TRANSPORT MINISTER Dr John Reid this week ordered an urgent report on the state of the UK's trunk road bridges after a 3t weight limit was suddenly slapped on part of a key motorway interchange in north London.
Inadequate shear capacity of a concrete viaduct, which forms part of the busy intersection between the M11 and the North Circular Road, has been blamed for the problems. Repairs are expected to take up to a year to complete.
Highways Agency engineers imposed the limit on the westbound sliproad on Monday after the viaduct structure, which carries it over the motorway, failed a detailed assessment. Around 1,500 vehicles an hour normally use the sliproad at peak times. Now only cars and light vans will be allowed on the viaduct.
The three-span insitu concrete structure was being assessed for its ability to carry 40t HGVs. Normal reinforced concrete construction was adopted, with a voided slab design. The centre span is 47.5m.
At the piers, slabs are unvoided and 2.5m deep. 'The problem is that the structure was designed before 1973, when shear design rules were changed,' said Agency civil engineering divisional director Alan Pickett. 'We finally concluded that we couldn't accept the original assumptions on shear capacity, and therefore had no alternative but to impose a weight limit.'
WS Atkins is currently developing a strengthening scheme, Pickett added, and work could start within two months. One option being considered is to reinforce the viaduct with carbon fibre composites before inserting additional steel bars in the understrength zones. This work is expected to take up to nine months to complete.