Watermark Westquay Footbridge, Southampton.

The new pedestrian footbridge connects the new Watermark retail centre to the existing car park. The bridge spans 40m at 8m above the busy Harbour Parade Road. 

We worked closely with ACME and SH Structures to develop a footbridge with a clearly efficient approach. A box truss structure with a pattern in the side bracing was the final agreed solution; it provided an efficient structural approach to both giving an enclosure and achieving the spans and also the opportunity to form a strong visual identity and some architectural expression. 

Steel was chosen as the preferred material for the bridge superstructure as it provided the opportunity to have a visually and physically lightweight structure that could be quickly erected from prefabricated component parts, minimizing the need for temporary works and road closure. 

The bridge was developed in 3D CAD which provided both a useful check on proportions of the various elements used and an ideal way to communicate the design. The challenge with the detailing was to achieve a uniform appearance throughout the length of the bridge. Thicker RHS bracing elements are used at the support locations with subtle local stiffening. The dynamics of the lightweight bridge and its potential for pedestrian induced vibration were assessed by Expedition Engineering. The straight span between supports and relative stiffness of the box truss resulted in a highly efficient form that performed well in a dynamic context.