Saturday, February 19, 2011

Lending A Hand

We had a challenging time figuring out how to build the dry stone greenhouse design I came up with a few years ago for a good client out west. The design called for two, four foot thick, tapered gabled walls, spaced proportionately to create the feeling of an old stone potting shed. A beam would be added later for sheets of clear glazing to be fixed to as a permanent covering and so still provide light inside. I really liked the angle design and was excited about being able to do it this year.

Several wallers, when I showed them my sketchup design (see above) at the Symposium in Ventura, said it wouldn't work and that it would spread apart or cave inwards. I invited walling expert Sean Adcock to join us on this project and together we took a lot of time thinking out how we were going to do it. At least a day and a half was spent measuring and setting up our guide posts and lines. We knew the quoins and voussoirs needed to be beefy, and that there could be no variance of the batter, or the 45 degree angle of the stone coursing, and that all the rows had to be 90 degrees to the pitch of the wall, if it was to look as good as I envisioned it.

Sean had to leave before the project was completed but I wanted to include here some of his thoughts he wrote in an email concerning the various tensions and pressures exerted on this unusual structure from the pics I sent him of the continuing building progress. Those of you who have met Sean will know he has a complex mind and analyzes things quite thoroughly in his own inimitable way.


The master with his tools- levels, squares, lines and batter frames.

"First, further thoughts re pressure on jambs. Several of my copes just sat there until I built UP to them. held in place by friction. Friction means that not all the weight/mass of stone on the incline is necessarily acting on the diagonal. Basically if its not sliding, there wont be a diagonal force. It's in equilibrium, even if it slides a little its still not 100% down the diagonal. However it still has mass which is going somewhere. Gravity suggests this is down. Basically there must be a vertical component its just a question of if/when this is transferred into a shear force by a FIXED diagonal. Do we have fixed diagonals, maybe.

If the force is vertical rather than sheer it might just mean it adds to the force becoming sheer on the next diagonal and ends up in the jamb lower in a more concentrated form, but it might just as well end up downwards. Even if it is more concentrated this should not be a concern provided the mass of the arch above the new point of action is greater than the mass acting on it from the side. If the point has moved down then there is more weight from above so it should sort of cancel.

Some of this might have implications on the bottom. The forces on the end /base boulders seem very complex. However if we assume 20" around the arch is pretty much a stable fixed mass, (we're banking on it) then looking at pics there is really very little stone in the diagonals to worry about (as it develops I am increasingly confident there is little to worry about in this respect). But as you see it develop I look at the stone and think that it's more likely that a mass could come to act just on those bottom stones.

If you tilt all the stonework in an image manipulator to form a column, then all the coping and a band of stone are acting on the bottom sandstone boulder. That is not what is happening because of the incline. However, I cannot get my head around what is happening. Some of the force from the stone cannot be acting diagonally. It must have a vertical component; how much and how this is transferred is beyond me. My gut feeling at the moment is that the end stones have greater potential to move than the arch (which would actually mean there is less pressure on the arch). I need to think more, but I've run out of whisky....

I think it is a good idea to have the heavy beam linking the apexes of the pyramids. Whatever might or might not happen roof wise. This will knit the structures visually and will also increase the load on the arch theoretically making the arch more stable. Of course I'm not that worried about the strength of the arches anyway... "



I would like to thank Sean for all his help, and of course Peter, Evan Oxland, Akira Inman and Dave Claman, who all lent a hand to complete this first stage of the potting house structure.






1 comment:

  1. Fascinating. My thought is that if you think of each half acting like a buttress then that is very similar to the supports you see in ancient Cathedrals. So that should last at least a few hundred years.

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