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Lines of thought

I was pleased to read Colin Davies' praise for the operation to replace one of the damaged bridge spans at Potters Bar (NCE 21 November).

Among the adverse publicity surrounding the railway industry it is good to note just one of many examples of the excellent work railway engineers continue to produce week in, week out.

With regard to Mr Davies' comments on the aesthetic aspects, it is difficult to argue that the clean and simple lines of the old bridge did not have a certain appeal. However, when deciding on a bridge design there are many considerations to take in to account.

Health and safety factors demanded a walkway and, due to track alignment and substructure constraints, this had to be cantilevered. As for the external stiffeners, the main damage caused to the adjacent centre span by the derailed train was the buckling and fracture of its main girder internal web stiffeners. But these can be prone to fatigue and to corrosion as they act as traps for ballast and litter so external stiffeners were used.

The design also allowed the bridge to be fabricated and installed in such a way that disruption to both the highway and the railway was at a minimum.

Richard Bell (M), regional structures engineer, Network Rail, 28-40 Blossom Street, York, YO24 1GJ

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