18 August 2019

Crowdfunding for Cody Dock Rolling Bridge

I thought I'd give a quick mention to this unusual bridge proposal, in case anyone has missed it.

Designed by Thomas Randall-Page, with support from engineer Tim Lucas at Price and Myers, it's a highly unconventional moveable pedestrian bridge, proposed to span Cody Dock at Canning Town in London.

There are three standard types of movable bridge: bascule bridges, which rotate about an axis parallel to the centreline of the river (or other obstacle crossed); swing bridges, which rotate about a vertical axis; and lift bridges, which move vertically without rotating.

There are also examples of retractable bridges which move horizontally, and a handful of bridges which move by transforming their shape, but very few examples of bridges that rotate about a horizontal axis perpendicular to the river centreline. The best known example of the latter is Gateshead Millennium Bridge, which like the Cody Dock design rotates to lift its deck sufficiently clear of the water to permit navigation. There are also a couple of examples in Belgium.

The Gateshead bridge pivots about its base, and so its weight is unbalanced in almost any position, requiring substantial machinery to operate.

The Cody Dock rolling bridge is instead proposed to be operated using a simple hand-crank, requiring its weight to be well-balanced in every position it rotates into. This is achieved by placing its centre of gravity exactly halfway between the floor of the bridge, and its overhead portals. When operated, the bridge rolls sideways like a giant pinion on a specially arranged rack; cog-teeth control the bridge's position.

This could be achieved by enclosing each end of the bridge in a large circular ring, which would roll along a horizontal rack somewhere below the floor level of the bridge. The half of the circular ring which is normally above the bridge deck would need to contain sufficient ballast to exactly balance the weight of the deck.

The Cody Dock proposal uses square portals at the end of the bridge, but rotating about their centre in the same manner as the circular ring. The centre-of-gravity remains horizontal, which means that the support rack has to be shaped to match the path which the rotating square traces in space. I think there will be quite a bit of ballast to pack into the upper part of the portal frames to make this work!

The cost of the bridge is estimated at £197,848, with a detailed budget cited which, perhaps worryingly, includes no contingency or risk allowance. There is currently no public funding for the project, nor any corporate sponsorship, so the project team are resorting to a crowdfunding web page to try and fund the scheme. This is currently sitting at around £62,000 pledged, with only 10 days left until their self-imposed deadline.

It's a very imaginative idea, and I wish them well.

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