In July 2002 when a cylindrical roller bearing cracked over pier V the Agency had to act quickly to replace it. But the riveted steel beam over the bearing was too weak to sustain the jacking loads required to lift the deck off the bearing. Stiffeners had to be bolted on to the 6m deep deck beam section before jacking could take place.
'But this was very time consuming, and wouldn't be [physically] possible to do in all locations, ' says John Martin project manager Pete Wilkinson. This is because the steelwork arrangement below the deck is very dense leaving little room for temporary works.
When eight further bearings needed replacing in random locations in January 2003, a different method was employed. Then, an A-frame structure was bolted onto temporary stiffeners on the existing deck beams. But even this design required specially designed 'legs' for each location depending on the slope of the beam.
'When we realised we'd have to replace all the bearings, we realised we needed a quicker solution, ' says Bridle.
With future maintenance in mind the latest temporary steelwork is designed to be simple and quick to install.
Permanent stiffeners are bolted on to existing girders to receive the third generation jacking frame. This involves using a 4.5t steel unit with four jacking legs which bear onto four temporary sliding spherical bearings. Each jack can take 100t and lifts the beam up to 3mm.
Since this activity takes place about 25m above ground level all components of the jacking frame have to be small enough to be winched up from the ground and man-handled into position.
'Using this method, we only need to remove traffic from that lane when the beam is being jacked, ' says Brindle. So far 30 units have been built at various locations to speed up the operation.