How to draw/figure the door arch?

[KEmerson]

Hello,
With an opening 19" wide x 12"high, how do I figure the arch? I want to build the form for setting the arch brick but it isn't a clean hemisphere, it's more elliptical. So I guess that's my question: How to figure out the arc of an ellipse. A quick Internet search turns up that I will need at least a rudimentary MIT education but I have to believe there's another approach. How about eyeballing it?
Thanks
Kim

[ThisOldGarageNJ]

Re: How to draw/figure the door arch?

Hi Kim,
I made my own compass rom an old piece of wood, Made a spot for a carpenter pencil on one end and a sharpened screw on the other, Drew up the bottom and 2 sides of the arch on a piece of plywood, Using my homemade compass to form a nice looking arch across the top, Moving the compass up or down on the center line changes the height of your arc,, Sounds a little confusing, But I may not be explaining it as well as i should be, Hope the pictures help

[fxpose]

Re: How to draw/figure the door arch?

I think the degree or style of curvature of the arch should simply be pleasing to your eyes.

[RTflorida]

Re: How to draw/figure the door arch?

I think most of the forum would tend to agree with fxpose, appearance and fuctionality are the key.
Is it just me, or is the recent new wave of builders a little too focused on the math and creating the "perfect" (based on their intense calculations) geometric shapes?
Just my observation.
RT

[KEmerson]

Re: How to draw/figure the door arch?

I just solved it myself mostly by eyeballing it. Looks good enough. thanks

[christo]

Re: How to draw/figure the door arch?

May not work for all, but I drew the rough shape of the arch I'd like on cardboard. Stuck a nail through the cardboard at each intersection of wall and arch, tied a loose string between the two, set it upside down so the string would hang in rough approximation to my sketch and corrected my sketch to match the cantenary curve of the string.

Even then, I think I may have deviated a bit from that curve to make something a bit more pleasing...

[jmhepworth]

Re: How to draw/figure the door arch?

I eyeballed mine on a piece of cardboard and then folded it in half to make it symmetrical.

Joe

[ThisOldGarageNJ]

Re: How to draw/figure the door arch?

hey JM

simplicty..... I think you have the best and easiest idea

Mark

[RTflorida]

Re: How to draw/figure the door arch?

jm, thats simplicity at its best......does anyone else remember in kindergarten when they taught us to fold our paper in half before cutting to make a symmetrical shape?

RT

[Spunkoid]

Re: How to draw/figure the door arch?

I am making the arch the same curvature as the oven dome and floor and am using the floor template as a guide for the arch. I cut a piece of plywood the width of my door then drew the arch on them tracing the arch from the floor template.
I have not built my arch yet but it should work.....

[Lars]

Re: How to draw/figure the door arch?

Hi Kim,
I just draw the arch using the bottom of the doorway as the center of a circle. Sometimes there's an issue with the sides ( will the arch, if too low, push the sides out) and then you might think about moving the center of the circle up to make the arch higher ( more downward force, less 'lateral' sideways force). Draw your bricks to scale, then just support a flexible piece of wood in there ( 1/8" masonite, or 1/4") and support it well with stacks of anything ( concrete block, bricks, wood frame)

There are MANY ways to implement an arch... and many pictures on this forum.

Good luck.

lars.

[kebwi]

Re: How to draw/figure the door arch?

Bear in mind as you read my response that I am, at the current time, a pure theoretician on ovens (I haven't started building my own yet). That said...

An arch/dome can be designed with various goals in mind. In short, there are multiple conflicting goals and restraints which must be satisfied on a per-design basis. Here's one possible breakdown:

GOAL: perfect structural stability
DESIGN: A tall catenary such that the forces at the floor are oriented almost vertically
PROBLEMS: not nearly the best shape for cooking (not bad for a kiln or an architectural dome however)

GOAL: cooking pizza
DESIGN: From what I've learned on FB, a low flattish dome is ideal, i.e., a relatively eccentric ellipse on soldiers, or even better, a low catenary on soliders (theoretically more stable than an ellipse)
PROBLEMS: Lateral forces on the soliders may require buttressing (apparently, the Pompeii has been designed to alleviate this problem, nicely done), low domes have tighter tolerances on brick wedge-shape to achieve a proper load-bearing arch without bricks falling through (mortar "glue" support not with-standing)
NOTE: In 2D instead of 3D an incomplete arch on vertical soldiers is called a "segmental arch", especially when a catenary is used. This is by far the most common oven-door and oven-entry shape on FB. The same buttressing problems apply of course, especially with a heavy flue/chimney.

GOAL: cooking nonpizza
DESIGN: a higher curvier dome, but still segmental on soliders because a full catenary is way too high for cooking. Segmental arches/domes conforming to a catenary, atop soldiers, will still be the strongest shape, my understanding is that ellipses are always acceptable if the catenary representing the force distribution remains within the middle third of the bricks (this rule of thumb is true of any arch: keep the true catenary within the middle third of the actual arch bricks).
PROBLEMS: less ideal for pizza (theoretically, although I question my own ability to discern the difference), same old buttressing problems on soldiers.

GOAL: aesthetics
DESIGN: whatever the heck you please
PROBLEMS: structural instability, cooking inefficiency

An important final note is that all discussion of structural stability is with regard to mortarless arches in which all force distribution and load support derives solely from brick-to-brick interactions. Ideally, no mortar is required. In reality, it is obvious that mortar can be entrusted with a significant fraction of the dome's load since many FB ovens are built to fairly inexact standards and there are surprisingly few reports of major structural failures. Note that an oven built from truely rectangular bricks, thus with no wedging whatsoever, must rely very heavily on the glue of the mortar to hold the dome together. The bricks still provide vertical support from the apex curving down the sides (transfered from brick to brick through thick mortar wedges of course), but more seriously, mortar (and friction in general) is the *only* thing preventing rectangular bricks from sliding along their radial axis toward the interior of the oven and falling inside. Since many ovens are built with rectangular bricks (and the Pompeii was designed that way), it seems obvious to me that mortar can be trusted to make up for a significant degree of imprecision in brick shape.

Follow-up, clarification, and corrections are welcome!

Cheers!

[stidik]

Re: How to draw/figure the door arch?

I bought a door from FB, then built my opening arch around the door. The opening is larger than the door, but the reveal is slightly smaller.

[Lars]

Re: How to draw/figure the door arch?

Stephen,
I think I spent the night in a tent near San Luis Obispo on my bicycle trip so many years ago down the coast of California ( SF to LA)

Kebwi: You need to start building. So many things become clear in the reality of bricks and mortar, and many other ideas and concepts get squished to infinitesimal size. There is much to learn from the process and it is, as you can tell, a lot of fun. I know in my case, things got much clearer as I encountered the process in reality. It starts with a slab of concrete. After 18 bags of 80lb concrete, things get a lot simpler. ( I should have said..."It is my lowly opinion that it would help you to begin construction"...sorry about that)

[kebwi]

Re: How to draw/figure the door arch?

@Lars: I'm going as fast as I can. :-) I'm on vacation this week so I'm not at home; I can't *do* anything except continue to research. My research is virtually complete however. I have a very clear picture of my goals and designs (which I will post once I get home). I hope to start digging up the yard this Sunday, fingers crossed.