# Arch question



## dom-mas (Nov 26, 2011)

I've asked many folks and looked in every book on masonry I've had and can't find an answer. I figure with all the smart folks that this site seems to have, someone can give me an answer

Is there a minimum height that a masonry unit must have for every foot of span of an arch. Example, I would use a brick in a rowlock position for a short span, less than 3' say. I would use nothing less than a brick in a soldier position for a span of 3-4' and rowlock on top of soldier and vice versa for 4-6'. And for stone I rarely use anything less than a foot. 

But it's all something I do just because it doesn't look right. Any rule that anyone knows of?

Thanks for any help guys.


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## fjn (Aug 17, 2011)

*arches*

Check out the B.I.A. web site ( gobrick) they have a tech. note about arches that may be of help.


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## dom-mas (Nov 26, 2011)

Hmm, tried gobrick. Put arch into the search and nothing came up. Went to technical noted and there were so many pages. I sort of asked here because my searching hadn't really worked. Also not only interested in brick. I'm sure that all masonry units are about the same but not sure.


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## Rockmonster (Nov 15, 2007)

It sounds like you have an arch that doesn't look right? Or on the plans it looks........too flat? There are of course flat, or jack arches. And of course segmental arches, which can be made from radii of unlimited all the way back to a semi-circle, or Roman arch. Of course the buttressing of the arch needs to be adequate to support the pressures, IF an arch were to fail, that is going to be your weak link, so to speak, but I have never actually seen one fail. Of course, you can always put relief above the arch as well, you should be able to find info on that fairly easily.....


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## dom-mas (Nov 26, 2011)

Alrght, tried again. Gave a figure of 1" for every foot and a minimum of 4" for segmental and roman arches. For jack arches (not allowed without steel in my area unless a rebuild) it's 4" + 1" for every foot with a minimum of 8". 

Nothing on gothic, moorish, tudor, elliptical etc...

Good info tho, thanks FJN


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## Rockmonster (Nov 15, 2007)

Now I've just re-read this, and is your question about the minimum height of the masonry_ unit_? What is your concern, is it the size of the joint at the extrados? So you want to use smaller units? Clarify please........


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## stuart45 (Oct 7, 2009)

Here's a bit of info.


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## dom-mas (Nov 26, 2011)

No I guess it isn't about the unit at all, it's about the depth of the arch. And I'm asking the question just for my knowledge. When I do arches I'm generally given free reign. and I don't believe I've ever built an arch that wasn't suitable it's just that I've always wondered if there was a minimum but have never come across the answer through asking other masons or looking in books.

Never seen a failed arch?? I've seen dozens. Mostly jack arches, but other types as well that have either a voussoir or keystone that has dropped due to deteriorated joints.


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## stuart45 (Oct 7, 2009)

It's 13 inches up to about 25 ft.


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## dom-mas (Nov 26, 2011)

Thanks stuart, not doing any bridge work in the near future tho.


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## artisanstone (Nov 27, 2007)

I have this info in a book somewhere. The answer has more to do with the relation of thickness to radius, rather than to span. Off the top of my head, I remember that an eggshell has a ratio of 20:1 or so. Modern dams are the closest man-made object to that, and historical arches have a ratio of 5:1-10:1. I wouldn't guarantee those figures, but generally for a <6' span arch it is irrelevant anyway.


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## dom-mas (Nov 26, 2011)

Artisanstone, your talking of the need rise of an arch? I was taught that a jack arch, which has the least rise (but does have some) you need a rise of .5" for ever foot of span. The other arches (except segmental) design dictates the rise per span.


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## Tscarborough (Feb 25, 2006)

I am not sure I understand your question, but if it has to do with the size of a masonry unit in an arch, it makes no difference at all for normal or reasonable size units and arches.

If you are asking about the design of arches, the the tech notes at BIA will answer your questions, provided you can do some simple math. There is a relation to width and depth of arch (from the springline up) plus the amount of masonry being supported, but it has to be calculated for the different types of arches as they carry the load differently.

As a rule of thumb, If there is masonry greater than the width of the opening above the arch, it only has to be designed to withstand 50% of the weight of masonry within that area of the wall.


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## dom-mas (Nov 26, 2011)

right, masonry unit wasn't the right word. Depth of arch was/is. And yes the info from the BIA was perfect, except that it only mentioned roman, segmental and jack arches in that respect.


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## stuart45 (Oct 7, 2009)

The jack arch here has a rise of 1/8 inch for every foot of span. This is only to make it look flat though and is not for structural reasons. 
A jack arch is really a segmental arch with the tops and bottoms of the bricks extended to make it flat.


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## Tscarborough (Feb 25, 2006)

What type of arch are you interested in then. Pretty much every other arch is either a derivative of those, or of a type that is not a true arch like the Mayan (corbel).


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## dom-mas (Nov 26, 2011)

stuart45 said:


> The jack arch here has a rise of 1/8 inch for every foot of span. This is only to make it look flat though and is not for structural reasons.
> A jack arch is really a segmental arch with the tops and bottoms of the bricks extended to make it flat.


Sorry, I was just coming back to correct myself. .5" per foot is nuts and that's what I was told also, just so it looks flat, like entasis on a column so it looks straight.


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## dom-mas (Nov 26, 2011)

Tscarborough said:


> What type of arch are you interested in then. Pretty much every other arch is either a derivative of those, or of a type that is not a true arch like the Mayan (corbel).


elliptical and tudor. I guess they are derivative of roman but since there are different centres, which size circle do you use?

I guess it doesn't matter, use the same ratio as roman/segmental and it should be fine.


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## Tscarborough (Feb 25, 2006)

Another rule of thumb: You can't over engineer an arch, only under engineer it.


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## artisanstone (Nov 27, 2007)

My point was: the height of the arch band, or shell thickness if you will, is more properly related to the radius of the arch rather than the span.

To get more specific, the line of forces does not follow a true radius ever. The point of having a certain shell thickness is to keep the line of force contained within the arch band. The normal rule is to keep the line of force in the middle third of the arch band. 

The line of force as it related to an arch follows a shape called a catenary. One example of this would be the arch in St. Louis. Another huge one would be the work of Antonio Gaudi.


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## stuart45 (Oct 7, 2009)

There's a few on Bath Abbey, just up the road from me. I'll take a close look at them for cracks next time. I don't think the Luftwaffe got any direct hits on them.:laughing:


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## dom-mas (Nov 26, 2011)

5000psi mortar. Yikes. 5000psi masonry units, where??

I used rowlock because thatis the position of that brick. the voussoirs alternate with a soldier on bottom and a rowlock on top the vice versa. They aren't plumb but they're much closer to a soldier and a rowlock than a stretcher or a header.


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