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.

 

[phamENG]

You only need what? 2.1 or so inches of support to stay above plate crushing bearing pressures, right? According to NDS 3.2.1, the span is face to face of supports plus 1/2 the required bearing at each end. So I'd go in about 1" and see what the depth of the member perpendicular to the grain is at that point and measure use the notch equation.

For the overhang, use the full notch depth for the compression side.

 

[lexpatrie]

Welcome to Tahoe.

The TJI has documents on birds mouth cuts, or did, and your C[sub]D[/sub]=0 is a typo?

C[sub]D[/sub] = 1.15 for snow typically unless th le local amendments change it due to longer term than normal snow loads?

Simpson makes adjustable "seats" with published loads that wouldn't need the notch if you wanted to explore that avenue.

 

[Aesur]

I have kind of gotten a bit lost on where this post has headed so I'm going to offer my opinion on the design.

With those kinds of snow loads, I wouldn't even consider IRC and would only use IBC.
If shear fails, you need a larger shape or maybe you can sister the areas where shear fails.
I would check the shear in the member for the reduced section for the birdsmouth.
If shear is that high, you may also have a bearing failure for Fc(perp). If this is the case, maybe something like a Simpson TBE can be used to increase bearing.

 

[XR250]

I'm suprised TJI's could be made to work at these kind of loads. They are sketchy enough at 20/10