Principle Rafter and Plate Scarf Location

[routestep]

A few weeks ago I read a very interesting discussion about where to place a scarf joint on the plate. I think the situation was tie below the plate.
Some responses were to put the splice right above the post and some were to put it off to the side of the post a ways.
It looked to me that the splice above the post supported the plate very well. But now I'm thinking about my principle rafter coming down into the plate right at the post location too. That got me thinking. My plates are nine by nine so I only get half of that for the top of the scarf and I have a post with the tenon going into the top spline somewhat. Then I have to remove wood to set the rafter and put in a peg or two. Can the rafter push so hard on the top half of the splice so as to bend it out or crack/brake it off? Has anyone seen this happen?

[Jim_Rogers]

First of all these are the reasons you don't put a scarf over a post. You need as much solid wood there as you can get, for just such things.
Read or search this forum for scarf joints and you'll find all kinds of info about where to place your joint, beside the post usually supported by a brace is best by far.

The reason I say this is that traditionally it was done this way. We know this because we see it in standing barns. We don't see this over a post, because this failed and the standing barn is standing any more, it's gone.....

Can the rafter push so hard on the top half of the splice so as to bend it out or crack/brake it off?

Maybe but the entire roof system may need to be reviewed to understand how much load is coming down the rafter to the plate....

Jim Rogers

[moonhill]

As prof to Jims post check out the second photo down on post #38 in " How to tear down a 34'x48' barn".  A scarf over a post .  And that barn won't last much longer.  If my logs won't allow the timber to streach another 3' than over the post it goes, But I try to place them as Jim suggests.  Tim B.

[routestep]

Yes, I see the scarf over the post. This barn looks like it has common rafters. I don't know how they are tied into the plate, but the rafters do not meet the plate at the scarf, at least in this picture.
I'm using 8 by 9 principle rafters that will be lapped stepped (?) into the plate. Hence I'll be removing wood from the plate. So I think I will put the scarf over to the side away from the post, maybe over the brace and spread out the various joints. Also, I'm using a pair of struts in each bent so that should take up some of the thrust exerted on the wall. I wish I had a way to guestimate how much load the struts will absorb.

[witterbound]

I thought principal rafters and purlins were used in bent frames, and common rafters are used in post and plate frames.  Don't know whether it's an engineering issue to mix and match.  It does, however, seem like a waste of a long plate, as nothing is going to be sitting on it except at the posts.  If the purpose of the plate is just to hold the posts in place, it would seem to me to be easier to just add shorter timbers (girts?) running from post to post.  At the very least, if you're going to do it, you need to get it engineered.

[routestep]

witterbound
Amen to that. I like the fact that a plate locks the bents together but the roof system is a hybrid of sorts. Since I thought the foot of the principle rafter could crush the plate or snap the post tenon I decided to use a strut or jamb de force to spread the load to the cross tie. The Germans have a more complicated strut system then the French. Drawings of both are in the TF Guild volume 2, but your right - Ed Levin's diagram is a rafter to post connection.

[Jim_Rogers]

I thought principal rafters and purlins were used in bent frames, and common rafters are used in post and plate frames. 

There are frames that have principal rafters with purlins that have continuous plates. Again this is to tie all the bent tops together to make a strong stand alone frame, that doesn't rely on the sheathing to hold it all together. Some of these frames had principal rafters purlins and then common rafters in between the principal rafters:



In the above barn frame there are principal rafters with a purlin in between them and then common rafters attach to the purlin and sit on the continuous plate.

It does, however, seem like a waste of a long plate,

At the time of building some of the large barns with long continuous plates, there was no lack of long straight pieces of wood.

Again short connecting girts can be used if they are designed properly and the joints are strong.

Jim Rogers

[routestep]

Jim, the picture you have posted is what I'm working on, a single purlin and common rafters in between the principle rafters. This way I don't have to find twenty pairs of small but long rafters.

Do you recall how long the purlin was on that frame?

[witterbound]

Thanks for the information, Jim. 

[Jim_Rogers]

Do you recall how long the purlin was on that frame?

I have the drawing here somewhere, but I'd say it was around 12' as this barn was 36 feet long and 38' wide. So that would be a four bent frame with 3 bays of 12' each.

Jim Rogers

[Don P]

Not sure if it is adding to or taking us down a side path but I've been doing some reading on cantilevered beam connections from the gluelam folks. They don't do a scarf but they do have a beam situation similar to what is being discussed.

Here's a quick sketch of one scenario, a cantilevered beam supporting another beam. Uniform loading.


This is the math that accompany's that, the first condition in the scan. Notice the hinge point and supports.


That looks a little rough at first; Down the left side Moment, Shear,Reaction, Deflection. Across the top the columns correspond to the labelled points in their sketch.

Formulas at the bottom of the chart were these;
M=CwL²
V=CwL
R=CwL
Deflection=CwL⁴/48EI

C is the appropriate coefficient above
w is load in pounds per lineal foot on the beam
L is span in feet


I've been working on some polebarn math so had some numbers in my head to apply to an example;
Assume the beams have a 16' span each.
First thing is the inflection point is 0.172 X 16' from the post, or 33"

On the polebarn we figured double #2 SYP 2x12's breaking over the posts to meet code. The beams had roughly 180 pounds per lineal foot on them.

I'll work through the Moment formula to show how it works;
M=.086 X 180 X 16²
  =3962.88 ft/lbs or multiply X 12 =47554.56 in/lbs
The SYP has an allowable Fb of 1121 psi so divide that into the max moment and we get a minimum section modulus required of 42.42 "³
A double 2x12 has a section modulus of 52.73"³ where a double 2x10 has a Section modulus of only 35.65 in³(too small) so it checks in the same range with what I was doing another way.

The other formulas use the same methods, if you're doing it yourself here's some inputs and solutions
E on the SYP was 1,600,000 psi
I on a double 2x12 is 296.6 in⁴
A solution, came up with R of 1192.32 lbs on the left post-3375.36 lbs on the midpost-1192.32 lbs on the right post. Add those up and backcheck by multiplying 32'X 180 pounds per foot, the load on the posts should add up the same.

I know some of you are allergic to steel but I think I should show the connector they use at the hinge that accompanies this math so you can decide for yourself how applicable this all is;

[Don P]

I bought a copy of "Design of Building Trusses" James Ambrose, it came today. While thumbing through there was an excellent section on this. Start on page 90 of the link below, read through section 6.13, Figs 6.43 and 6.44 are worth really scrutinizing. It might be good to back up and read section 6.12 starting on pg 81 to get the background and full context.
Beams with Internal Pins

Pg 192 is where the heavy timber truss section starts  ;).