I read with interest Andrew Mylius's article (NCE 24/31 August) on the East London Line extension and, in particular, details of the proposed bowstring arch bridge across the Regent's Canal.
I was in discussion with the designer of this bridge during the initial stages and understand that it is of steel construction. This would certainly make it the first steel bowstring railway bridge for heavy rail in this country. However, two concrete bowstring bridges already exist, over the M56, dating from 1970, and of 68m span.
The 32 hangers on these consist of 100mm diameter steel bar machined from a solid (150mm) bar with the ends profiled to avoid stress concentration at the junction with the threaded end.
Careful consideration was given to fatigue conditions in the design of the hangers and the acceptable working stresses were calculated to be 120N/mm° in the body and 78.7N/mm° at the root of the thread diameter.
The bridges are regularly examined and continue to perform well in service. The use of hollow section for the hangers on the Regent's Canal bridge is a different approach, but the inability to inspect inside these critical members would still be a concern.
John Parker (M), Johnhcparker@hotmail. com