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Cracks expose flaw in Scandanavian mega-bridge


URGENT STRUCTURAL repairs are being carried out to the 7.8km Øresund Bridge linking Sweden and Denmark after cracks were discovered in concrete at 50 different locations late last year.

A aw in the installation of the bridge's reinforced concrete girders at the eastern approach to its central cable stay section has been blamed for the cracking.

'The girders were designed to be installed with a gap of 10mm between them [to allow for dynamic and thermal movement], ' said Bjorn Zellea, project manager for Sundlink Contractors, the m that built the bridge.

'At the western end of the bridge this gap exists, but it is not there on the eastern part of the bridge' said Zellea.

The bridge was designed by Danish consultant Cowi.

More than 150 trains cross the bridge each day. The east-west alignment of the bridge means the southern side of the bridge is subject to signicant solar heating, while the northern side is always in shade.

'There has been friction between the two structures, ' said Zellea.

The trough girders were constructed using high strength C60 concrete 'which can be more brittle' than ordinary concrete, contributing to failure when subject to shear forces, Zellea added.

Cracking and scaling was found in the girders at the eastern end of the bridge during a routine inspection, said Svensen, project manager for bridge owner and operator Øresundsbro Konsortiet.

'There were patches of blistered and cracked concrete approximately 200mm to 250mm long, 100mm to 120mm wide and 15mm deep at 50 locations, ' con ed Zellea. Sundlink Contractors, is responsible for structural defects under a 10 year warranty, which expires in March 2010.

Crack patching has already been carried out. Sundlink is to carry out further work, costing up to £135,000, over 7km of the bridge to eliminate the defective detail that has led to the damage.

It will use a circular saw, operated from the bridge's maintenance gantries, to create the missing 10mm slot between the trough girders, starting at the bridge's eastern end.

The work is due to start this month and will take until June.

Spans approaching the central cable stay section of the Øresund Bridge comprise a longitudinal reinforced concrete box truss, through which trains run.

This is topped by a road deck (see diagram). The box truss for the central cable stayed section is steel.

A pair of reinforced concrete trough girders support the rail tracks between the box truss's cross members, which are at 20m centres. They also provide train crash containment.

The 370mm thick trough girders measure 4.5m wide by 1.8m deep.

It is the lower inside corners of the girders that have suffered damage, Svensen said. 'Scaling of the concrete has been caused by contact between the structures, ' he said.

But he emphasised that the damage did not pose a major structural hazard. 'There is 80mm of concrete cover to the reinforcing steel.

'The maximum depth of the cracking and blistering is 15mm.'

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