In Sobon's, Build a Classic Timber Framed House, he uses 2"x10"x 20' attic joist which span the 18' wide building.

My question is if the building was 36' wide with interrupted joist, 2 pieces, simply attached to a spine beam at the mid point would there be any thrust on the joist wanting to pull them apart? Bent spacing 12' and less. To simplify most questions everything else is the same as in the book.

In Sobon's plans the principle ties and the common joist are handling the spread from the rafters. In the outline I presented the principle ties are the only members resisting the thrust. Are they enough? Am I looking at this correctly.

Tim

The raising plate in that frame is what the thrust is dumping into and it is transfered from it into the joist ends through nails if I remember correctly. In your revision, the raising plate would be acting like a simply supported beam (turned on its side), and it would be transferring all the thrust into the principal ties at the bents, and "spanning" the distance between bents, assuming we assign the interrupted joists no tying capacity.

I don't know if that arrangement is sound, without doing more analyzing than I can do tonight.

If it isn't, you would need to "shorten the span" of the raising plate by making one or more of the interrupted joists have tension capacity again, either with concealed strapping or exposed iron strapping.

It was early this morning I was thinking that the connection at the spine beam would look similar as the joist to top plate connection, each joist would meet in the center, as the spine was going to be at the plate level with a splice nailed on the side connecting each joist.

But, this does not work for the spine needs to be in plane with the ties for simplicity. The tie, spine and joist will all be flush on the top. A steal strap on every other or all could work as well. The joist being reduced on the ends and slipped into drop in pockets.

I have though of adding two spine beams and having the joist cog over them but this gets complicated.

It was in a recent thread where a similar example was shown, an English Tying joint with principle rafters and common rafters in between. The example I put forth is similar but a step above in that it does have connection through the building all be it a weaker connection than full length joist. The top plate is looking to be in the 10" wide range, affording additional resistance.

I tried looking in HATJ which doesn't show the detail of how the intermediate joist were attach on wider buildings, it is detailed just at the plates.

Tim

Is there a post up to where the spine beam would join the tie? If so you could drop the spine down the post to the same elevation as the plate and have them cog over like they do at the plate.

There is not a post at this time. The tie is the top cord in a steal queen rod truss. The truss clears the 36' and is 8' deep for a usable second floor over a 12' high hall, under it.

As I look for examples I see a lot of principal rafter and purlin systems with common rafters, these examples are on larger buildings and the truss systems are in the roof. I see a common rafter roof system as being the least costly to install, which is a factor for the job. And being that there is not a structure in the roof system adding one is additional work. The rafters are 25', tapered poles with a collar beam about half way up. There are many common rafter roof systems locally, some converted from purlins, the daps are still present. One in particular is English Tying and common rafters, 36' span and very sound, an interesting aspect is the common rafters are all leaning in one direction. The story goes that it was in the midst of a roof repair when a large storm moved through tweaking the roof, they just boarded it over with out straighten the rafters, just stories who knows really why?

In Sobons example there is no teasel tenon to resist wracking from the wind bracing, in effect lifting the tie off the top plate. This should not be an issue in the structure I put forth as the tie is connected into the building via the queen rod truss.

I hope you all don't mind me thinking out loud and any and all thought are welcome.

Tim

I'd consider adding a purlin plate to support the commons and eliminate some thrust.

It would be a solid, reasonable solution, insurance.

Canted queens or an H assembly? I lean toward the H assembly constructed of light material, 6"x6" stock using 1-1/2" mortices, this all would allow the roof to be braced as well. I will have to look into the canted queens, nothing in the way down the middle .

Two piece rafters or single? Probably single piece rafters.

This building is 96' long, 9 bents.

Tim

I was thinking just set the purlin plate on top of your queen rod truss -- on top of the straining beam very close to where the principal struts join it, just inboard of the queen rod.

This is a pretty massive frame with some nice challenges. Is the queen rod truss to allow usable room upstairs? I would probably use a queen post instead of a rod, so I could brace the purlin plate to the queen posts.

Hanging a floor from the roof like that, you will need serious tension capacity at the QP to tie connection. Maybe a forged stirrup strap or a wedged through tenon or both. I've seen a few wedged dovetail tenons that are no longer working because the wedges fell out -- be sure to think long term.

I know we say this plenty, but a frame of this size I would be sure to have reviewed by an engineer. Is this to be a house?

An engineer will be working on the truss. I may have him look at the common rafter issue.

The truss is below the eve line, the top cord is the tie beam, the bottom cord is the second floor binder. The purpose of the truss is to have a clear span on the first floor/ground floor. The area in the truss will be used for fiber arts at this point.

This leaves the roof area totally open, no structure as of yet.

I thought the rods would replace the wooden queens and avoid the lost wedge issue. There is also something mysterious to seeing steal in a wooden frame, to me. I see a lot of frames in my area switched to steal pieces at the turn of the century.

Tim

Ok, I'm starting to see.

So you have a parallel chord truss holding the floors up, and you are wide open up top.

So as long as the parallel chord truss can handle the roof load, you could do either an H or canted queens to support a purlin plate (just as you said earlier). I like the canted queens -- simpler joinery for the rafters, they look cool, open it up a bit more up top. (and I haven't done one yet)