I'm not sure how doing the layout and cutting a brace with your method gets you more bearing.
Let me show you what I learned at Heartwood school when I took the engineering of timber framing joints class in 2005.
Laying out a brace using the surface of the timber as the hypotenuse of the right triangle will get you a brace that fits into a brace pocket that looks like this:
Instead of this:
In the first drawing above, it's a little hard to see, but you can see the shelf created by the housing. And that the surface of the brace does not meet the surface of the post.
Here is a larger view of the two braces, side by side:
Some may say: "I'm ok with that...." I would say fine with me if you're ok with that.
Now let's say you have 2000 lbs of load coming down that brace into that pocket where there is no bearing area at the housing.
Shown here as the highlighted area in this wire frame drawing:
This highlighted areas is 3" x 2" or 6 square inches.
If you cut the brace with the 3/8" layout line as the hypotenuse of the right triangle you get a bearing area like this:
Of course both timbers have to be nearly the correct size, not 1/4" under as you have suggested.
In the second example the 2" thick tenon is 3" long and there is a 1/2" wide by 4" shoulder bearing on the housing.
In the first example we have 6 square inches, and in the second example we have 8 ((2x3=6)+ (1/2x4=2) square inches.
Ok, so you have two more square inches, what difference would that make?
Well, let's look at the difference in what the bearing areas can support.
In the first example the bearing area is 6 square inches. Using eastern white pine strength values from the NDS we see that compression parallel to the grain (as we are bearing on end grain in the post) is 325 lbs per sq inch for grade 2 post and timbers.
So 325 x 6 = 1950. This is less than the 2000 lbs we got coming down the brace. What will happen? The fibers of the brace will crush until it can bear, if it ever can.
In the second example we have 8 square inches of bearing area.
So 325 x 8 = 2600. We now exceed our load by 600 lbs or about 1/4.
To me it's much stronger and there shouldn't be an fiber crushing.
Next, let's say our post did come in undersized by 1/4".
What does that do to our bearing area?
Now we have ((3x2=6)+(1/4x4=1)=7. Also, 7x325 = 2275 which will still support the 2000 lbs we got coming down the brace.
Does this make sense to you?
Jim Rogers
