Rafter Notch at Supports in Snow Country 2

[SHEAR_FORCE]

I am hoping to have some corroboration or denial of a new outlook on birdsmouth/seat cut notches for rafters. The approach has always been a max notch of depth/4, which is currently supported in the Wood Frame Construction Manual (WFCM) 2.5.1.1.4. The 2018 NDS mentions in 3.2.3.2 that the stiffness is unaffected by a notch at depth/6 but I can't find anywhere else in the NDS that allows for a prescriptive shear allowance of depth/4 at supports.

We are in snow country in California where snow loads are almost always above the WFCM design limit of 70psf, so we can't use the prescriptive method, correct? This would mean that every rafter with a birdsmouth/seat cut notch would need to be designed for the lower shear value using the NDS equation 3.4-3, which is very restrictive in comparison to the prescriptive depth/4. It doesn't seem like a burden to make the change but I would like to be sure as I have never seen another engineer take this approach for sizing all rafters.

 

[Eng16080]

Perhaps I don't understand the question, but NDS 2018, Section 4.4.3.1 (and Figure 4A) allows an end notch up to 1/4 the depth.

Assuming you had a notch of this depth, then you would use Equation 3.4-3 (as you note). Did you actually run the numbers on this because in most cases, although the shear capacity is reduced a lot, in my experience the design usually still works?

Maybe draw a quick sketch. Pending that, I might have some additional commentary.

 

[SHEAR_FORCE]

You are right, @Eng16080, I missed that reference. Thank you.

I have a 1-3/4x9-1/2 LVL @ 16" OC spanning 10' with a 366psf snow load, 15psf dead load, Cd=0, Cr=1.15. I am getting that this rafter is ~40% overstressed in shear w/ Fv=285psi.

 

[Eng16080]

366 psf snow load? Really?! You weren't joking about snow country. I didn't realize the loads were that high anywhere!

 

[SHEAR_FORCE]

It is 366 at 16" oc spacing, so the roof snw is only 285, my bad. That includes sliding snow from above.

I could see using equation 3.4-3 for notches within the beam span but at the supports it seems excessive. Only a 1" notch would be allowed for a 9-1/2" LVL.

 

[jhnblgr]

Could you double up the rafter at the ends to get it to pass, need to likely check the screws or nails capacity.

 

[SHEAR_FORCE]

Yes, @jhnblgr, that would likely work. I ran into this single issue but I am now wondering if I should be changing the way I calc all of my rafters, which would have a noticeable impact on project cost.

I just noticed that NDS 8.4.1.1 actually calls for a depth/10 max notch at supports, yikes. 8.4.1.2 does refer back to EQ. 3.4-3, though.

 

[jhnblgr]

What if you slope the top plate to match the roof slope.

 

[Eng16080]

I missed the part about it being engineered lumber. You're right, Section 8.4.1.1 controls.

Personally, I find it debatable whether or not a birdsmouth rafter cut should really be classified as a notch, assuming the "notch" only occurs over the support. Is this what you're calling the notch?

 

[SHEAR_FORCE]

Yes, we do that or use VPC's when contractors request using TJI's at the roof but then there are other issues involved such as inward thrust of the walls. Seat cuts are nice because they help transfer the load downward and negate the horizontal component of the load. I didn't realize how restrictive the NDS is for using seat cuts.

 

[SHEAR_FORCE]

Yes, @Eng16080, I was assuming the birdsmouth is a notch. It would be great to have clarification on whether it is or is not a notch.

I would still assume that the depth/4 for sawn or depth/10 for engineered lumber applies to the birdsmouth.

 

[jhnblgr]

https://www.jlconline.com/how-to/framing/sizing-the-birdsmouth_o

I think if you check how much bearing is required and see the minimum depth of over that distance.

Are you using the ASD with the duration factor for snow?

 

[Eng16080]

Probably the consensus is that it is a notch. I recall a figure in the IRC code showing it as one, although I personally question this. I suppose it comes down to exactly where on the top plate most of the force is being resisted because if the load is all going to the inside edge of the top plate, it seems unlikely that there would be a splitting risk.

 

[SHEAR_FORCE]

@jhnblgr, I think the question we have now is whether NDS EQ 3.4-3 applies to birdsmouth/seat cuts that can use the prescriptive depth allowances of depth/4 and depth/10. My understanding is that notches and birdsmouths create a reentrent corner with a high stress point that can cause the member to fail before it reaches the uncut shear strength or bearing capacity.

 

[jhnblgr]

https://www.accessengineeringlibrar...0128673/toc-chapter/chapter6/section/section6

See if you can access this

 

[Eng16080]

So, here's a "code-compliant" birdsmouth detail for an LVL rafter, at least in terms of the notch depth. Of course, this almost certainly doesn't work if you check bearing perp. to grain. The JLC article linked above is basically showing a similar sort of thing.

I would feel much better with the inside edge of the rafter bearing on the inside edge of the top plate. Based on the notch depth limitation, it basically becomes impossible to frame a 12:12 pitch roof with conventional lumber and certainly with engineered lumber.

FWIW, I don't recall ever seeing a rafter framed like this nor do I recall seeing the typical detail (with rafter cut at inside face of top plate) as having any splitting problems. Also, if you're considering the notch to occur across a bearing surface, then the typical rafter to ridge board compression detail doesn't work since there is technically a notch for the full rafter depth.

OP, I think you touched on "another one of those things" where there perhaps isn't a clear answer.

 

[jhnblgr]

I would say check the shear with 3.34
Unlike a notch though the are of splitting will be on the overhang cantilever. Specifying a drilled hole at the notch corner should reduce stress and not hinder construction.

 

[jerseyshore]

That load is insane. I'd double check that but regardless, I don't think I'd be worried about a birdsmouth cut like a regular notch. I'd just check bearing length and shear for the reduced depth. If it doesn't work still you could try to add something to the end like jhnblr mentioned.

 

[Aesur]

It is 366 at 16" oc spacing, so the roof snw is only 285, my bad. That includes sliding snow from above.

Lake Tahoe area? The last few projects we did in that area had by far the highest snow loads I have seen, and we do projects all over Alaska as well.

 

[JLNJ]

Cutting a birdsmouth does not necessarily reduce the horizontal load. The horizontal load is a function of the global action of the superstructure.