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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.

Don P

In your first post example A is bearing on the lower part of the rafter primarily but as the rafter shrinks the upper level cut takes load and concentrates stress on the inside corner of the notch. Examble B is bearing on the level cut, hanging the lower part of the rafter in air and really concentrates the stress on that re-entrant corner, it will almost certainly split.

A step lap is supporting the lower face of the rafter, that is the bearing. It is buried into the plate by the end grain stopping cut, that is the thrust restraint, so wood to wood, the fastener is simply holding the rafter down for uplift not for shear. The sloping cut beyond can be lowered to give shrinkage relief underneath the overhanging portion of the rafter to avoid concentrating stress on that inside corner of the rafter notch.That is difficult to make and make look good in the truss heel as shown in my response to your first post, here it is easier to do and is not seen as a "loose" joint.  As canopy said it is a good solution. My caution there is to check the reduced plate section caused by the removal of wood for the step lap. A simple birdsmouth does not remove wood from the plate so its full section is available in bending. If there is a properly sized ridge beam there is no thrust restraint needed, the rafter is hanging from the ridgebeam. Nothing wrong with doing a step lap there but it really isn't structurally necessary. Part of the thinking I'm using here is in the first page of Sobon's guide.

doug2500

When I looked up step lap rafter joints this morning, somehow I ended up on a photo that only showed the lap. The step hadn't been cut yet. Now that I've found a better photo, all the things you said are obvious. Sorry about that. The joint looks like a lot of fun to cut, ;) and my wife needs a roof for her precious horses by November. So if I don't need to do that cut I will go ahead with the standard birdsmouth (violating that rule you mentioned about not taking so much wood out of the rafter.)  That rule makes no sense at all to me. As far as I can tell it would only reduce my bearing surface on the plate and give me less would to put screws through.

Don P

The older code rule (code is for dimensional lumber) said that for an overhang up to 2' leave at least a 2x4's worth of material above the notch. Not great but generally that would pencil out if you do the check for a beam overhanging a support. It does introduce a potential grading issue if the maximum defect occurred in the notch zone. Use common sense, don't put a substantial defect there. But the powers that be don't operate that way. Around 10 or 12 years ago they changed that to read that you could not remove more then 1/4 depth, which is the notching rule for a notch out in span. If I squint real hard I can see a little bit of logic, but only a little bit. With a max defect in that zone and a 1/4 depth notch it would still violate the assumed design strength, hmm, back to needing common sense :D.

Here it is merely an inconvenience to you, think about that rule and framing hips and valleys, then the hair pulling really starts.

Edit; If you don't have a copy, this is a link to
Historic American Timber Joinery

doug2500

Thanks for the link. I took a look and will read it before my first real timberframe building. I have another question before I actually start building this roof. I have tried using a rafter thrust calculator online.

 

Are you familiar with this and do you know if it is reliable? If so I would assume this value would be for each rafter and so the actual thrust on the top plate would be times the number of rafters connected to the plate. With 30 rafters and 5 posts I would think the thrust load at each post (30 rafters/5 posts = 6) to beam would be 6 times the rafter thrust = 3,300 lbs. Which brings me to my question. Here is how I initially planned to connect the beams to the posts. There is one on each side and the screws are not all in yet.



 

Those are 8x9's on 8x8 posts, 1/4" steel and 4" GRK screws in the holes. I found a pull out equation in the
USDA Wood Handbook that comes to 141 Lbs per screw. (seems like it would be more for a 3/8" screw). But with a thrust load of around 3,300 lbs on each connection it looks like it would fail. I am considering running 16" 3/8" GRK screws down through the beams and 8" into the top of the posts to add strength to the connection. They shear at 3,695 lbs per their specs so four of those at each location should be over kill. Does this make sense, or is there a better way to do this?

Don P

Yeah, that calc is pure genius

Uhh, its mine :D, nevertheless, its good anyway.

You are checking your screws backwards. The 4" GRK's, check their shear capacity. If you use 16" down thru the top the failure mode will be more of a top plate roll rather than a shear, use the withdrawal numbers there. The T plate is restraining that roll by putting the 4" grk's in shear, better. Their website will have the better numbers. With proprietary fasteners, first look for an ESR number on the box (code approval) that engineering services report(ESR) contains the allowable numbers to use with that product. In the future for generic but code approved stuff use the connections calc on the awc.org website or the tables in the NDS on that site, both will yield the same, code referenced, results. The wood handbook is generally the source raw data (excellent info), the nds is the construction allowable values.

That is also the tension force in the tie to post connection and the load on the post cantilever above the tie (View it as a beam overhanging a post with a point load on its end, that calc is in the toolbox at the bottom of the left sidebar.)


doug2500

I don't see the directions. If rafter thrust is horizontal and outward, wouldn't that cause the beam to want to move outward sliding on the top of the post until it fell off. And wouldn't the load be in the direction of pulling the outer tee plate with it, therefore pulling the screws out of the wood post? And the inner tee plate would remain connected to the post with it's upper screws pulled out of the beams?



I don't want to dream about this tonight.

Don P

That is the horizontal component of the axial load, running down the length of the rafter. I could certainly be looking at it wrong but I think the top plate is going to want to roll off the post.

doug2500

So a more effective solution to insure the screws hang in there might be to drill a couple holes in each of the three legs of the TEE and put through bolts with washers and nuts?

Don P

That should take care of either direction.

doug2500

I just looked at the price of GRK screws. I was going to use four 3/8" x 10" screws to toenail each rafter to the top plates. $812 for 300 which gets me 6" into the top plate. $515 for 8" screws which gets me 4" into the top plate. I have no idea how to decide how much is enough depth other than common sense. Any thoughts?

Don P

I find the ESR, a quick google and I found the one from ICC, the code people who wrote it, this is the source if an inspector has a question;
https://icc-es.org/wp-content/uploads/report-directory/ESR-2442.pdf
Table 1 has the shear, table 2 has the withdrawal and head pull through numbers. Always read the footnotes. The withdrawal numbers are per inch of full thread penetration into the plate, so not the tip, measure penetration from the first full thread. Table 5 has more on edge and end distances, these reports answer more questions than you ever thought to ask :D

doug2500

That's a lot of fancy information I just read. I was hoping you would just tell me what to do . ;)

Do you have any idea why the shear strength and the tensile strength in the ESR is about 1/3 of what is shown in their web page info sheet? Diameters vary a little also.

Don P

I haven't looked at their site, check to see if those are ultimate numbers, these are the allowables.

doug2500

They both say Allowable Steel Strength. I can't find anything that would explain why they have significantly derated the values in the ESR.

Don P

If its the table here;
https://www.grkfasteners.com/getattachment/126d6b66-8378-40a0-8be3-f68373c97375/RSS-Product-Information

Scroll to the bottom of the table and look at the right bottom corner, those are ultimate design values not allowable. Hey a double entendre :D. If your looking at another table post a link and lets see, I was actually starting a contact ticket when I spotted that on the bottom of that table.

doug2500

I probably would have seen that if I was  paying attention. So we have to use allowable. Bummer, I liked the other numbers better.

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