Barrett Byrd Associates

An elegant steel footbridge is being built in the Anglesey port of Holyhead to encourage ferry passengers to visit the town and help stimulate regeneration, reports Mike Walter

Two million people travel through the port of Holyhead in north Wales each year, but few visitors choose to spend time or their money in the town. Lack of an obvious and direct route from the port into the town centre means that even those determined to experience the charms of Holyhead face a long and uninspiring walk over a busy roadbridge. All that is about to change.

Structural engineer and consultant Gifford has been commissioned by Isle of Anglesey County Council to design a striking new pedestrian and cycle bridge to take visitors arriving from overseas right into the heart of the town. The bridge, to be known as the ‘Celtic Gateway’, will span redundant railway sidings and a busy road and is to join up with a recently completed pedestrian causeway that crosses a dock to link with the port terminal.

The Celtic Gateway bridge forms a crucial part of an ongoing regeneration scheme known as the ‘Holyhead Transport & Environmental Package’ that includes extending the A55 into the town, constructing a new port entrance and relocating a railway maintenance depot. The hope is that the new footbridge will attract investment into the town and help generate employment opportunities for the 12% of adult residents in unemployment.

The Celtic Gateway will feature two slender arches made from stainless steel that will lean away from a deck suspended from stainless steel hangers. The bridge will have a main span of 68m and a deck that is over 200m in length and four metres wide, that will gently spiral down a ramp to ground level on the port side. The choice of stainless steel for the arches is both aesthetically pleasing and should reduce the need for bridge maintenance in such an aggressive marine environment.

Principal engineer Andy Marginson of Gifford says that his company was asked for a bridge design that was far from just functional. “Our brief was to come up with something unusual and we have designed a spectacular bridge that will catch the eyes of people arriving at the port and hopefully encourage some of them to visit the town. The bridge will be the main pedestrian gateway into Holyhead and a large steel tail will rise up from one end of the bridge to provide a focal point,” he says.

Pedestrians who join the bridge from the port will do so six metres below those who walk onto the bridge from Market Street in the town. Use of a spiral ramp on the port side takes into account this steep drop in height, provides easy access for wheelchair users and, if viewed from above, resembles part of a Celtic knot.

Marginson adds that the slender appearance of the bridge belies what will be a fairly heavy structure. Much of the added weight will be in the deck, which is to be formed from a steel trough filled with concrete and finished with stainless steel nosing. “The client was keen that vibration from pedestrian movement on the bridge was kept to a minimum so we decided to design a heavier deck. This should help to quell fears that the structure could oscillate in a similar way to the Millennium Bridge in London when it first opened,” he says.

The steel tubes for each arch are currently being fabricated and bent into shape by Italian firm Cimolai and will be brought to site in 12m lengths by sea container. Segments will then be assembled on site and lifted into place this autumn.

The hollow stainless steel arches measure one metre in diameter and have a 25mm wall thickness. They are designed to lean back from the vertical at slightly different angles to one another. This means that for the deck to finish level, following self weight deflection, it has to be precambered to different levels on each side to accommodate the different arch deflections. It will be jacked up to this twisted shape before being connected to the arch via nine steel hangers along each edge. When released from the jacks, the deck will settle into a level position.

Each bridge arch will cantilever from a springing point at one end that is to be anchored into a 10m square concrete pile cap, tied into 16 reinforced concrete piles (see box). Six vertical steel columns will support the descending spiral ramp.

Later this summer, main contractor Laing O’Rourke will install seven temporary piers along the route of the bridge to support the deck and two 14m high supports will be used to install each steel arch. End sections of each arch will be installed first before the central section is lowered into place.

Access to the new bridge from Market Street in the town centre will be made possible only after a commercial building is removed from a terrace. Laing O’Rourke will demolish the building and install a steel portal frame structure between occupied buildings to carry the end of the pedestrian walkway.

Illumination for the deck and the steel arches has been designed by Kevan Shaw Lighting Design, which has been commissioned by Gifford through the NEC form of contract. “The client has a ‘dark skies’ policy to minimise light spill so the bridge lighting has to be very focused,” says Marginson. “A viewing platform is likely to be installed at the end of the bridge and information boards set up to direct people to and from the town.”

Project manager Arwel Roberts from the Isle of Anglesey County Council says that he has high hopes that the Celtic Gateway bridge will entice more people into the town. “Holyhead is a typical port town in that it has high levels of unemployment and we are desperately trying to turn its fortunes around.

“The Celtic Gateway should also attract cyclists. It will form the start of the new Anglesey Coastal Path and will be a key node in the National Cycle Network. A local artist will be commissioned to design bronze inlays for the deck and others will be commissioned to display artwork on the spiral ramp,” he adds.

GROUNDWORKS BOX

Large diameter piled foundations to support the Celtic Gateway bridge were completed in June and involved use of a powerful down-the-hole (DTH) hammer to create 47 sockets in the underlying bedrock.

Foundation specialist Roger Bullivant Ltd (RBL) created nominal 400mm and 600mm diameter sockets up to a depth of 5m in the rock using a DTH hammer and air flush system to remove broken rock from the bottom of the hole.

RBL contracts manager Adrian Mercer said: “The down the hole hammer acts like a masonry drill, it rotates and hammers at the same time, smashing down into the rock. There are very few companies with this size of equipment and this was only the second time this hammer has been used by us in the UK. It performed very well.”

RBL’s rock socketing followed the installation of 750mm and 450mm diameter thick wall steel tubes to depths of between 5m and 11m by Commercial Marine & Piling. Tubes were driven through alluvial silts and clay to the rock head using a top drive hammer, which then allowed RBL to auger out loose material overlying the rock.

Completion of each rock socket was followed by installation to full depth of heavy duty reinforcing cages made of 12 T40 steel bars. The pile tube was then filled with concrete.

FACT BOX
Client: Isle of Anglesey County Council
Main contractor: Laing O’Rourke
Structural engineer and consultant: Gifford
Steel fabricator: Cimolai
Foundation contractors: Roger Bullivant Ltd
Commercial & Marine Piling
Construction value: £4.8M
Revenue streams: Welsh Assembly transportation grant, European Regional Development Fund Objective One, Isle of Anglesey County Council, Welsh Development Agency, Lottery Funding

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