Is there a best joint?

[Thehardway]

OK, here's a couple quick sketches that show the bents with a scarfed top plate.  Ideally scarf should be over a brace but placing it close to the oversized braces and it spanning only about 3 feet give me a comfort level I could sleep with.  Beams below give some additional bracing/support options if you don't like the large diagonal braces. I advise you have an engineer look at it and not take my word for it.  As Jim said, scarf location are tricky and are dependent on a lot of variables and one is the length of timbers you can obtain. This design would require that you be able to produce or purchase  4- 22' timbers for the 4 posts in #2 bent and an additional 6 for all the scarfed top plates.

I think the exposed scarf joints would give this frame a great point of visual interest, especially with the large braces and the central stairs ascending near it. 

Not sure what this does as far as your glazing area for passive solar gain.  It might give you a starting point though.

I like the layout.  This frame could be cut, assembled and erected in a hand raising.  Common rafters dropped in after frame is raised.

The spans all look reasonable and it gives you nice ceiling heights and dimensions for your rooms.


Crosssection:





Bent 1



Bent 2



Bent 3




What are you thinking of using for thermal storage mass?  A masonry stove would probably work nicely in this design.

[jtbartlett]

Hey! Thanks very much for the drawings, they do help. My problem lies with the length of my mill. As I stand right now the longest timber I can cut is 18'. If I do some creative sawing I might be able to get to 22'. That restriction has been the real dilema I've been battling with. The posts don't fit right with that length.  I would have to change any plans from 24X24 to 24X40 so there's no sense in buying plans I would have to change. Here's (yet another) question: since the lower ridge beam are essentially connecting girts (right term?) which carry the common rafters and not a true beam ontop of a post, would this work for the top beam as well? If I do hand raise the structure I could erect the posts and drop in the girts individually. I am not a purist on construction so I was thinking about using plates to secure the top beam (girt actually) to the post. It would be have a half housing and tenon or spline to the post. If I rout a 1/4" pocket for the steel plate I could bolt the thing together. The shed roofs are the same size and should carry the same loads. If it's ok at the lower beam would it also work for the top? Thanks again for all the help. There's a learning curve I am getting real familiar with right about now. 

[jtbartlett]

Oh, for thermal mass, it's a slab on grade with PEX in the floors. I have a 3' wide, 18" deep center footing running the length of the house under the posts. The plan is to slip form a stone wall between the 14' posts on one of the bays. It will break give me a lot of thermal mass inside the house and close enough to the south wall. If I put a wood stove in front of it I can get double duty. Has anyone seen the $1000 solar water heater ideas on builditsolar.com? Large (1000 gal.) water storage. I'm hoping to get a lot of thermal mass in a smaller house with lots of insulation.

[Jim_Rogers]

There are some standard rules for scarf locations and sizes.

One that is important, at least to me, is the amount each timber overlaps the other timber it is joining. I used to use 3 times the depth of the timber. But a while ago, I asked Jack Sobon what he uses for a standard. And he said he uses 4 times the depth.

So if your timbers are 12" deep then you need a four foot scarf.

The next standard rule is the location of the scarf. As nice as those drawings are, that thehardway did, that location is not the best. It could be one of the worst.

This picture below is a graph of the stresses on a plate when supported by several posts.

To read this graph you look at the line labeled "Shear". Here you will see that the shear is highest right over a post. This is the reason you don't want to have your scarf over a post. This is where the highest shear point is and you have cut your nice 12" deep beam in half.

Next look at the line labeled "moment" this is where the most load will be pressing down on the plate. It is in the middle between the supports. You don't want your scarf here as you have cut your nice 12" deep beam in half again.

You want to make your scarf someplace where there is a balance between these to force/stress points.




The reason for setting the scarf joint just off the post is that is will receive less stress there.

Look at this drawing:



Between the posts the unsupported timber will sag or deflect. Over the center post the timber will crown.
This causes this "exaggerated" wave shape effect in the timber.
At the point where the sag or deflection changes to a crown is called the point of "inflection".
This point is where the timber is neither sagging nor crowning. It has the least amount of both forces.
This, I am told, is where the joint should be made. If I understand all this correctly.

I hope this helps to explain why joints shouldn't be made over posts or directly between the posts.

Jim Rogers

[beenthere]

Again Jim, well presented and described.  8)

[Jim_Rogers]

Again Jim, well presented and described.  8)

Thank you.

And I might add, changes subject to engineer's review.......

jtbartlett:

I do know some very good timber framing engineer in the greater Burlington, VT area.
One is the engineer who helped me with my last frame design. And there is another one as well who was at the timber framing conference I went to last weekend.
I can pass their names and contact phone numbers on to you, if and when you want them.

Jim

[Jim_Rogers]

Now having said all that, I'd use timbers the length that he can easily mill, that is 18' pieces and I might construct the wall (bent) #1 like this:



This puts the scarf off the side of the post and it is easily supported by a standard three foot brace. No giant braces to get in the way of any windows on first floor.

Jim

[jtbartlett]

Thank you very much Jim, that second drawing with the 2 scarfs (scarves? lol) over the braces is what I have been drawing over and over but I talked myself out of it more than a few times. I did not have the engineering evidence and to my untrained eye, having a 'floating beam' -again for lack of abetter term, without a post under it just looked wrong.  I really do appreciate all of the help you have given freely.  Hopefully I will someday have some more  experience and the opportunity to pass it forward.  I have a feeling I'll be doing a bit of searching the archives here.

[Thehardway]

As usual, Jim has bested me again ;D  His design meets all of your objectives.  It definitely simplifies assembly and use of shorter timbers. Jim, thanks for the 4:1 scarf ratio.  I was using the 3:1 as well but I will change for future.  I think directly over the post is the absolute worst location for any scarf.  I have seen many of them failing due to the crowning effect you mention.  The midspan seems to fair much better although they are highly dependent on good braces and a proper amount of timber on the opposite side of the post making it essentially a cantilever. The scarf at the brace is superior in all but a few isolated cases and is certainly a better option in this case.

  As to the question of using an interrupted top plate, I would advise against it.  The continuous plates over the posts give the frame structural stability that you would not have otherwise. It also makes assembly and the raising an easier affair. It will also require more joints be cut, effort which could be put into cutting or even hewing the longer timbers. This is just my opinion and others may feel differently.

If you can only cut 18' with your saw you may still have an issue with cutting posts for the center bent.  Using a 6:12 pitch you will be over the 18' mark unless you use less than an 8' ceiling height at the eave.  I would not want to do scarfed posts and interrupted posts creates a whole new set of issues with bracing, joinery etc in that center bent.

If you plan on using solar collectors at any point on the roof, make sure you add in the weight of panels in your load calculations.  Additionally, the optimum incline angle for PV panels in the Burlington area is listed as 30 degrees or roughly a 7:12 pitch.  Might be nice to think about this when you build your frame, this would make panels more attractive as they would lay flat on the roof and optimize efficiency and cost effectiveness.  It costs virtually nothing now to make the small pitch adjustment. 6:12 is a 26 degree angle.  This would increase power output is the summer when you need it less and lower it in the winter when you need it more.


I should probably carry that discussion to a new thread though as it has nothing to do with scarfs!