Santa:
AFIK the Tinsley viaduct lane closures were due to the increases in permitted lorry weights in the late 70s from 32 tons (32.5 tonnes) on 4 axles, to 38/40 tonnes on 5 axles (depending on configuration) and, later, to 44 tonnes on 6 axles.Many other bridges were partially closed, some of them for years, for the same reason. The A40 roundabout over the M5 took a decade to strengthen.
All the bridge closures at that time were due to the box girder construction method which was found to have inherent weaknesses and needed to be replaced/modified