iDRY Vacuum Kilns

Sponsors:

Cross brace/knee questions

Started by peter nap, December 25, 2007, 07:51:19 PM

Previous topic - Next topic

0 Members and 1 Guest are viewing this topic.

peter nap

These are pretty stupid questions, but then again, I'm pretty stupid! :(

Other than the looks, is there any difference between a knee brace and a straight cross brace? Is there any particular benefit of one over the other?

Is there a formula to determine the correct length of the brace to give the maximum support?

Thanks!                 

Jim_Rogers

Quote from: peter nap on December 25, 2007, 07:51:19 PM
is there any difference between a knee brace and a straight cross brace?
You'll have to define what you're calling a straight cross brace. I've never heard this term before.

QuoteIs there any particular benefit of one over the other?

That depends on the answer above.

QuoteIs there a formula to determine the correct length of the brace to give the maximum support?

Not sure again, it depends on your definition.....

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

peter nap

 cross brace is a straight piece of lumber, mitered on each end and inserted at the intersection of a horizontal and vertical beam. Most do not a tenon a tenon.

Jim_Rogers

Peter:
I'm not sure if I'm understanding you.
But you may need to spend some time reading the glossary of terms listed at the top of this section, so that you can use the correct terms correctly.

A beam is a horizontal piece of wood there is no such thing as a vertical beam. If a timber is vertical it is either a post or a column, and in timber framing it's usually a post.

A brace is mitered on both ends and usually goes between beams and posts, and usually at a 45° angle, with tenon on both ends. Sometimes these tenons are pegged to the post and/or beam and sometimes they are not.

The spacing of the horizontal and vertical distance from the 90° corner created by the brace which is at a 45° angle is decided by the designer based on several different factors. Such as looks, head clearance, shape (curve cut or straight), and height of beam off finished floor.

A brace or knee brace's job is to prevent the frame from racking. Racking is rocking back and forth in the wind, making the frame stiff or preventing sway. Most horizontal beams are not expected to get any support from a brace, and are sized as if there is no brace there at all. It needs to support it's own load by itself.

Unfortunately I don't understand what "Most do not a tenon a tenon." means. Somehow you've forgotten a word or two....

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

peter nap

Sorry Jim
I'm on clearwire and we're having a storm.  It's hard to type in between glitches.

I think the problem I'm having is my terminology. I have gone over the glossary but it didn't have cross brace in it. It may be regional slang here. I did a lot of boat building several years ago and a Knee was always curved. A cut knee was mostly for looks but a true knee was curved to follow the grain of the wood.

The Cross brace I was referring to was just as you described it. A knee or knee brace is the same as the brace you described, but curved, (Remember, this is my terminology).

The first part of the question is: Is a curved brace/knee any better than straight?

Part two is: Is there a standard way of determining the length of the brace/knee?

I have always used those braces for shear bracing. Longer to a certain degree is stronger, but I'm not sure how long is enough.

Jim_Rogers

Well most timber framing had it start in ship building. And in ship building knee braces were used to gain head room.
Here is a picture of a knee brace:



This knee brace is made from a knee which is a piece of wood that you get from cutting a trunk/stump of a tree and the root/or branch of a tree. This way the grain goes around the corner.

If you want to make a curved brace there are several ways to do it.
You can take a wide piece of straight lumber and cut two curves on it, one on one side and one on the other to make it curved. But if the piece isn't wide enough you may have some areas where the grain comes out to the surface and in a loaded situation the brace may break cross grain. This isn't the best way, in my opinion, to make a curved brace.

The other way to make a curved brace is to make it out of curved stock.

Here is a shot of two pieces of curved brace stock, I made out of one curved log to make a pair of book-matched curved braces:



What we did is we used these to create two opposite curved braces.

To do this you first need to start with a regular brace and place it correctly on your saw horses so that you can see where the tenons will be when you layout your curved stock.

Such as this:



Next you draw a line or use a straight edge to create the brace layout line. In the above photo the piece of square tubing is the hypotenuse of the right triangle which the brace usually is.
From this straight edge we laid out our 45° tenon shoulder lines. From the tenon shoulder lines we then laid out our tenons and cut away the waste.

We cut both the post and the tie beam to have regular brace pockets at their regular locations and then custom cut longer mortises to fit the longer tenons for these curved braces:



Here is a shot of the brace mortise on raising day:



Here is the brace in place:



Why we did it was for fun, but which is better?

A curved brace gives you more head room. I could walk under the brace with my arm against the post and not hit my head on the brace. I couldn't do this under the other straight braces.

You asked: "Is there a standard way of determining the length of the brace?"

Yes the brace layout is measured from the 1st working point, which is the point where the two legs of the right triangle meet. This is the 90° corner.
Each leg is usually a standard length and each leg is usually the same length as it has to be the same in order for the angles to come out to be 45°.

You can have right triangles that don't have the other two angles both being the same, 45°. Some people do use braces where the angles aren't the same on each end but this is usually advanced timber framing.

Some of the standard layouts for braces are 24", 30", 32", 36", 42", 48".

I believe I've written a story and posted it here about how to layout a brace.

Jim Rogers

Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

peter nap

Thanks Jim. I'll do a search for the story.

Jim_Rogers

I just bumped it to the top of the current list.....
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

stumpy

I've been following this thread and just had to comment.  I think the skill it takes to do timber framing is about as impressive and any craftmanship I've seen.  My hat's off to all of you.
Woodmizer LT30, NHL785 skidsteer, IH 444 tractor

peter nap

I saw where you had bumped it right after I posted.
Thanks again. Both threads are very good and I'm going to bookmark them in my what to do folder.

Greg

When you say cross brace do mean a diagonal between the upper corner of a vertical post and horizontal beam?

And when you say knee brace do mean a diagonal brace in the lower corner?

Functionally they have the same purpose I believe, whether they are low or high. Neither one is structural or load bearing. I believe the only thing they provide to a frame is stiffness from wind shear, so a calculation isn't really needed.

No expert just regurgitating what I've heard from (real* experts over the years...
Greg

Jim_Rogers

bump to the top for Raphael....
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Jim_Rogers

Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Dave Shepard

Are you just accenting a normally straight brace? If so, I would axe close to the line and clean up with an adze and a spoke shave. For very curved pieces, you will want to use a naturally curved piece so that you don't have grain runout.
Wood-Mizer LT40HDD51-WR Wireless, Kubota L48, Honda Rincon 650, TJ208 G-S, and a 60"LogRite!

Thank You Sponsors!