DAVID DEXTER ASSOCIATES
S H STRUCTURES LTD
SIR ROBERT MCALPINE LTD
THE BRISTOL ALLIANCE
Cabot Circus Footbridge fulfils a utilitarian brief to provide the seamless linking of ‘satellite’ parking with new retail development. The bridge was planned as a continuation of a long sweeping pedestrian boulevard that cuts longitudinally through the new 2,500 space car park. Its long curving form is clearly related to the curving plan of the car park and can be read as having been ‘extruded’ out of the void created within the building.
Externally the bridge is viewed almost exclusively from Bond Street South below, and generally from the confines of vehicles passing beneath. The so-called ‘third’ elevation of a bridge – its underside – takes on particular significance in this context, and the design of the wide bridge soffit is critical in defining the visual signature of the structure. The careful use of concealed deck splices along the deck also provides the required continuity on plan.
The deck structure comprises a closed steel torsion box which is essentially triangular in section and provides a smooth soffit plane. The section varies along the bridge length such that the soffit seam meanders from side to side along the structure providing a fluid threedimensional form. Deck splices were bolted with TCB bolts which were accessed through hatches in the deck.
The deck is supported at mid span and at each end on tapered cantilevering raking steel columns, orientated in alternating directions such that the deck section at each support is identical but handed. These support arms from the main car park columns also house an arrangement of pot bearing, to accommodate differential movements on plan, while providing local vertical support to the bridge.
Internally, the changing deck geometry exerts an influence on the enclosure, defining the internal space of the bridge. The side plates of the deck are variable in inclination, and this variance is translated through to the steel portals, which incrementally ‘rotate’ relative to each other along the length of the bridge. These portal frames support wall and roof panels, which form warped planes of glass to provide a dynamic spatial experience for the bridge user.
Since the external and internal geometries of the bridge are visually complex the detailing of all elements is minimal to ensure a legible and ‘clean’ aesthetic. Services are subtly integrated, with lighting strips set flush into the portals internally and placed at varying heights to further accentuate the warping geometry.
The steel frames are portalised laterally to provide the principal axis of stability while, out of plane, the frames work compositely to antilever from the main deck. This stability system, with the aid of variable stiffness sealants, can accommodate the vertical and lateral deflections of the bridge, while maintaining stability of the glass panes required in an overhead condition.
The limited access and restricted number of road closures available during the construction phase meant that employing a structural steel solution enabled the bridge to be fabricated in large sections, which were trial assembled for fit and alignment before being delivered to site for installation.
The bridge provides a striking pedestrian approach to this major centre. The sweeping form of the box deck, sculpturally curved and twisted, supports the glass enclosure. The careful detailing, masking of the deck splices, the integration of the services and the high quality fabrication, all ensure an impressively clean appearance.
High standard of work has ensured an equally high quality result.