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Rafter Connection to Top Beam

Started by doug2500, August 17, 2020, 09:40:02 PM

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doug2500

I've attached a drawing showing two possible ways to attach a rafter to my top beams. I'm curious what you all think are the pros and cons of doing it either way. I'm wondering if the drawing on the right would be more likely to crack in the corner.

These are 2" x 8" x 17' rafters and 8" x 9" beams. Eastern Hemlock beams. Red Pine rafters. 24' wide building. Rafters on a knee wall beam so no way to tie them together with a full length joist. 12/12 metal roof in Central Vermont. Collar tied at about 5' down from peak. No ridge board. All dimensions are full size, not Home Depot 2 x 8's. These will be assembled on the loft floor and then raised in place.


Any thoughts are appreciated.

 

Don P

Both are prone to splitting, B more. I saw an engineer put a name on a tie raised like that the other day "rafter buster". Keep ties low, code says in the lower third of roof height and gives rafter upsizes as the tie is raised within that lower third, better all the way down at the plate.




I have successfully used that joint multiple times but I try to use dry material.

doug2500

What does it mean to intentionally leave a gap between the top and bottom chords?
The joint you have successfully used is the one pictured? If I run a tie at the bottom I will have no space in the loft.

Don P

Let's back up, I'm not fully understanding what you are trying to do. The loft floor joists, if tied to the rafter feet form the rafter ties to restrain them from spreading. Collar ties or straps over the peak are necessary in the upper third of roof height to hold that upper portion together.

What the description in the pic above is alluding to is that when you notch a timber and create an inside corner the stress will concentrate on that corner. If you dangle part of the timber in air or try to create a double bearing that is probably going to end up with is as the rafter shrinks it will lift off the lower bearing and the stress will again concentrate into that corner and want to split. Back out and do a cross section view of the roof and floor detail.

Then let's check your rafter size to see if that is large enough. What is the design snow load for your area. Another thing to consider is that when using full dimension material the nails usually need to be upsized as well so that they can run through your thicker lumber and still get sufficient bite into the secondary member. For instance using a 3-1/4" sinker nail through a full 2" stick only allows 1-1/4" into the timber you are attaching to.

doug2500

Here is a sketch of the North or South Elevation, rafter metal plate and a photo of where I am so far. 24' x 48'. 10' side posts. The knee walls are 20". The rafters are 2" x 8" x 17'. Red Pine mostly with a few Spruce. 19-20" O.C. but I could change that if necessary. I'm in Central Vermont where I think the snow load is 50 lbs/Sq Ft. But winters have sure been different with the changing climate. The posts are 8" x 8" Hemlock and the Top Beams are 8" x 9".  Cross beams are 8" x 8". I'm using 3/8" GRK RSS screws of various lengths for the braces and for all the metal plates that you don't see yet (plywood right now), and I thought I would use them for the rafter to beam connections also. I was even thinking I would run a 1/8" x 4" steel plate across the top of all the rafters to clamp them to the top beams using GRK RSS screws. I don't use nails. Everything else is assembled using 3-1/2" x 10 deck screws. Thousands $$$ into metal plates and fasteners, but the wood comes from my back yard.


 

 



 



 
 

Don P

I hate it when the computer eats a post.
Rafter spacing is 1.66' x 12' horizontal span x 60 psf load (50 snow + 10 dead)= 1200 lbs/rafter
Enter everything here;
https://forestryforum.com/members/donp/ddsimplebeam.html
I get a pass in #2 eastern hemlock. With the kneewall and raised tie I suspect the bending stress is some amount higher, that's above my pay grade.

structural screws and nails have ductile failure, deck screws have brittle failure in shear, they are not for structural use, I've replaced many snapped deck screws over the years. The rest of that is really engineer land

doug2500

In this calculator, what does the size factor mean? And when you say the bending stress may be higher due to the kneewall and raised tie, does this calculator assume you have a joist tie at the bottom of the rafters?

doug2500

Is the "section input" the capacity you actually have?

Don P

Yes, section input is the section modulus you actually have, the equation for that is;
bd2/6 notice depth is squared, so when you come up short of what is required going deeper will get you there far quicker the going wider.

Size factor, in the dimensional lumber base design tables Fb (bending strength) and a couple of other values are based on 12" deep lumber. In breaking tests they found that shallower members had greater bending strength in relation to the base values so this is an adjustment factor to multiply the base design values by to correct for that. At the bottom of the calc the left side gives the base design values out of the book. The right column gives the adjusted design values which the calc uses after multiplying the base numbers by all the adjustment factors you are clicking.

Here's that page from the NDS;




I would put it that you probably have a more complex loading scenario than a simple beam supported at each end with a uniform load

doug2500

So when I put the numbers in the calculator I chose 2 & 3 x 8 for size factor because these are actually between 2x8 and 3x8 by current lumber standards. (I'm assuming the data being used is for 2x8's that are actually 1-1/2 x 7-1/4). Was that the correct number to choose?

Also I'm curious about a part of this that I never see discussed on the internet. The snow load here is 50 lbs/sq. ft. But snow is mostly air except when it is very wet snow or packed down well. It doesn't pack on these roofs at all. This is my third building with a 12/12 metal roof. The first two were 12' and 18' wide. There are no hips or valleys. Snow does not accumulate on these roofs more than about 6-8" before it always slides off. North side or south side, the north just takes a little longer to fall. Fresh snow takes more than a few feet to reach 50 lbs/sq. ft. So do you think the snow load factors for this type of roof are a little unrealistic? And is it possible they were created based on much shallower roofs or just to provide a large safety buffer?

doug2500

Oh, and by not having a joist running across the bottom, does this calculator still work? If my knee walls are essentially un-bendable isn't this the same as a joist tie if the connections are reliable?

Don P

Yes the 2&3 x 8 tab is correct. That calc is actually checking a beam between two simple supports. There is a bit more going on there.

You'll need to satisfy yourself on those last two posts, I'm not here for confirmation. On a job I'd call an engineer, this is outside of my comfort zone.  This is the engineers caution,



There are reductions for slope and whether the roof surface is slippery or not. I try to build for the one bad day in a century rather than the blissful sunny days the entire rest of the buildings life.  I've had 3' of heavy consolidated snow stick to one of my 12/12 metal roofs I don't reduce ground snow load. It's your call, there is a pretty light penalty for being conservative where there is a severe penalty for being unconservative. I see that light bulb come on way too late way too often. 

nick_m

Quote from: Don P on August 19, 2020, 04:21:59 PM
 This is the engineers caution...

Are these annotated illustrations (your initial and most recent responses) engineer's notes from a specific project, or are they from a book or other published document? Looks like a good collection of information - especially the shrinkage issue on the bird's mouth.

Don P

Jim sent it the other day, its another new publication from TFEC
Design Guide for Timber Roof Trusses TFEC 4
A good read and lighthearted for dense material. I see a good bit of Ben Brungraber in it, you can find various webcasts and articles from him on the net, good stuff.

doug2500

Thanks for the help. I am a PE, electrical, but I understand forces, moments, etc. all of us had to take those basic Statics and Physics classes. What you've said makes a lot of sense. Especially the part about different angle cuts having different drying in the grain. I'm going to go with standard bird's mouth cuts because of that issue to get the majority of the rafter on the beam. Everything is green. Sometimes within minutes of milling.

Thanks!

doug2500

So after a lot of thought I milled some 14' 6x6 posts and will mill some 2x10(or 12)x12 ridge beams. I drew a free body diagram and made a model to test it out. It appears that adding the ridge beam eliminates rafter thrust. Am I correct in this or is it not that simple?

 

Don P

Yup, pretty much that simple, if the ridge cannot sink there is no thrust.
A ridge BEAM, as opposed to a ridge BOARD, does eliminate rafter thrust. The typical ridgeboard is simply a 1x or 2x non load capable nailing convenience so that's why I'm making sure there is a distinction understood there. 

The rafters with a ridgebeam are basically hanging from the beam so the loads become vertical. To calculate the ridgebeam size, first identify the tributary area that loads the ridgebeam and the trib area that loads the plates. For the ridgebeam it is carrying half the span of the rafters on each side of it, so 1/2 the building width X the span from ridgebeam from post to post. That will give you the trib area loading the ridge. Multiply that x the total design load (LL + DL) and you have the beam load to enter into the simply supported beam calc.

The plate load is the lower half of the rafter span + the overhang, so half the ridge load+ overhang.

Don P

I just noticed in your right hand birdsmouth sketch, code now considers the birdsmouth a notch so it is limited to 1/4 rafter depth. I bite my tongue and try to comply :D.

doug2500

So here's my math.

24' wide building and 12' span for the ridge boards.

That means the ridge board is supporting 12' width and 12' length for 144 sq. ft.
The load is about 5lbs/sq ft max. for the structure (rafter, nailing boards and metal roof).
Snow load is 50lbs/sq ft, although from experience this is around 3 times more than we will ever see here on these slippery 12/12 roofs. I have never seen more than about 6" accumulate.

55lbs/sq ft X 144 sq ft. = 7,920 lbs on the ridge board.

Obviously a 2x12 would fail your calculator.

But, at 50/3=17 lbs/sq ft X 144 sq ft = 2,448 lbs on the ridge board it passes in all categories.

Not a fully supportive ridge beam, but I am reducing the rafter thrust and top plate load significantly.

I don't expect anybody to bless this. I could also increase the boards to 3 or 4" thick, the logs haven't been milled yet.




When I install braces from the 6x6 posts to the 2x12 ridge boards, does that shorten the span for the ridge board?

Do you typically toenail the rafters to the top plate? I would use GRK structural screws.


Don P

As you say, I can't bless it but you are aware that you just juiced the numbers. 1 day in 100 years, that is the 1 day you are designing for. We just had a 5.1 earthquake, it has been 100 years since the last one, that's the design day cause it might be tomorrow, we don't get to say when your sticky snow day happens.

Braces do not decrease span but if you go down that path its time to investigate combined axial and bending load on the posts, heading into the weeds and dubious. The top connection to the ridge is the working joint. Simpsom , I think they are LS framing angles, strongtie.com. Check load to connector specs. That's one way but there are others. Your structural screws at the bottom end should be fine for uplift. Might be ok up top but check their shear capacity in your species if you go that route.

canopy

Looking at the last drawing just wanted to add that you might want to look at using a step lapped rafter seat.

doug2500

Based on what Don P was saying earlier, that would set up two bearing surfaces on the top plate that would potentially dry differently resulting in only one making contact after drying. What would be the advantage other than keeping more wood in the rafter as it crosses the top plate?

Don P

It can be easily relieved to just bear on the bottom and end of the notch. It is another form of thrust restraint into the plate but is obviously more time consuming to cut than a simple birdsmouth. Bear in mind it also weakens the plate which can transmit accumulated thrust, if present, to a post, splitting it. There are good discussions and sketches in Sobon's "Historic American Timber Joinery". Not trying to be constantly critical just mindful, as with all things, there is a balance in there somewhere.

canopy

In that text Jack Sobon says: "The best solution to connecting rafter and plate, at least to this author and builder, is the step-lap rafter seat." Just thought it was worth mentioning a joint some consider to be of high quality. I prefer the step lap joint and so does the engineer I have used. There was also a good thread made here some years ago with pictures on laying it out real easily with some magic of the framing square.

doug2500

Can you tell me what you mean by "easily relieved".

Also, this new notch suggestion is the opposite of the notch I posted to start this thread. That notch was supposed to provide better geometry between the rafter and the top plate to prevent a connection failure due to rafter thrust. That notch would not increase rafter depth, but it would concentrate more of the force between opposing wood surfaces rather than relying on the fastener strength at the joint. Using a bird's mouth, or this step lapped rafter seat puts all of the strength of the connection on the fasteners.

Sorry if I'm confused, I have never built anything this large before.

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