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Bridges: After the flood

Workington’s Northside Bridge is one of the last piece of infrastructure to be rebuilt after flooding devastated the town in 2009. Declan Lynch reports.

November 2009 remains fresh in the minds of Cumbrians. It was then that catastrophic flooding devastated the county.

The small town of Workington bore the brunt. Two of the three road and pedestrian crossings of the River Derwent were swept away as it burst its banks. A third bridge was severely
damaged.

Since then one of the damaged bridges has been replaced, and the severely weakened Calva bridge has been repaired. Engineers are now close to completing a replacement for the third crossing, the new £11.5M, 152m long, three span Northside bridge.

Original crossing

Scour undermined the original crossing, built between 1903 and 1904, causing its collapse.

Motorists were faced with a 29km detour following the collapse, until August 2010 when Cumbria County Council installed a temporary crossing manufactured by Jansen. The council then started looking at options for a permanent replacement.

“We looked at eight options,” says consultant Capita Symonds supervisor Mike Briggs who is the client’s representative on the scheme.

The council eventually chose a 152m long structure with a 68m central span and two 42m side spans supported by two reinforced concrete piers. The deck is a composite structure with a reinforced concrete deck supported by six longitudinal plate girder beams each made of five sections, welded together.

Capita Symonds began the design of the new structure in August 2010, while contractor Morgan Est removed the remains of the original masonry arch crossing.

“We lifted the beams in pairs as it was more stable”

Phil Dilworth, Birse

Principal contractor Birse and piling subcontractor Balfour Beatty Ground Engineering (BBGE) began building the new crossing in August 2011.

Roughly three weeks of site investigation work was needed to locate the site’s utilities and services, which were not positioned quite as expected.

Then piling began on the northern abutment. BBGE was due to install 16, 750mm diameter up to 25m long bored piles on the northern abutment. Forteen piles were inserted on the south bank, and 21 piles beneath each pier.

Piling was complete in December 2011 and efforts in 2012 have focused on creating the bridge’s superstructure.

The 700mm wide beams supplied by Mabey Bridge have a curved profile which makes them 3m deep at the piers and 1.3m deep at mid-span.

Engineers began installing the plate girder beams during a weekend possession in January. The girder sections for the side spans are between 22m and 26m long and were initially supported on temporary trestles positioned on each bank between the abutments and piers.

Crane lift

A 700t crane lifted the girder sections into position before they were welded together.

During the same possession the crane lifted another six 22m long girder from the trestle over the first pier cantilevering out over the River Derwent.

“We lifted the beams in pairs as it was more stable,” explains Birse site agent Phil Dilworth.

Welders then joined the sections together forming continuous 46.5m long beams, and removed the temporary trestles. The same procedure was repeated on the northern abutment leaving a space over the River Derwent for the central section.

The final lift required an 800t crawler crane to place the 47.5m long central section. The central section rested on the bottom flange of the cantilever sections and was clamped before
being welded into position.

With the superstructure in place, engineers installed permanent formwork across the beams to form the underside of the deck. The 250mm deep reinforced concrete deck was then poured on top.

The bridge is due to open next month.

Construction team

Client: Cumbria County Council

Main contractor: Birse

Designer: Capita Symonds

 

Piling contractor: Balfour Beatty Ground Engineering

Steel fabricator: Mabey Bridge

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