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Cracks expose flaw in Scandanavian mega-bridge

Urgent structural repairs are being carried out to the 7.8km long Oresund Bridge linking Sweden and Denmark after cracks were discovered in concrete at 50 different locations late last year.
Cracking and 'scaling' was found in the bridge's twin reinforced concrete trough girders during a routine inspection, said Bjorn Svensen, project manager for bridge owner and operator Oresundsbro Konsortiet.'There were patches of blistered and cracked concrete approximately 200mm-250mm long, 100mm-120mm wide and 15mm deep at 50 locations,' confirmed Bjorn Zellea, project manager for Sundlink Contractors, the firm that built the bridge.Sundlink Contractors is responsible for structural defects under a 10-year warranty, which expires in March 2010. The bridge was designed by Danish firm Cowi.The damage is at the eastern, Swedish end of the bridge.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.Oresund Bridge consists of a longitudinal reinforced concrete box truss, through which trains run. This is topped by a road deck.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 trough girders measure 4.5m wide by 1.8m deep. Concrete is 370mm thick.It is the lower inside corners of the trough girders that have suffered damage, Svensen said. 'Scaling of the concrete has been caused by contact between the structures,' he said.'The girders were designed to be installed with a gap of 10mm between them,' Zellea said. 'At the western end of the bridge this gap exists, but it is not there on the eastern part of the bridge.'The gap was needed to allow differential movement between the girders, caused by dynamic and thermal loading. More than 150 trains cross the bridge each day, while the east-west alignment of the bridge means the southern side of the bridge is subject to significant 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.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.'Sandlink will use a circular saw, operated from the bridge's maintenance gantries, to create a 10mm slot between the trough girders, as per the original design.The work is due to start this month and will take until June.

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