Services

Cornmarket Street Pedestrian Bridge

An open competition was held by Cork Corporation in conjunction with the Institute of Engineers of Ireland to arrive at the design of the new bridge at Cornmarket Street. A total of 37 entries were received from Ireland, the U.K. and Europe. The entry from McGarry Ni Éanaigh Architects with Muir Associates Consulting Engineers was selected. The design selected is a visually attractive light steel structure with two symmetrical arches and a timber deck. The bridge was designed to look equally well in daylight and at night-time and will incorporate a lighting scheme to emphasise the view at night. Work on the bridge is scheduled to begin early next year and is expected to be completed in July 2000. The National Millennium Committee has awarded Cork Corporation £500,000 for the Millennium footbridge.

Structure

The primary issue to be resolved in a flat arch is its vulnerability to "spreading" of the supports. Although the ground conditions are defined for the purpose of the Competition as being good, the capacity to resist the substantial thrusts from a flat arch is obviously in doubt. The decision to tie the arch ensures that the only significant load to be resisted at the abutments is a vertical load which allows a simple foundation to be designed integral with the quay walls.

Two canted 273 mm diameter CHS arches (on either side of a walkway) springing from MHWOST and with crest of arch at top of handrail level. The arches are laterally supported by the balustrade verticals which also provide some additional torsional resistance and are tied using a steel plate arrangement to raise two pairs of tie rods. The tie rods are located at the level of the handrail and at the level of the deck.

Strategy

The bridge provides a direct and easy connection between the north and south quays - the deck slopes evenly to accommodate the different levels resulting in a 1:50 gradient - flat in terms of disabled access. The bridge has a recognisable visual identity - two shallow tied symmetrical arches. The shape and elevation of the arches are determined by holding the feet of the arches out of the water and keeping below the sight lines of those using the bridge. The elements of the bridge are legible - arches, tie rod, steel brackets, deck balustrade.

The walkway deck comprises balau plants (balau is a tropical hardwood from a sustainable source) - 100 x 75 mm spanning transversely from edge beam to edge beam. The edge beams span 3 metres between vertical plate hangers. The balustrade is canted inward so as to dissuade would-be climbers and is made from 50 x 6 mm stainless steel flats - the orientation of the flat section minimises obstruction when viewing the bridge.

Lighting

There are two forms of lighting - one for pedestrian ambient light and the other to illuminate the arches. A series of light strips at 3 metre centres coinciding with the steel hangers run horizontally across the bridge deck and vertically between the steel brackets. There is an individual light source concealed within the edge beam at three metre centres. The light source illuminates a fibre optic lamina between the steel brackets - providing an even glow of light appropriate to pedestrians. The luminaries sources are directly accessible for maintenance from the deck. Overall general lighting of the arches is achieved by directional narrow focus floodlighting from the quay walls. The profile of the arch is therefore read.