How to find out brace size and length

[jfl]

Hi everybody,

I fear this might be a well know fact, but I haven't found the answer while searching in the forum:  How should the braces in a timber frame building be engineered: I mean what size, what length and how many?

I guess this depend upon the size of the building (height, width), the wind load, the strength in compression and tension of the wood...  Anybody knows where I could find a guide on how to design that?

Thanks,

jf

[Jim_Rogers]

First rule of braces is that you can't have just one, they come in pairs. Each brace has an opposing opposite brace, in the same plane of the frame.
Next is placement. You should have them in every exterior wall surface, at least at every corner as a minimum.
One timber framer near me, puts them in every rectangular opening of the frame. What that means is between every pair of posts in both directions left and right and front and back. But this can be too extreme and you may end up with interior posts having braces on all four faces.

To properly place braces you have to take into consideration window and door locations so that the braces don't interfere with the doors and windows.
That means proper planning of every door and window location in advance.

I have posted these pictures before, at least I thought I did, but here are two examples of what I mean:



And:



In both of these photos, you can see that the house was under construction.

In the first photo not only the brace is in front of the window but the timber the brace is hooked to is also in front of the window.
How are you going to hang a curtain in front of that window?
How are you going to trim that window out with wood?

In the second window which was way up high in this house, you may not need a curtain but you'll still have trouble cutting and installing the trim boards....

You need to think about the placement of everything and how it will effect everything else.

Next you've asked about size.

Mostly I've seen either 4x6 or 3x5 inch braces. However in very large timber frames for very large barns sometimes 6x6 or even 6x8 braces are used.

What length? Well again, placement come into play. If the brace is too long then it will interfere with the door and windows, make it shorter so that it won't.

On a regular framing square, a good one, and not necessarily an old one, on the tongue, you'll find some brace layout lengths. They are 24", 27", 30", 33", 36", 39", 42", 45", 48", 51", 54", 57", and 60". These are the spacing lengths of the two legs of the right triangle of the brace layout. Also, on the framing square next to these numbers, is the diagonal dimension of the brace layout, as a decimal number. For example, next to the 24-24 layout is 33.94 so that's 33.94" to convert the .94 to a fraction so that you can lay it out with your tape measure you multiply the .94 times 16 and you get the number of 16ths .94 is. The answer is 15.04, rounding off to the nearest 16, that's 15/16ths. So the diagonal measurement of the 24" x24" brace layout is 33 15/16".

These lengths were considered standard lengths by the framing square manufacturer so I'd have to assume that they were standard lengths used by timber framers who informed the square makers that they needed this information for their layout. And good companies have continued making these framing squares with these numbers on them.

I think I have posted this information before on the "framing square" story thread, but maybe not.....

Well, I've discussed what size, and what length, and how many really is a frame specific question....

Hope that helps....

Jim Rogers

[Andrew951]

Unfortunately, my framing square doesn't list brace layout lengths.  Can you please tell me what the diagonal measurement of the 27" and 30" brace layouts would be?  I can't find a list of standard layouts with corresponding diagonal dimensions anywhere.

[Andrew951]

Never mind, Jim.  I found my answer on a math youtube channel.  Dividing the brace layout measurement by .707 gets it every time.  So,
24/.707 = 33.94
27/.707 = 38.18
...and so on.
I also found the reverse.....if you cut your knee brace first and have that length set, what measurement would the two legs of the right triangle be?

Starting with a known knee brace length of 33.94 (33 15/16th), for example:
First, 33.94²:  33.94 x 33.94 = 1151.9236
Divide by 2: 1151.9236 / 2 = 575.9618
Then, using the square root function on your calculator, square root of 575.9618 = 23.999.....or 24"

[btulloh]

Trigonometry and geometry. Who knew they would be useful some day?   :D :D

[Don P]

Yeah, they should be taught in shop class rather than by a bored teacher who cannot answer the question "what is this stuff good for?"

For a right triangle with equal length legs, that is 45 degree angles (or a 12/12 roof), the magic numbers, carpenters call them line length ratios, to have etched into your brain are .707 and 1.414.

Like you just did with Pythagoras, you can use that to find the line length ratio of any right triangle.

Braces are installed as pairs whenever possible because in wood they are really only effective in compression. In a racking situation one of the pair goes into tension, the other is in compression.

There is structurally no such thing as too long, ideally from floor at one post to plate at the next is good. If you look at post framed buildings, engineered pole barns, there are brace bays where they make a large X using threaded rod and turnbuckles from the bottoms of the posts up to the plate level. As a brace gets shorter it applies side load to the post and also begins to jack the plate off the post in a racking situation. The posts and beams become long levers around that fulcrum. There is an opportunity for more trig and geometry there :D

The sheathing on most buildings is actually the bracing element. It forms the longest, stiffest brace. Load goes to stiffness, you can't fool the building, it knows the load path. Take advantage of that when possible. In Jim's second pic that could have been a no brainer, use the sheathing and omit the braces.

[btulloh]

Very useful numbers to have in memory for sure, DonP. If you're running out of storage space in your brain, you can get by with just one of them since 1.414 and .707 are reciprocals of each other. (Divide 1 by either, and will give you the other one.)

I wonder if kids are learning any of this these days???  seeing as how most of them can't make change for a dollar???   ;D

[timberframe]

The problem with general guidelines is knowing when they don't apply.  

But one thing I've heard often about braces is that they should join the post at "the third point", meaning 1/3 of the way down from the horizontal member to the bottom of the post.  Like Don says though, many times longer is better and I've gone longer when they wouldn't be in the way.  

[Ljohnsaw]

I guess you could say I'm into "overkill" territory with my cabin.  Here is my model from February, 2017.


 
My cabin roof is based on a 3-4-5 triangle.  That is, the bottom (horizontal) is 3, the height (vertical) is 4 and the angled side (hypotenuse) is 5.  So I made the braces the same so they look good with the rafters and just continued them around the building.  Yeah, a bit much with the knee braces but my goal was to have NO shear wall (aka, plywood) in the building.

I'm in a very high snow load area (341 lb/sqft) and very high winds along with seismic activity.  With all of that and this overkill on bracing, my engineer gave it the green light in his stress modeling software.  The brace and horizontal girt stock is all 4x6.  There is also vertical "girts" on each side of the windows where the 2x11 "frame" is attached to house the windows through the thick insulation.  In hindsight, I probably should have stuck with shorter brace lengths and that would have made it look a little less cluttered.

That being said, a number of my braces are 5' long so layout is pretty straight forward.  The longer ones you see on this wall are 7.5' (1.5 times 5'), so the other two measurements are 4.5' and 6'.  Pretty simple.  As Jim mentions, plan for your windows and doors.  The left and right windows are 5050 (5' x 5') and the center (bathroom) is 4050 (4' wide x 5' tall).

[Brad_bb]

Everything Jim said.

If there isn't a door or window consideration, the most common brace layout is 3'x3'.  I like to use 4x8 stock, but you can use 4x6.  4x8 gives you enough material to cut a 1" curve in one side if so desired.  Also when doing brace layout, take into account if you are housing your braces/joinery.  A 1/2" is typical for a housed brace.  Your layout of your diagonal shifts on the brace to account for the 1/2" housing.  It's pretty easy to do once you layout one.  It also looks so much better in the frame.

I also use 6x6 braces in certain locations.

Consider this also, when putting braces on an interior wall bent, a wall that will get infilled, Make the brace wide enough so that it won't get covered by the wall.  You want to be able to see those beautiful braces.  So if your bent is 8" thick and you're infilling with framed 2x4 wall, make sure that brace will show on both sides by making it thicker than the wall framing plus the drywall or whatever.  If you're doing a complete wall infill, consider making the brace the full 8" or close to it.  Another option if only using 4" thick braces is to locate on one side of the bent (not centered) so I will show on one side at least.

[Mad Professor]

You can layout your posts, ties and braces just using a divider, chalkline, and plumb bob.

To layout "Star of David" use intersections of three circles, one radius apart, along centerline.  Connect intersections of the circles, with the circle centers of the two outer circles.

The equilateral triangles give points to square off ends and sides of timbers.  The ends and sides will be perfectly 90 degrees square.  

Note that timber/brace stock need not be square to start with.

Braces are also laid out using a reference circle from the centerline of the depth of the brace, it's diameter being the depth of the brace.  This same reference circle is also used to locate mortise pockets of the timbers being braced.  In effect using a 1, 1, root 2 triangle.  But you just measure with your divider.  The brace length uses the hypotenuse , the brace pockets the triangle sides

[Mad Professor]

Is anyone still interested in traditional layout?

What I gave here was given to me.

You can layout 3/4/5 braces with, just,  a compass/divider. But a little more complicated. Not what I want to TRY to draw/post.  You need more stock for layout also.

1/1 /root 2, 45 braces are easy.

I'm on a too ancient computer, to post here.

But I don't need a square, tape, ruler, fancy tools, to make things.

Dick B. taught me, sometimes over a beer or five- six.......  God bless him.