Framing A Groin Vault?

denver 2

I'm a mason by trade and I generally build my own templates for arches. I want to build a groin vault ceiling with wood framing then apply a rough coat of cement then apply thin brick for a finish ceiling. My question is how to figure the diagonal arches. If let say my wall arches are 10ft wide with a 16in. spring, and the room measures 14ft diagonally accross, would I simply build the 14ft arch with the same spring and then the intersecting diagonal arch and then attach the perlins from the center to the outside arches? When sketching the wall arches I would simply use two tape measures to find the radius, then draw the arch. I guess the same applys to the diagonal arch as well? Am I right?

D.Foster

There was a recent thread about this in the framing a knee wall category under framing. Tim Uhler posted some pics and described how he did it!! Hope its helpful.

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lukachuki

Also ask this over in the masonry section...there are some skilled guys who have done this with pics as well.

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Jack Rafter

D2,

The first question would be whether the vaults are segmented arches or are they elliptical?

Segmented = http://www.josephfusco.com/Junk_Drawer/barrel_vault.gif

Full Elliptical = http://www.constructionforumsonline....r/vault_00.jpg

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denver 2

The vault will be a full elliptical. I guess what you call two intersecting barrel vaults. I understand that the rise of the arches are all the same, but from looking at photos of this vault in the framing stage I see that plywood arches are spanned diagonally accross the room with perlins being attached from the center outwards on all four directions attached to the outer wall arch templates. I know how to draw a the elliptical wall arches by useing the two measuring tape method. Is the same method used when figuring the diagonal arches?

denver 2

Thanks D. Foster and Lukachuki for your suggestions which I have already looked at. Tim Uhler's pics and Master Mason's pics are helpful.

kpatrix

The diagonal arches can't be figured using the "two-tape" method. It will be close, but no cigar. The easiest way to arrive at their solution is thru lofting the measurments of the main barrels.

Tim has some good pictures on how this is done, and it's very simple. Just divide the 10' wall span into one foot sections and use a framing square to determine what height the arch intersects the square at each point. Write these numbers down.

Then when you plot out the 14' side, divide it up by the # of divisions used in the 10' side. Again use the framing square and mark the corresponding measurements you wrote down earlier, connect the dots and you are done.

look at the first few pics in this gallery http://picasaweb.google.com/KirkpatrickFramer/20070709#

john

Jack Rafter

D2,

The reason for asking is to determine whether the elliptical hips would be full of partial. If you are using a segmented arch then the hips would be partial unless the arch was a semicircle.

Since the arches in your case are full ellipses, that means that they have a major axis of 120" and a minor axis of 32. The semi major and minor axis' would be 1/2 of their full lengths.

To get the elliptical hips at this point is just a matter of determining their major axis length since their minor axis is the same as the other elliptical arches minor axis'. The easiest way to find this length is to just use the Pythagorean Theorem with each of the major axis lengths of the elliptical arches as the "a" and "b" components to the equation and then calculating the "square of c" as the length.

Since both the interesting barrels are the same size elliptical arches this becomes a "square" problem and you could also find the length of the major axis of the elliptical hip by multiplying one of the major axis' by the sqrt(2) or 1.4142.

As John pointed out, the two type method might be just a bit understated for this.

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denver 2

Thanks John K. for explaining in detail what lofting is. You made what seemed as a difficult math problem into something a masonry contractor can actually understand. I veiwed the pictures in the gallery and now understand the process. You have been of great help friend.

Jack Rafter

D2,

There is nothing like being "enlightened" and being able to see something in easier terms, but don't forget you'll still have to "draw" the first ellipse to be able to project the second from it.

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denver 2

Thanks Jack R. for the encouragement and advice. The way I was taught to draw an elipse was to find the center of a openning and then determine the rise and then from the top of the rise and either far side of the openning take two tape measures follwing the line of center and when they both meet will establish the radius, from which a string and pencil can be used to draw the arch. Perhaps there is a better method of which I'm not aware of. Any advice I'm more than willing to listen to and give it a whirl.

Jack Rafter

D2,

What you describe gets you the radius of an arch, which in realty is a special case of an ellipse where both foci occupy the same place.

If you knew for sure that the curve you where working with was an ellipse you could find its foci or for that matter reconstruct it like this: http://josephfusco.org/Flash/Ellipse_layout.html

This uses the major axis length and the semi minor axis length to determine where the foci are. Using 1/2 or the semi Major axis length as a radius, place the point of that radius at the top of the semi minor axis. Swing the arc until if cross the major axis, where it does is the location of the foci.

Once you know where the foci are and the top of the minor axis is, you can then by a couple of different methods form the ellipse.

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denver 2

Thanks Jack R. for explaining how to find the foci points which by the way I did not know. I guess I spent to much time in math class looking at pretty girls in short skirts back in the 70's. Now the string method makes sense, and I look forward to put in practice.