Groin Vault

[oceancity]

Talking the GC into having a groin vault ceiling was easier than I expected. I showed him the JLC article about it, and he said, "Yeah, I like it, let's do it"
In fact I got more than I bargained for. I was only suggesting putting one in the dining room, but he wants to put one in the master bedroom, too. I have my doubts about that because the master bath door is in a wall that cuts into the room at a 45 degree angle. I'm not sure I could make that look good. Could I? I think its better to stick with the tray thats on the plans.

But here are my questions.

First, the dining room is slightly rectangular. The article I saw had the same situation, but the carpenter kept the vault square. I'm inclined to have the soffit uniform around the room and have a rectangular cross vault. Any opinions on that?

Second, today before leaving I quickly marked out on the floor the corners if I had a 12 inch soffit. That would give me 17' 4 diagonally across the vault. That's more than eight foot for my hips... I might be able to make the soffit bigger, but I really want to know how you pros would go about making that hip longer than 8'? Nail and glue the plywood together?

Third, it appears that the hips in the article are drawn with a constant radius. It strikes me that if two barrel vaults intersected each other, the hips would actually be elliptical?? And that there really shouldn't be need for a bevel cut on the 2x4's he frames the ceiling with. At the same time I wonder, if I'm right about this, it still might look better to do it his way? Any feedback on this?

Oh yeah one more. He builds the soffit with what we call a chicken ladder on the bottom (horizontally), a piece of 3/4 nailed to the side, and a 2x4 nailed to the ceiling joists at the top. Is this sufficient, or would there be a better way to frame it?

Thanks,
KB

[Joe Bartok]

Re: Groin Vault

Groin Vault Thread
Groin Vault Math

[kpatrix]

Re: Groin Vault

KB,

where did you see the article in JLC ? I haven't seen it and would like to.

Joe posted 2 good links. read thru them and you'll realize rectangular is fine. You can make them from any even numbered/parrallel sided shape. I've done one where a 45 wall cut into a square room. Treat it like you would a valley cutting thru.

Here's a pic of the first one I framed that had the 45 wall with door. http://new.photos.yahoo.com/kpatrix2...804030226908/3

I left the 45 wall out and ran the groin first, then came back and framed in the doorway. That was all done w/out Joe's math, and it was a chore.

Don't worry about the length of the valleys either. Glue and scab and make sure they are connected to the ceiling joists well. All the blocks create a strong diaphram.

john

[oceancity]

Re: Groin Vault

KB,

where did you see the article in JLC ? I haven't seen it and would like to.

Joe posted 2 good links. read thru them and you'll realize rectangular is fine. You can make them from any even numbered/parrallel sided shape. I've done one where a 45 wall cut into a square room. Treat it like you would a valley cutting thru.

Here's a pic of the first one I framed that had the 45 wall with door. http://new.photos.yahoo.com/kpatrix2...804030226908/3

I left the 45 wall out and ran the groin first, then came back and framed in the doorway. That was all done w/out Joe's math, and it was a chore.

Don't worry about the length of the valleys either. Glue and scab and make sure they are connected to the ceiling joists well. All the blocks create a strong diaphram.

john

http://www.jlconline.com/cgi-bin/jlc.../View/0406fram

That's the link to the article.
Thanks for the picture. You convinced me. Your work looks awesome.
It looks like your using 1/2 inch on the sides and two pieces of 1/2inch glued for your hips? or is it 3/4? can i use osb or should i use plywood?
I can't see from your picture how you attached the hips to the ceiling joists?

Thanks for those links Joe. I was kind of confused until I got to page 5 and then it all started making a little more sense when I saw those three diagrams.

The best way I can grasp it... It seems like I'd be starting with a semi circle on one side, and then stretching it for the adjacent and hips.

"The Arch Width / Horizontal Axis should be constant for all barrels"

That makes sense.

In the article the author gives and equation for determining the radius of a semicircle, and I believe the implication is that would also work for the hips, which struck me as not quite right. It isn't then?

"The vault is formed by the intersection
of two barrel vaults. Both the arches
needing to be framed — the barrels and the intersecting
diagonals — can be quickly found with
the simple formula shown."

"r = (w^2+ 4h^2)/8h"

That's from the article. It's not implied, that's what he says. Don't do that, huh?


Thanks
KB

[kpatrix]

Re: Groin Vault

KB,

Check out this thread by Tim. It's very informative also.
http://forums.jlconline.com/forums/s...pictures+groin

The one in the pic I posted was 1 1/8 subfloor. The others I've done, I have used 3/4" cd ply, and OSB. I like the OSB better because unlike regular plywood, it doesn't deform as bad when wet, and there are no grains to keep track of.

That article is pretty straightforward, but I recommend using Joe's math for solving the radius and ellipses. The higher the rise, the more important it is to follow use that formula.

In that first one, I went from 7' to 10', and I used regular radius for the valleys. I had to cut them in "the air" so they would "fair out" from side to side. This is really important when you have consecutive groins.

It can be done just using a radius, but it won't be perfectly level, or cylindrical.

Good luck, and keep us posted with pics OK !!

John

[rustynail]

Re: Groin Vault

because I can't get my head around Joe's awesome formulas I have to use a mechanical method, on a rectangular groin vault I always make the short wall a radius then the long wall and hips become ellipse's. I snap the edge's and hips on the floor, then divide the short wall radius baseline into normally 6" segments these I use to establish heights at each 6" mark then I square these 6" marks out to intersect the hip line this gives me the height points for the hip, then I turn 90* and cross the long wall baseline to establish height points for it. basically stretching the short wall radius into the long wall and hip ellipse's this is probably not very clear, I have a drawing on my other comp I'll dig up and post, I also remember seeing this same method in "A roof cutters secrets"

Rusty

[Tim Uhler]

Re: Groin Vault

Rusty,

That is a great method. I think it is also in George Collings book. If you don't have that book, it is a great one. I just found it and other framing books I've had for awhile.


John,

I cannot get Sketchup to work, but as soon as I do, I will try and take Joe's formulas and put them to a SU slide show or something like that.

Since I did that groin vault a year ago, I can't remember how to do them and have to review all the info :-).

By the way, thanks for posting that link to the thread from last year, I forgot about that. It was fun going through the pictures.

Also, you need to add some more pics to your yahoo site. I drool over all those pics and you need to put new ones on there. :-)

[Joe Bartok]

Re: Groin Vault

In the article the author gives and equation for determining the radius of a semicircle, and I believe the implication is that would also work for the hips, which struck me as not quite right. It isn't then?

"The vault is formed by the intersection
of two barrel vaults. Both the arches
needing to be framed — the barrels and the intersecting
diagonals — can be quickly found with
the simple formula shown."

"r = (w^2+ 4h^2)/8h"

That's from the article. It's not implied, that's what he says. Don't do that, huh?

That's the formula for "back calculating" the radius of a semi-circle if you want to make one of your barrels semi-circular. The Valley will be an ellipse. Since the vault is rectangular in plan view the remaining barrel will also be an ellipse.

The best way I can grasp it... It seems like I'd be starting with a semi circle on one side, and then stretching it for the adjacent and hips.

That's the mamma bear. The widths (major axes) of both ellipses are based on the diameter of the circle being projected through the plan angles, exactly as you would do a Valley roof: Comparison of Cross Vault and Cruciform Valley Roof

You can make all the barrels ellipses if you wish (in fact any conic section will work). You may begin with the width of either barrel or even the major axis of the Valley, provided the remaining widths are all based on the first width.
The arches may be the full semi-circle or semi-ellipse or they may be segmented arches. This is an example diagram of Intersecting Elliptical Barrels employing only portions or segments of the curves.

[oceancity]

Re: Groin Vault

That's the formula for "back calculating" the radius of a semi-circle if you want to make one of your barrels semi-circular. The Valley will be an ellipse. Since the vault is rectangular in plan view the remaining barrel will also be an ellipse.

That's the mamma bear. The widths (major axes) of both ellipses are based on the diameter of the circle being projected through the plan angles, exactly as you would do a Valley roof: Comparison of Cross Vault and Cruciform Valley Roof

You can make all the barrels ellipses if you wish (in fact any conic section will work). You may begin with the width of either barrel or even the major axis of the Valley, provided the remaining widths are all based on the first width.
The arches may be the full semi-circle or semi-ellipse or they may be segmented arches. This is an example diagram of Intersecting Elliptical Barrels employing only portions or segments of the curves.

Yeah, I've checked out the thread you posted a link for, and checked out your website. Exactly what I needed to know.
I was leaning toward making one side semi-circular.
This is what I come up with based on your math:

Main Length:12
Adjacent Length:13
Vault Height:1.33

Main Radius= 14.19883
Main Diameter= 28.39767

Adjacent Length= 13
Adjacent Major Axis= 30.76414
Adjacent Focus= 5.916181

Valley Length= 17.69181
Valley Major Axis= 41.86717
Valley Focus= 15.38207

So do you guys think thats to shallow? Looks like I would need about 72 feet of string to make the valley? Can that be done? Might be better to use Rusty's method to layout?

But if I go with 2 feet for my height ->

Main Length:12
Adjacent Length:13
Vault Height:2

Main Radius= 10
Main Diameter= 20

Adjacent Length= 13
Adjacent Major Axis= 21.66667
Adjacent Focus= 4.166666

Valley Length= 17.69181
Valley Major Axis= 29.48634
Valley Focus= 10.83333

It gets a little bit easier the more I increase the height...
I have ten foot ceilings, but I'm not sure how far I can go down right now, because there are columns set in the entrance to the dining room.

I made my own calc with qbasic, to figure this out. I saw yours on your site, but its for a completely elliptical vault, or am I missing something? Do you have one that can figure out the purlins for a vault with one side semi-circular? I'm open to the elliptical vault, has anyone done one? pictures?

Sorry, guys this is new to me. Thanks for your answers.
Is 16 inches too shallow for a vault this size?

thanks
KB

[kpatrix]

Re: Groin Vault

Rusty,

The technique that you mention is found on page 288 of Roof Framers Secrets. It actually looks like a much quicker way to layout the arches.

For a 4'x6' cross vault, start with the 4' side. We'll have a 12" rise.
With a CM pro v.3 calculator I divide 48 by 12 to get 4" OC . I store that as 4" OC.

With a 48" run and 12" rise I have a 30" radius.
I continue to hit the arc button and I get results of 11 3/4, 10 15/16, 9 1/2, 7 3/8, 4 3/8, 0

Now I take the 72" side and divide it be 12 and get 6". I use those same numbers on 6" centers instead of 4" to layout this arch.

My valleys are 7-2 9/16, and I divide this by 12 to get 7 3/16". This will be my OC spacing to layout the valleys with the same heights as the other two.

I know this sounds confusing, but the picture in Will's book makes it clear.

I haven't tried this method yet, but I think I will check it versus Joe's math the next chance I get. I have been laying out my arches this way for a while now, it is much easier than trying to strike the radius from a center point. I can work alone on the horses as opposed to having a helper and a big area to scribe with.

Joe, do you know of any way to plot an ellipse like an arch can be plotted ?
I hope I don't consume too much of your time with that question ;-)

John

[Joe Bartok]

Re: Groin Vault

Joe, do you know of any way to plot an ellipse like an arch can be plotted ?
I hope I don't consume too much of your time with that question ;-)

Yeah, just plug the numbers in my Ellipse Calculator and let it crunch the numbers.
That doesn't take much time ... :) Or were you looking for the formula?

[oceancity]

Re: Groin Vault

Rusty,

The technique that you mention is found on page 288 of Roof Framers Secrets. It actually looks like a much quicker way to layout the arches.

For a 4'x6' cross vault, start with the 4' side. We'll have a 12" rise.
With a CM pro v.3 calculator I divide 48 by 12 to get 4" OC . I store that as 4" OC.

With a 48" run and 12" rise I have a 30" radius.
I continue to hit the arc button and I get results of 11 3/4, 10 15/16, 9 1/2, 7 3/8, 4 3/8, 0

Now I take the 72" side and divide it be 12 and get 6". I use those same numbers on 6" centers instead of 4" to layout this arch.

My valleys are 7-2 9/16, and I divide this by 12 to get 7 3/16". This will be my OC spacing to layout the valleys with the same heights as the other two.

I know this sounds confusing, but the picture in Will's book makes it clear.

I haven't tried this method yet, but I think I will check it versus Joe's math the next chance I get. I have been laying out my arches this way for a while now, it is much easier than trying to strike the radius from a center point. I can work alone on the horses as opposed to having a helper and a big area to scribe with.

Joe, do you know of any way to plot an ellipse like an arch can be plotted ?
I hope I don't consume too much of your time with that question ;-)

John

Wow, that sounds easy.
Now, I'm not Joe, but I do want to try this... so, it seems like what you want to do is lay out your valley and adjacent on 4" oc, too? (because what you would have plotted at 6inch centers would be an ellipse, but on different centers, right?)
Couldn't you use the same 48inch run 12inch rise, but change the O.C. to
48in x 4 in / 72 in = 2.667 o.c. to find what the 4 in o.c. layout measurements would be on your adjacent, and
48in x 4 in / 86.533 in = 2.219 o.c. to find the 4 in o.c. layout on your valley?? (pulling your tape from the outside)

KB

[kpatrix]

Re: Groin Vault

Joe,

Thanks. That was quick. I'll stay away from the formula for now. The calculator is all I need.

Sometimes it's hard to draw an ellipse when you only need the top 14" of a 12' semi minor axis with a 36' major axis and I've needed an easier way to plot these.

Keep up the good work.

John

[Joe Bartok]

Re: Groin Vault

I'll stay away from the formula for now.

Too late, I typed all this up and don't want to waste it so ...
There are links to code for "back calculating" segmented elliptical arches at Groin Vault Math. I show the math on these web pages.
The easiest formula to work from when plotting an full ellipse is:
(x/a)² + (y/b)² = 1
x = measurement along horizontal or level from the center of the ellipse
y = height at given value of x
a = semi-axis on horizontal or level
b = semi-axis on vertical or plumb
For a vault, the semi-axis designated "a" will also generally be the semimajor-axis. We also need to rearrange the formula.

Squaring all terms … x²/a² + y²/b² = 1
Solving for a common denominator … (b²x² + a²y²)/a²b² = 1
Cross multiplying … b²x² + a²y² = a²b²
Transposing the term with x² … a²y² = a²b² – b²x²
Dividing both sides by a² … y² = (a²b² – b²x²)/a²
Collecting terms … y² = b²(a² – x²)/a²
Taking the square root of both sides … y = Square Root(a² – x²) × b/a

The calculator is all I need.

Use a Programmable Graphing Calculator to plot the ellipse; the "Trace" function will show the points as the cursor moves along the curve.

[kpatrix]

Re: Groin Vault

Well, I guess my grass is gonna grow a little taller today because I'm going to spend some time figuring out how to plot my ellipses.

Tim,

Maybe you need to upgrade to windows vista and try sketchup from there. The newest update works on my computer but the previous one didn't.

John