A3 hook Underpass Bridge

Hook underpass footbridge is an 18.6m steel structure crossing the A3 trunk road at Hook Underpass, South West London. Stirling Maynard designed a steel superstructure comprising three steel boxes, spanning longitudinally onto the underpass retaining walls. The design was complicated by the lack of available construction depth with tight headroom restraints to the carriageway below and restrictions to gradients on the footpath above, making overall construction depth less than ideal. This lead to potential issues with vibration of the structure when used by pedestrians. A number of options were considered, but eventually a mass damped solution was adopted whereby the central 12 metre portion of the 3 No 500mm wide by 400mm deep box girders were filled with concrete after the structure had been lifted into place. Observations made on site during installation of the bridge demonstrated that the mass damping successfully reduced the liveliness of the deck. Alternative solutions involving heavier steel girders were investigated, but were rejected on the basis of the additional cost of steel and the heavier crane that would have been required for installation.