Rafter Connection to Top Beam

Don P · August 31, 2020, 10:00:38 AM

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 · August 31, 2020, 12:10:14 PM

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 · August 31, 2020, 02:23:19 PM

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 · September 01, 2020, 07:54:02 PM

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 · September 01, 2020, 08:40:19 PM

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 · September 01, 2020, 08:56:47 PM

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 · September 01, 2020, 09:19:00 PM

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 · September 01, 2020, 09:28:07 PM

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 · September 01, 2020, 10:10:45 PM

That should take care of either direction.

doug2500 · September 08, 2020, 09:11:33 PM

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 · September 08, 2020, 09:56:37 PM

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 · September 08, 2020, 10:54:11 PM

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 · September 09, 2020, 07:33:19 AM

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

doug2500 · September 09, 2020, 07:40:58 AM

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 · September 09, 2020, 04:07:34 PM

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.