Bridges

Sean O’Casey Pedestrian Bridge over River Liffey

The Dublin Docklands Development Authority (DDDA) held an International Competition in late 2002 for a new pedestrian bridge, capable of opening, across the River Liffey in Dublin. The width of the river crossing, approximately 100m, and the competition requirements for maximizing head room and an opening bridge to accommodate marine navigation were some of the key structural design criteria which led to a solution comprising two leaves of an opening swing bridge.

Following an open international competition, the joint submission by O’Connor Sutton Cronin and Brian O’Halloran & Associates Architects was nominated the winning entry.

The winning entry and resulting structural design evolved on the basic principle of a “balanced cantilever” which is to function both in the in use condition whilst remaining stable in the bridge open or out of use condition. Each bridge leaf is 44m long, rotates about a bearing mechanism over the river piers 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 4.5m wide aluminium bridge deck is supported on two 600mm diameter longitudinal 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.

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 minimize 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 was designed and delivered, complete within a 30 month programme commencing With the appointment of the design team in December 2002, obtaining statutory consents and foreshore licence, preparation of E.I.S. and construction on site culminating with the official opening in July 2005.

The bridge was the awarded the Pedestrian Footbridge of the year in 2006 by the Institute of Structural Engineers.