Notch at End of Wood Rafter 4

[DaveAtkins]

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?

DaveAtkins

 

[COEngineeer]

Wait the minute Dave, I assume you are notching the tension side of the rafter (not at the support). You can only notch 1/6 of depth. You cant have a notch in the middle third of the span.

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.

 

[COEngineeer]

2x12 is 11.25" so theoratically also, 3" exceeds 1/4 rule. I think you should go ahead and notch it but sister similar rafter next to it.

 

[COEngineeer]

Also dave, if you do deviate from NDS, for example if you notch 2x12 5" at the bottom or the top, you can just assume you are using 2x6 instead of 2x12. (11.25-5=6.25.. close enough to 2x6)

 

[SlideRuleEra]

DaveAtkins - This quote from a USDA Forest Products Lab patent provides insight why notches are a problem for wood:

Wood has relatively little strength perpendicular to the grain in comparison to its strength along the axis of its grain.
For example, a sample of Douglas-fir might have parallel to-
grain tensile and compressive strengths of 15,600 and 3,470 PSI respectively. but perpendicular-to-grain tensile and compressive strengths of only 360 and 340 PSI, respectively.
The strength of a wooden beam in a spanning application derives from the fact that the forces experienced by the beam when loaded are primarily oriented along the grain (tension, compression and shear) with essentially no cross-grain tension.
This assumes, however, that the beam is supported underneath its ends and that the beam is of essentially uniform moss section without cuts or notches. This latter assumption may not always be true in practice. Beams may be cut or notched in various places to run utilities or to fit against other structural members. Notches that extend a significant distance into the beam may be an unavoidable part of the building’s design or may occur from poor construction techniques.
Generally, a notch in a beam causes some of the loading of the beam to be manifest as cross grain tension, a mode in which wood is relatively weak. Additionally, the stress concentration at the notch re-entrant corner produces stresses to initiate and propagate a crack. As result, if a spanning beam is to be notched, it is necessary to use reduced loading figures for that beam resulting in the need for larger or more beams than would otherwise be necessary. In renovation projects, where beam number and size is fixed, notching of the beams may not be allowable.

Here is the link to the complete text of the patent

http://www.fpl.fs.fed.us/documnts/pdf1998/solti98b.pdf

http://www.SlideRuleEra.net

 

[HouseBoy]

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).

 

[DaveAtkins]

This "notch" is at the support--and it is really a tapered notch, because the rafter is sloped 3:12 (that is, the rafter is 11.25" deep over the supporting masonry bearing wall, there is a vertical 5" cut at the inside face of the wall, and there is a horizontal 22.36" cut in to the bottom of the 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

 

[RARWOOD]

SlideRuleEra has provided excellent information on this problem. In addition do a search on this site, since this topic has been covered several times before.

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.

 

[HouseBoy]

I'd take RARSWC's comment one step farther, instead of "probably" I'd say that you DEFINATELY will need to ignore the shear capacity of the 2x12.
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")

 

[DaveAtkins]

Thanks for all the help--especially the information on knots.
I "sistered" a 1-3/4" X 5-1/2" Microllam each side of each rafter.

DaveAtkins

 

[RareBugTX]

How about a steel plate on both sides of the rafters. They could be joined together with through bolts. In other words the size and geometry of such plates (on both sides) could take the shear and as one of the other guys suggested above, neglect shear capacity of the notched roof rafters altogether.

My first choice would be, though, to notch the hood. (make it work with whatever is already there)