Rafter to ridge BEAM connections

Started by Jeff922, July 27, 2013, 09:29:14 AM

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Jeff922

I'm messing around on DraftSight with my barn/shop plans.  I'm in the process of figuring out my engineered posts and columns.  For the ridge beam, I want to use 3-plys of 1-3/4" x 11-1/4" LVL supported by two columns across 34'.  The question I have is the connection of the 2 x 12 (24 O.C.) rafters to the LVL beam.  What I've drawn is the rafters resting on top of the beam, overlapped, and nailed together as described in Wagner's book on pg. 154 for an exposed Glulam beam.  I should mention that this is a cathedral ceiling.  I seem to remember a thread on this subject.  What do you guys think? Is this an acceptable technique? Is there enough bearing surface in the birdsmouths?




"They don't grow trees so close together that you can't ski between them"

John Raabe

I have seen that overlap technique used on several projects here in WA state. Because the rafters span from the beam to the pony wall they are not pushing outward at the heel.
None of us are as smart as all of us.


Jeff922

Oops, I should have been more specific - the small "birdsmouth" at the top of the rafters where they sit on the LVL is the bearing surface I was concerned about.  Each rafter would have 2-5/8" of bearing in the downward and lateral direction (5-1/4" total).  12:12 roof so equal force in both directions.  Guess I should just do some math and figure the load on each rafter.
"They don't grow trees so close together that you can't ski between them"

Don_P

That will work, keep the birdsmouth up top minimal to avoid splitting. You can also run a bevelled ledger nailed to the lvl's under the rafters to help in that regard. another detail is to cap the top of the beam with a 2x bevelled to the roof pitch that unnotched rafters rest on, that eliminates the splitting possibility created by that corner of the notch.

Jeff922

Those are both really good ideas Don, thanks for the input.   
"They don't grow trees so close together that you can't ski between them"


SouthernTier

You probably remember that discussion from my cabin design thread.

I have similar ridge beam and rafter material in my design.

Don had a similar comment for me about skipping the birds mouth because that would put the lower part of the rafter under tension and split it.

Since I needed the headroom in the loft, I decided to revise the design to hang them from the face of the LVL, although that will cost me in simpson brackets.

http://countryplans.com/smf/index.php?topic=12801.msg167800#msg167800

busted knuckles

"That will work, keep the birdsmouth up top minimal to avoid splitting. You can also run a bevelled ledger nailed to the lvl's under the rafters to help in that regard. another detail is to cap the top of the beam with a 2x bevelled to the roof pitch that unnotched rafters rest on, that eliminates the splitting possibility created by that corner of the notch."

This beveled ledger will not cause outward pressure? I was wondering about the rafter wanting to "slide" down the beveled board?  This seems like a much simpler way than a birds mouth.  I am considering this on a build up ridge beam.



you know that mugshot of Nick Nolte? I wish I looked that good.

Don_P

The beveled ledger is all mine, I can't recall a prescriptive reference. When tied to its mating rafter I'm comfortable.

In the pic in the original post, if the framing allows you to lap rafters over the ridgebeam that works well. A metal strap from top of one rafter, over the top to the top of the other rafter in that couple also works. A plywood or 1x gusset across the faces of the rafters is another way.

Following prescriptive details that you can point to these are some things I can think of to look up;
Review section R802.3 in the IRC for the code language.

Fig 3.10a in the WFCM shows a couple of methods, a lightly birdsmouthed rafter on top of the ridge beam, and rafters in hangers attached to the sides of the ridgebeam. It might be worthwhile to poke around for language in there as well.

While you're reading section 803 in the IRC look up to R802.1.4. It references AITC, American Inst of Timber Construction. Notice the IRC references the AITC glulam standard in chap 44, the chapter of code referenced standards. Their "Timber Construction Manual" has more construction details generally aimed at building with timber ridgebeams and timber rafters but in most the rafter could be light framed as well.  I jumped beyond a direct referenced document...but, when it goes to engineering the manual the engineer reaches for is that one. They have some sections of that manual for free view under the publications tab at glulam.org, I think I saw the chapter of good/bad construction details on there. That chapter is where I've seen the canted top cap.

busted knuckles

Thank you for all the information, Don. I think I will go with this method.
you know that mugshot of Nick Nolte? I wish I looked that good.