Wednesday, March 27, 2013

Building on top of the form

The two keystones are in place and the lowest pair of voussoirs are fitted.

The other large sandstone voussoirs we shaped are then lifted on by hand, one by one, and fitted carefully along the outer sides of the form so there were no gaps.

As we built up the outsides, the centre of the arch was infilled with micca quartzite builder stones from Sidney Peak Quarry. They were laid face down in an array of radiating courses spaced properly near the upper edges with the ever important, yet tiny, shim stones.

Watching out for fingers!

Removing the stones from the temporary form was not as difficult as you might imagine.
The concrete outer shell broke out nicely too.

Finished Arch - Strong bones that stand ready for 'fleshing out' into a bridge when the time comes.

Thanks to Sean Smyth, Sean Adcock, Alan Ash and Patrick McAfee for all their help on this project.


  1. John, do you think the fact that the keystone runs the full dimension of the barrel ADDS strength by acting as a through (at the most important location) or diminishes overall strength by forcing the other voussoirs to adjust to it (as opposed to allowing the individual keys contribute to the adjustments? I suppose if it super tight (as it should be) before the form was removed it doesn't matter, but I've always assumed some tiny adjustment as the load makes the subtle transition from form to barrel. I apologize for the lack of beer for this discussion.

  2. It is hard to see in these photos but that wasn't just a 'single' long keystone in the new bridge, but a pair of keystones. These two keystones were made from the original full-length keystone that was actually longer than the width of the bridge.
    In answer to your question. I think voussoirs including keystones shouldn't be too long. If you imagine a bridge as a wall that has been laid on its side and then raised in the middle to form an arch, you will realize that, as in a dry stone wall, it is much better to have the stones going into the wall, not along the wall. (which is called tracing) Stones are more likely to fall out if their length runs 'along' rather than 'into' the interior of any dry stone structure.

    Yes it would be good to have a beer and discuss this more.

  3. Ah, I see it now and agree! Looking forward to that soda.js