What style of arch?

[Minkeycat]

I'm replacing an old arch that was in an old fireplace. The chimney stack was removed long ago and is blocked over at the top. It was necessary to expose the old fireplace because the way it had been filled in was causing a lot of damp in that end of the house. The arch is 131cm wide with a rise of 13cm in the middle.
Question is, over the fireplace was an old brick arch. I've bought some bricks that match the old ones and have a template over which to create the arch. We took photos of the old arch but they're not actually clear enough to show the "layout" of the bricks that formed it because the bricks were too crumbly and old and covered in dust etc.
I've laid out the bricks over the template and can't help thinking that bricks placed vertically on their ends next to each other to form the arch look a bit too even. Would I be better off making the keystone at least (and perhaps say, every 4th brick) a kind of 'T' shape by putting in a vertical half brick on its end with another half brick at right angles across the top to create the 'T' and which will force the arch shape? Will this make it stronger?
Any helpful suggestions much appreciated.

 

[Kearn]

I'm not quite sure that I am visualising this as you describe?! But in my mind, what I see looks a bit odd losing the uniformity of a brick arch?! :? Maybe I'm seeing something different! Do you have any example pics or a sketch? Or even the old photo.

Is there a reason why you don't think the direct replacement won't be strong enough? From what I think you're describing, my initial instinct is that the load would be spread less easily with the T additions.

Just my initial thoughts... Keen to see more!

 

[88v8]

Our smaller inglenook has a brick-on-edge arch 48" across the springs, with a 4" rise.
The bricks are uncut other than the keystone and I think that was only cut because it's just half a brick wide.

But...... it's sitting on a length of wrought iron 2" wide and ¾" thick that continues at least 15" into the wall each side. I wouldn't trust such a flat arch without support.

The other point of course is that these brick arches are functional They were not usually meant to be on display. Ours would have been rendered over &/or covered by fire surround*. So their appearance was not a matter of great concern,

Ivor

* and will again be covered by a fire surround if I ever get around to making one.

 

[steveb]

Ours looks like this now, just bricks on end, but supported by an iron strap which you can just see underneath. Probably how it was built but not as it was originally intended to look - however it's a vast improvement on what we inherited...

I would guess that the strap could come out and physics would hold it up but I'm much happier with the strap in. The openings upstairs are about half the width and the bricks are used on their sides to create the smaller arch.

It might help you visualise if nothing else.

-Steve

 

[MikeG]

You don't need a strap. You also don't have a brick on the centreline, but one either side of the centreline:

 

[stuart45]

Mike, how would it look as an axed arch with equal 10mm joints, looking bonded with 2 bats every other and a keybrick.

 

[MikeG]

Odd, I'd have thought. Axed arches usually have very very fine joints.

OK, not too bad, I guess, but not the way I'd go. It has the look of a Victorian factory window head rather than a country fireplace.

 

[stuart45]

Thanks Mike, much quicker than I could build it, or even draw it.
A gauged brick arch has really tight joints, normally rubbed brickwork.
An axed arch normally means the bricks are cut to form even roughly 10mm or so joints.
A rough ringed arch is usually when the joints are tapered instead of the bricks.
Always good to have an architect knock up a pic to look at before you decide what to do.

 

[MikeG]

A gauged brick arch has really tight joints, normally rubbed brickwork.
An axed arch normally means the bricks are cut to form even roughly 10mm or so joints.....

Ah, right. I thought (erroneously) that axed arch = rubbed brick arch. Thanks.

 

[Flyfisher]

I understand how an arch provides support but have never quite been sure how to determine how the supported mass translates into a lateral/sideways force.

For example, that photo of the chimney breast arch looks fine but imagine the supported mass trying to 'push' downwards, 'flatten' the arch and force the sides of the chimney breast apart.

An arched bridge is usually braced against the side of a valley or similar and there is clearly a huge mass to push apart before such an arch could collapse. But the sides of a chimney breast are typically just one or two bricks (as in the previous photo), so not much mass to move. What is going to hold the sides in place?

I guess builders of olde (and perhaps even today) know about such things from experience (ie, it fell down last time so we'll build it stronger next time!) but surely there must be some method to calculate such things? Or even a rule of thumb?

 

[MikeG]

You have to remember that the arch is only supporting the triangle of bricks directly above it, not the entire weight of all the brickwork over (see diagram below). It is doing, therefore, a good deal less work than most people think. Also, the weight of the chimney is pushing down on the piers, resisting any outward thrust from the arch. So much so that I reckon this would stand up dry (ie without any mortar) if it weren't subject to weather.

 

[Flyfisher]

Fair points, and I'm sure you're right about the chimney breast in question because I've also seen many similar ones and they've not showed signed of failure.

I just wondered if there was an actual methodology for 'proving' such a design, or are such things too trivial for structural engineers to bother with and 'rules of thumb' are adequate?

 

[MikeG]

Structural engineers will have a calculation for this, certainly, but not at this level. This is so trivial that basically any arch which survives having the temporary support removed post-construction is going to be strong enough. If it was site-constructed non-domestic, and was taking serious or moving loads, or spanning anything more than, say, a double door, then I'd get an engineer to prove it. The thing is, in the circumstances I have just described arches these days are typically built off-site, bonded to a steel lintel/ former, so a calculation isn't required.

 

[Flyfisher]

Interesting about modern builds being done offsite. The ones I've seen are certainly not modern but have a relatively thin iron former underneath the arch bricks. I've always assumed this was to support the bricks during the build rather than being a structural component as such.

I'm probably over-analyzing things (again!) and I'm sure period fireplaces did not have much structural engineering input! :lol:

 

[livias]

I went on a courea once where they said medieval builders had a rule for arch building which was not a good rule. It worked ok for narrow.arches but not for wide ones which had a tendency to fall down. It was only when a new rule.was invented that gothic arches started to appear
Frustratingly i dont know.what the rule was. The course was in italian and communication was difficult

 

[CliffordPope]

There's a story that when Brunel built his famous Maidenhead arched railway bridge the railway owners didn't believe it would stay up without the wooden former on which it was constructed, and he was instructed to leave the wedges in place until after the opening ceremony and passage of the inaugural train. Brunel surrepticiously removed the wedges anyway, and of course it didn't fall down.

 

[stuart45]

I went on a courea once where they said medieval builders had a rule for arch building which was not a good rule. It worked ok for narrow.arches but not for wide ones which had a tendency to fall down. It was only when a new rule.was invented that gothic arches started to appear
Frustratingly i dont know.what the rule was. The course was in italian and communication was difficult

A rule of thumb used by some surveyors is for semi circular arches the abutments should be 3/4 of the span, 1/2 for Gothic and all the span for Segmental.

 

[MikeG]

There's another rule of thumb, which is a bit more sophisticated. It even involves a little drawing:

Divide the arch into thirds. Connect the springing point to the nearest "third" point. Draw a circle around the springing point to the "third" point, and extend the line you've just drawn to meet this circle. This is the width of the buttress required. In the case of this arch, that's about 425mm, so a 2 brick buttress (440 is what you need).

 

[stuart45]

Thanks for that Mike. Is that a method you would use in design, or just a rough guide?

 

[Flyfisher]

Yes, thanks for that 'rule of thumb', though I hope it doesn't worry steveb unduly :wink: