rather buying (or cutting) a new one. Doesn't look like any reasonable damage source as the damage is severe and the neighbors are fine. Is the rafter displaced in any way? Is this an insurance claim?

______________
MAP

 

[msquared48]

Archie264:

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

 

[Archie264]

msquared,

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.

 

[PSUengineer1]

THANK YOU ALL FOR THE RESPONSES.

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.

 

[msquared48]

Actually, this gets into good connection design practice more than end bearing values, which would be covered by the current NDS. In my AITC Timber Construction Manual, the third edition, on pages 7-702 (Detail A5), and page 7-703 (Detail A7), this condition is specified as a definite NONO. Similar details can be found in the 4th Edition on pages 8-780 and 781.

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

 

[msquared48]

Commenting further, the better, structurally acceptable connection would have been to flat cut the rafters, bear them directly on top of the plate, and use angle clips or similar Simpson connectors to nail the rafter off to the plate.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[hokie66]

Strange detail. Hard to tell with the insulation there, but is the rafter double cut to bear on both plates (the wall plate which the ceiling joists bear on and the one on top of the ceiling joists which we can see in the photos)?

 

[dhengr]

Look in NDS for notching of the bottom of beams/joists/rafters, particularly at the bearings or in high tension regions. At the lower bearing plate, that rafter is effectively notched about 60 or 70% of its height, at least that is the effect of how it bears on the plate. The horiz. shear stress is very high at the bearings and all the more so when you have an actual notch or that notch affect. Your detail is literally tension perpendicular to (across) the grain, also not allowed. Is the crack up at the ridge in the same member? That is at least supported by a ridge board and would/should be less likely to crack. That may just be one bad joist member. But, if they piled large stacks of shingles right over that joist, that might have caused high shears on that one member also. I agree with MikeM on several accounts: the bottom bearing on those rafters should have been slope cut without a notch, and they should have been bearing on the plate under their lower edge, not at their upper edge. Then light angles applied btwn. the rafter and the bearing plate to pick up the thrust. You might want to rip one edge of a 2x6 or 2x8 to 8/12 to match the rafter slope. Lay this member flat on the clg. joists and push it in under the rafters and nail it off to the clg. joists, for some added bearing area on the underside of the rafters. This member doesn’t have to be a 2x8 as long as you can nail to the clg. joists under (along side) the rafters.

 

[woodman88]

The 2003 IRC (the most current one I have) Section R802.7.1 "Sawn lumber. Notches in solid lumber joists, rafters and beams shall not exceed one-sixth of the depth of the member, shall not be longer than one-third of the depth of the member and shall not be located in the middle one-third of the span. Notches at the ends of the member shall not exceed one-fourth the depth of the member...."
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.

 

[lardman363]

It looks like the bottom may have water damage/rot (white residue, dark colored wood). It also looks like the lumber was milled a little off, notice the ridge that runs the length. I have floor joist that have delaminated at these locations.

 

[PSUengineer1]

woodman88,
Can you provide me with a copy of the details?
thanks,
jimjxs263

 

[msquared48]

Jim:

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

 

[msquared48]

And the second:

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[PSUengineer1]

msquared48,
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!

 

[msquared48]

I tried, but it did not post. I will try again...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[Splitrings]

Are we sure there isn't something else going on? All of the ends of the rafters appear to have short cracks running parallel to the grain. I have never seen cracking in wood like this. Also as mentioned by lardman363, there is discoloration and white residue.