The strengthening contract to ‘Bridge MR46A’, originally constructed in 1960 and currently part of the Amersham branch of the Metropolitan line. The all-welded steel structure, which carries twin Railtrack and London Underground lines over Station Road, adjacent to North Harrow Station, is 9 metres wide and has a clear span of 26 metres - hence the requirement for the unusually long CFRP plates.

An assessment by London Underground Ltd in December 1997, found that in relation to the current loading requirements, the bridge cross beams were under-capacity in bending while the bridge main beams were under-capacity both in bending and to a lesser extent in shear. Consulting Engineers, Scott-White and Hookins, investigated various solutions including bridge replacement and conventional means of strengthening by the welding of steel cover plates. However, having considered the element of disruption to the railway and road below, best value and speed of installation, the use of Sika Carbodur DML Plates (fabricated by Sika Ltd/Devonport Management Ltd) was finally selected as the preferred system. At its thickest, the Sika DML plate was 30mm thick. The largest plates, which were 13 metres long and 250mm wide, weighed only 125kg each. The total length of all plates installed was about 250 metres.

The bridge refurbishment/strengthening works were carried out over an intensive 10 week period in winter whilst minimising disruption to the very busy road under the bridge. The works included the erection of scaffolding (whilst maintaining headroom for buses etc), removal of the existing lead contaminated coatings in strict control conditions (tenting, airlocks etc), bonding of CFRP plates to the bridge’s steel flanges using Sika epoxy adhesive and the application of new coatings to the entire structure.

The timing of the project was such that the bridge had to be shrouded in stages to maintain ambient climatic conditions during the plate installation works. Space on site was extremely tight. The site storage and full welfare conditions were set up within the bus stops next to the bridge.