Rebuild stone fireplace arch

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3 Mar 2018
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Hello All,

So I'm rebuilding my fireplace ready to install a woodburner. The deeds of my my house say it's 1701, but from local maps I think it's more like 1850. I've broken out the old fireplace. Because it's been covered in cement inside and out for, I'm guessing, at least 20 years, the front of the fireplace has become crumbly and the front collapsed. The arch that was in place was brick with mortar matching the rest of the front but had very little rise.

https://drive.google.com/open?id=1WtNj21PNLiQnKIOM4VyQhMVGKJfzvnuZ

I've cleaned (2% hydrochloric acid painted on and brushed off with a churn brush), chiselled out the mortar, replaced the wood 'bricks' with the same stone and repointed most of it. It looks like this at the moment
https://drive.google.com/open?id=1gqeu5xJoXSwws352uvKQf8cHV_2kXLV2

According to this

http://www.masonryconstruction.com/how-to/construction/designing-the-segmental-arch_o

the rise of the arch should be at least 1 in 12 (rise:span ratio of 0.083), but to be a 'minor arch' able to support floors and walls (the pillars support a joist and I may want to install a regulation grate in the fireplace above) should be no more than 1 in 6.5 (rise:span of 0.15).

So, this this arch should be fine which is a rise of 127mm over 1050mm span or 1 in 8.3 (rise:span ratio of 0.12). I guessed this, taking the floor as a striking point for the curve (which I figured will make checking the angles when I'm building it a whole lot easier). I've drawn the model for thrust forces and this seems fine i.e. the force line stays in the middle third of the voussoirs are under compression and won't pop up. I've assumed they're all the same size for this.

The only problem is that the skewback on the abutments are about 6deg as they stand, meaning this requires the skewback angle to be 27deg.

Any ideas on how I'd go about chipping the excess of the skewback stones where the springers will lay without fracturing them? It's red sandstone.

Also, does it matter if the voussoirs are all different sizes, i.e. the extrados will be jagged? They're paired, with each pair more or less the same width (within about 10mm), but some are much taller and different shapes.

Finally, how thick mortar should the mortar be between the voussoirs? (I've got NHL 3.5 hydraulic lime for this).

Thanks all!
 
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