Design and Engineering
The Seán O’Casey footbridge was commissioned by the Dublin Docklands Development Authority in 2002 and was built to improve pedestrian linkages in the rapidly expanding docklands area of the city. Following an international competition the successful design team were O’Connor Sutton Cronin, Consulting Engineers and Cyril O’Neill of Brian O’Halloran, Architects. The main contractor was John Mowlem Construction.
It is a three span swing bridge, spanning 97m and is 4.5m in width. The structural design of the bridge evolved on the basic principle of a balanced cantilever, where each bridge leaf or arm, 44 metres long, rotates about a bearing mechanism at the centre point of each span and closes onto a cantilever abutment to complete the transition with the river quay walls. The balanced cantilever approach, combining many structural materials, provides both an inherently efficient and transparent structural arrangement. The primary supporting cradle elements comprise four tapering steel fabricated box sections which merge at their base to form a circular drum containing a slew ring (or rotating) mechanism.
The 4.5 metre wide aluminium bridge deck is supported on two 600mm diameter steel tubes which are subsequently supported at their extreme ends via 100mm diameter Macalloy tension rods. Separate rods extend continuously on each side, articulated at the cradle tips via special continuity connectors, which are in turn tensioned down to the central cradle base. The abutments are also clad in granite. It weighs over 320 tonnes and sits on two Chinese granite piers. When closed, each bridge leaf is both locked together and to the cantilever abutments via hydraulic locking pins. The out of balance structural load conditions are resisted by these locking pins, suitably arranged to restrain vertical, lateral and torsional deck forces whilst allowing and accommodating bridge rotation at its ends.
The cradle base and slew ring bearing is supported via cast-in holding down bolts on stone clad elliptical concrete piers located approximately 27m from each quay wall. The pier configurations were geometrically designed to minimise their impact on hydraulic river flows. The concrete piers, supported on four large diameter bored piles were cast within a prefabricated steel caisson temporarily dropped into the river during construction.
The operation of the bridge is controlled by a remote radio transmitted hand held pendant key. The bridge is opened by releasing the locking pins in a predetermined sequence via a radio controlled signal to the ‘hydraulic rams’ at each slew ring bearing. The electrical distribution panels, hydraulic motors and pumps are suspended in the zone of main structure beneath the bridge deck. Readily accessible electrical switch cabinets, downloading sockets and navigation lights are integrated within the balustrade panels at the tips of each bridge leaf.
Dublin Port Company requested the bridge to be protected against collision from a pleasure boat or errant ship in the navigable channel. A bespoke system evolved during the design process incorporating a floating series of horizontal tubes with buoyancy boxes, supported and guided on vertical piles driven to refusal in the boulder clays. This system conceals the V-fenders at water level, rising and falling with the tide. The bridge was designed and delivered within a 30 month programme commencing with appointment of the design team in December 2002.