Gavin continues his story of building the Venus Baths Bridge in Australia.
The park managers' stipulation was that the bridge be large enough to cope with the stream's (at times) heavy flow while being low enough that it wouldn't need a hand rail due to fall height considerations. I thought a good solution would be to employ an elliptical arch as this would allow adequate water flow whilst being lower than a semi-circle or segmental arch of equivalent opening dimensions.
I am indepted to Pat McAfee for providing advise and cautioning me as to the particular tolerances associated with elliptical arches due to their low span/height ratios. Mortar is typically used in these arches to harden them and ensure good bed contact between the voussoirs. To help address these concerns in my drystone structure, I took particular care in shaping the voussoirs to achieve a snug fit as well as making the barrel thick to avoid the risk of having the line of thrust go outside its surface boundary and a 'hinge' developing - the extrados(outer curve of the arch) actually formed part of the tread, or walking surface, and is something I had come across on pack-horse bridges in the Lake District in the UK. Laying stone pitching for the remaining sections of tread would further increase the bridge's overall strength and thus tolerance to vibration and other forces.
Here is a very easy method for drawing an elliptical arch.
http://www.thisiscarpentry.com/wp-content/uploads/2012/02/Ellipsestring-method.pdf
Grant Rose assembles the four elliptical ribs of the ply wood former.
Gavin assembles the bottom cross pieces .
