Tuesday, August 24, 2010

How do we handle a wall with a twist?

Today we embarked on a project involving building 'spiral' dry stone pillars. They will form the posts for a small decorative fence which roses will then grow up on. There will be a middle pillar which is oriented 45 degrees to the line of the fence. The outer two are aligned with their bases parallel to the line of the rails but then turn a full 45 degrees by the time they reach the top.

Building spiral structures is always a challenge. I remember doing the first 'twisted arch' at the Fergus Highland games in Ontario in 2004.

The idea of 'twisting' an arch as it connects two ends of a curved dry stone wall sheepfold was a structural challenge I presented to dry stone wallers in N.A. Scotland and England back in 2003, to see if it really could be done.

Norman Haddow and I tried building a model of it at first with sugar cubes and potatoes cut into small twisted cube shaped voussoirs, to see if it could be done. We did this while I was staying with him in the bothy at Balmoral Castle when I worked with him there in 2003 . It worked briefly but the sugar cubes began dissolving with the juice from the potatoes. We built it again on the top of the television using only potatoes cut in cubes. It was a complete success. Norman announced triumphantly. "Look, it's on TV"

The next year at the Fergus games I and members of the DSWAC managed to do it. The twisting shape of a wall went over a 3 foot arch span and turned a full quarter rotation. Combined with the circular part of the wall it effectively created a dry stone 'mobius sheepfold'. It took a lot of right brain activity and considerable dexterity to build. We had a pretty exciting time attempting it, with many hands and minds working together to make it happen. Sadly the arch had to be taken down after the festival.

There were lots of design issues to figure out today with the twisted pillars so we didn't get much building done. We decided the spirals could not just twist as they went up since this would make the profile of the pillars look top heavy. They would have to taper too. If that was the case how large should the top be and how much should it twist? Would the vertical (angled) corners merely be straight lines leaning in or would they be slightly curved too. How much batter should it have? Would the corner stones all have to be slightly less than 90 degrees? And how would we set up guides in order to follow the shape we are building?

To help us we tried building models again. First with plastercene, then scrabble tiles, and then we tried doing 3D drawings on Sketchup.

Tomorrow I will post some of those drawings and I hope to also to be able to show pictures of how we are progressing.


  1. Do I feel another twisted arch coming on?

  2. Never mind the scrabble tiles John -- to model a "twist" properly you need to use chubby checkers. /Scott