Saturday, March 26, 2022

The Internal Batter

The lean of the outer wall of our design would have the typical batter of a dry stone wall. However, for the batter of the inner wall of the hexagonal temple, we decided to create a gradual concave curve that would come to vertical above the point it started from at 8 feet, with a maximum offset/deflection  of 6 inches, something subtle so visitors wouldnt necessarily notice straight away, so as to  not be immediately obvious.

Sean thought the best approach was to make a ‘profile’ or ‘template’ which could be used at any height to check the batter curves evenly.

This profile would in effect be part an arc, that is part of the circumference of a larger circle. 


Sean writes, “ Given it was an arc, the height was in effect a chord (8 feet) and given the known offset (6 inches) I was able to calculate the radius of the imaginary circle our arc would be taken from.  Then taking the edge of a sheet of plywood as a tangent to the centre of our arc,  I was able to calculate the distance of the arc from the tangent, via trigonometry and very possibly witchcraft… I can’t actually remember exactly what I did, and I have lost my original diagram, with calculations on it, I did however at the time transfer the computed measurements to a notebook, in case the notes got lost/damaged and we needed a new profile…  

To make my life easier I had converted everything into centimetres and gone for offset of 15cm and length/height of 240 cm (whilst imperial measurements of 64ths of an inch are more accurate than millimetres but using a calculator and having decimals makes metric more user friendly  - you don’t need to recalculate 9.82(cm) into inches and sixty fourths). I marked the offsets on the ply, every 2cm I think , where the change was noticeable, every 4cm where the change was minimal (ie towards top the bottom) and literally joined the dots. I handed the plywood over to JSR to cut with a skill saw along the marked line, as I didn’t trust myself. A straight batten was then attached to the cut plywood, parallel to the straight edge, to strengthen it and give a flat edge against which a level could be placed.  


We had marked the centre of the floor to give scribe internal circle footprint and carefully ran the first course of interior stones, using a wire from the centre point. Then placing the profile against the base of the wall and ensuring it was vertical and the surface plane pointed towards the centre of the circle , and thus  theoretically give the correct position for a stone at any point in (and up) the wall.  

This was of course not entirely true as straight stones don’t sit exactly on a curve, as , in an inside curve, only the centre of the stone is on the correct line, so care had to be taken to remember this for the first few courses before a more uniform curve was in effect created against which the profile could be ‘rested’. Even then stones occasionally got in the way, the surface was full of imperfections.


It had been clear for some time that our build would go beyond the maximum height of the profile we were planning to use,  and an extension would be needed.  I can’t remember if JSR or I suggested exaggerating this last bit of curve before building up to the upper ‘rim’ , in order to emphasize the subtlety of the original curve, and to really start bringing the upper ridge more towards the centre, to create an overhang  above people’s heads.  It was one of those many things that we had apparently both been mulling over, so that when one of us suggested it the other said ‘yes I was thinking that…’  Anyway I came up with a curve I thought transitioned well, and wouldn’t be entirely lost with the thickness of stone we were working with.