Monday, March 9, 2015

Ribs and Lobsters

The design I use to make the form , often called the 'centering' that goes under a dry stone bridge or arch (until the stones support themselves) looks a lot like a large lobster trap.

The one I am assembling here for the Hubb Creek dry stone footbridge project near Wellington Ontario is a twelve foot span.

The form was made in two sections, from 4 sheets of 3/4 inch plywood. The two halves of the form will be scabbed together with squares of plywood.

The arc of the bridge was to be a segmented Roman arch shape. I have used four sheets of plywood and equally spaced the curved 'ribs' to make a form with a twelve foot span and a width of six feet.  

Having determined that the distance across the bottom of the segment is to be 12 feet ( the width of the stream) and the height of the arc to be 3 feet on a flat surface, I fix an axis and draw a circle with a radius where the line of a twelve foot span segments the circle at three feet along a perpendicular axis. See tomorrow's post to see how this is calculated.

This gives me the height I want for the form and thus the shape of the ribs for the arc of the bridge. I lay each of the four sheets down (one at a time) with one edge along the bottom segment and a corner aligned with my axis and then trace the half rib shape on to my plywood. 

Before cutting the outside ribs, don't forget to draw radiating lines from the fixed axis while the plywood is flat, so you have guidelines for determining the angle that each of the voissoirs should be oriented when the form is made up and supported in place at the actual bridge site. It is better to draw the lines now than try to guess the angles later. 

Trying to re-establish the the centre point of the arc in order to draw radiating lines after the form is made can be pretty tricky. We had to turn this form upright and on its side (and then measure out so many inches to an axis point 6 feet high) to be able to draw the radius lines for our MacDougal Bridge form, because we had forgotten to draw them when the ribs were not attached. 

I cut the rib out with a jig saw or sometimes even a skill saw.
After I have cut one, by rotating the first rib around the plywood sheet I've cut it from, and placing properly I was able to get another full rib out each sheet of plywood. See tomorrow's post to see how this is calculated.

Since plywood only comes in 8 foot lengths, for the 12 foot span bridge, I made both halves six feet long at the base and three feet high. Keeping the form in two halves made it easier to transport in my truck too and also lighter to carry to bridge site. 
The four ribs (of the one half of the form) are spaced at 24 inches and secured with 2x4s and deck screws in five or six places.  A couple of braces are screwed in place diagonally between the ribs to stop the form leaning sideways with all the weight of the stones. Several 2x8 6 foot long boards are screwed to the bottom of the form to give it stability.  The other half is made the same way.

At the site the two halves are scabbed together and the form is then carried into place. To keep it lighter most of 2x4s that will create the surface of the arc will be aded to the top of the form later.

Next: how to temporarily support the form over the stream until the stone arch is built.


  1. I wonder what the union thinks of your scab labour?

  2. Has anyone also noticed that JSR looks like Mick Dodge on the National Geographic Channel.

    He is the Mick Dodge of Dry Stone Walling