‘In an overlooked part of east London, weed-strewn and still industrial – a district un-steamrollered by the giant apartment blocks going up in much of the old docklands, close to where the River Lea joins the Thames – two curve-cornered squares of steel frame the view. They look like heavy metal sculptures. They are actually essential elements of the recently completed Cody Dock Rolling Bridge designed by the architect Thomas Randall-Page and Tim Lucas of the engineers Price & Myers, which moves in a way that no bridge has done before.’ – Rowan Moore, architecture critic in The Observer, February 2023.
Since London won the rights to host the 2012 Olympic Games, the delta of the once-neglected Lea River in East London has undergone seismic change. Cody Dock is something of a remnant from that process – a large ex-industrial basin on the banks of the river that is now home to a dynamic artistic community who are transforming it into an epicentre of creativity and production.
Cody Dock Bridge is a hand-cranked, rolling steel bridge that spans the dock mouth, alternatively providing a crossing for pedestrians and a passage for boats. The bridge is carefully counter-weighted, so the centre of gravity is level, allowing the 13-ton deck and frame to be rolled along a threaded steel track using only a hand-cranked winch.
Like its Victorian forbears, the bridge design is tied to its functionality and the environment in which it sits. Most of the structure is Corten steel, which requires minimal maintenance. Oak bearing-strips fixed to the hoops roll on the undulating Corten track, whilst precision cut Corten teeth interlock with Hardox steel pins. The rolling and guiding interfaces are kept separate, and the materials chosen such that the softer component can be easily replaced within each interface, facilitating maintenance over its lifetime.
The footbridge is a simply supported structure with a monocoque steel deck, spanning 7metres over the dock mouth. Two 5.5m rounded square portals at each end allow it to roll along undulating concrete abutments which are cast into the existing masonry walls. The upper section of each portal is counterweighted such that the centre of gravity is raised to the midpoint of the frame.
The path geometry ensures this point remains horizontal when in motion so that the bridge weight is never lifted vertically. The bridge is driven with a cable, by a pair of manually operated winches on one bank, creating a safe, haptic mode of operation. The handrails are constructed from a welded lattice of steel reinforcement bars and can fold down, via a torsional spring mechanism, for additional clearance height when the bridge is inverted.
The geometry of the bridge track is loosely based on the ‘square-wheeled bicycle’ problem. Mathematicians Robison (1960), Wagon (1990), derived the path geometry that results from rolling various shapes along a horizontal trajectory. While the path geometry of a square has been demonstrated to be a set of inverted catenaries, a new solution had to be generated to derive the path of the bridge portals. This involved numerically integrating elliptic integrals to calculate the path shape around the rounded corners (as no analytical solution exists) and combining this result in 3D software to that of a square wheel. This gives a set of transformations that guide the movement from start to finish and generated the geometry of the teeth and track.
The bridge aims to be understated when resting but playful in its movement, creating a spectacle when operated. Part of the ambitious footpath and cycleway project along the length of the Lea River, the rolling bridge has become an important landmark and a symbol for the dynamic community growing here.
‘The simplicity of its idea requires fiendish mathematics and precise construction.’ – Rowan Moore, architecture critic in The Observer. February 2023.