Notch at End of Wood Rafter
Notch at End of Wood Rafter
(OP)
I have a client who wants to notch the end of three existing roof rafters (2 X 12 @ 24" oc) in an existing church, to install a Kitchen range hood. The 1997 NDS (Section 3.2.3.3) states that a notch at the end of a bending member cannot exceed 1/4 of the depth of the member, so I told the client a 3" notch is OK. But he wants a 5" notch.
Assuming the shear and bending stresses work (I think they will, but I have not checked yet), can I deviate from the NDS? Is there a reason for this empirical rule limiting a notch to 1/4 of the member depth?
Assuming the shear and bending stresses work (I think they will, but I have not checked yet), can I deviate from the NDS? Is there a reason for this empirical rule limiting a notch to 1/4 of the member depth?
DaveAtkins






RE: Notch at End of Wood Rafter
1/4 dept rule only apply if you rest the notch end on the wall or beam.
You can always beef up the notch rafter by sistering 2x next to it.
RE: Notch at End of Wood Rafter
RE: Notch at End of Wood Rafter
RE: Notch at End of Wood Rafter
Here is the link to the complete text of the patent
http
www.SlideRuleEra.net
RE: Notch at End of Wood Rafter
You mention 97 NDS but 2001 NDS increases the reduction of the allowable shear stress by squaring the de/d term (so you have the reduced shear area AND the allowable is reduced by (de/d)squared where it used to be reduced by only de/d).
RE: Notch at End of Wood Rafter
I like COEngineer's idea--I will see how it works out. If it doesn't work, I will ask for sister rafters.
DaveAtkins
RE: Notch at End of Wood Rafter
As houseguy has indicated the 2001 NDS has reduced the allowable shear at notches. It is my understanding that change was made due to testing that showed very poor performance at notches.
If you have to notch the existing member 5", you probably want to inore the shear strength of the existing 2x12's and provide reinforcement. One of the problem of notching a 2x12 down is that the 3 3/4" edge knot which would be allowed in the top of a 2 x 12, might end up taking up most of the cross section of your notched down section.
RE: Notch at End of Wood Rafter
DA your last description helps clarify, I think you are talking about a "level cut" at the bearing end of the rafter and there is no way that this can be justified without reinforcement. Perhaps external stirrups or "Oly log screws" could be used to provide shear reinforcement to the rafter ends.
I'm not familiar with any metal strapping that could be used as external stirrups but nmaybe someone else is. Some carbonfiber products are used (generally) on larger members. Anyone with experience using Oly log screws for shear reinforcement? (I've used them on glulams for a similar condition (5 inch wide member though not 1 1/2")
RE: Notch at End of Wood Rafter
I "sistered" a 1-3/4" X 5-1/2" Microllam each side of each rafter.
DaveAtkins
RE: Notch at End of Wood Rafter
My first choice would be, though, to notch the hood. (make it work with whatever is already there)