Back in about 2000 there was a programme on TV called Secrets of Lost Empires: China bridge. A group of American architects and some Chinese engineers got together to recreate an ancient chinese design of bridge known only from a 12th century chinese scroll painting. They worked out how it probably went together and then built it over a canal in China.

Inspired by this I set about working out how one would go about making a small scale ‘rainbow bridge’. 

A bit of O-level GCSE maths gives a straightforward trigonometrical formula into which you can plug the span of the bridge, the desired curviness (arc), the number of segments and the diameter of the cross poles and you can then work out the exact size of all the other bits.  We had some spare wood lying around so decided to have a go over a 4m wide inlet to the pond.

Unfortunately, the rainbow bridge design, though very simple and elegant in itself depends upon fairly solid footings at either side to make it rigid.  So we first had to make some concrete pads (abutments) for it to push against. These we slightly over-engineered... they each worked out at about 2 tonnes!

I cut the wood to size and assembled the bridge. We then carried it out to the abutments and bolted it down.  Almost a perfect fit!

We haven’t quite got round to making the deck and handrails yet.