Timber Framers Guild Forums General General Forum Questions Lapped Dovetail vs Tenon&Mortise Knee Braces

The area of supporting shear plane at the acute entry seems very slight, even though this is just a detail in isolation, my confidence is low regarding this joint.

ok.

I had actually 9 cm depth first, following the Tedd benson book Building the Timber Frame House where it's written 3 1/2 " depth.



But i have an issue anyway for the center post which has to receive two braces at the same level, there is no wood left! my center post is 16,5 cm (the perfect timber within the 17,5cm initially ordered).

My posts are spaced every 3 meter, also I can not change the lenght of the brace itself, I need to have enough room for openings (doors and windows).

Here is a closer look at the bent plan



as mentionned previously, my tie beam to post is connected with a wedged half dovetail through tenon joint and I have joists and purlins dovetail connected in between bents.

What about that shear plane Roger, how big would you draw it?
and why your confidence is low?

Do you have a copy of Jack Sobon's work? I think you need to step out of the confinement of just one book. You are following some of Ted's first work. Things have changed.

Accepting the limitation of 8 cm penetration of the halflap, I would square off the tip at 7 cm, the reason is that the shear plain increases geometrically from the acute entry as depth increases.

In the customs I am used to, where you have 2 laps on a single post joining at the same level there are 2 approaches to this. Either the laps are made to meet in the center of the post, or the are alternated, one on the inside of the post or one on the outside.

Also I personally would place laps on a post on sides that will be visible on the finished frame, this is a beautiful joint that you should want others to see for years to come

On the current barn project that I am working on, all of my joints are arranged for best visibility. I wish I was at the point of cutting the braces, I could show you the different methods used in different situations. The system has a but more dynamism that simple mortise and tenons

Right now, I'll try and whip of the drawing I promised you of the refined entry concept

Here we are then,

DL, when is your book coming out? It would be nice for another source on just such items. Good to see a variety of choices. The sharp point on the first one would be more tricky to cut than the others.

I stopped by a winery today, it had lapped braces through out the building, built around 30 years ago. I am betting the influence came from Ted Benson.

Book, oh well hopefully I'll get it started soon

As for the first one, in most cases where I see this, the point is all the way on the far end of the timber -that is, the lap goes all the way, making it a super-simple joint to cut, just saw two lines along the timber's side, then chisel or axe or adze out the waste, can be done in about 2 minutes.

When you get the the refined laps, those are awful complicated to lay out, the best ones -functionally and visibly- use some rather complicated geometry.

In good news though, I got my compass back out, and did some old-fashioned geometry today, laying out my barn according to the compass geometry. This thread inspired me, since I was looking at these laps and figuring out how one could drawn them with the compass to make effective layouts

When you have two braces coming together at the same point on a post, is it accepted practice to extend one all the way and have to other come up to the full lapped brace?

I just drew that out and it doesn't give much of a dove tail on the shorter brace.

2 minutes, no doubt. can't get much quicker than that.

It's done different ways.

let's look again at this:


These are examples of laps that come to the same point, this construction is called the Heidenkreuz, because it looks something like a cross, and because it's a practice (a framed gable) brought by the pagan Alemanni to the Christian Berner Oberland about the 7th century or so.

This in my opinion is the weakness of the lap dovetail.
The best solution in my opinion is to push the angle of the braces well past 45 degrees, typically they will be either 3:4 or 2:3 (I prefer 3:4, I think it looks nicer) This gives you more space to work with within the same width of timber. Generally the two will meet in the middle and form a point.
These examples both show a small bit of wood between them.

When building a frame, I like a different approach, it's an old one that has an interesting look to it. This is to make all of the braces come in to the timber at a different height, which lets me give each one as much space along the face of the timber it needs. I'm working on the drawings for my barn this week, I'll put up a picture of one of the posts with 3 braces at different heights when I get that drawing made