Hammersmith Flyover was built in the early 1960's and sits on top of 15 concrete columns or piers. As the 600m long concrete bridge expands and contracts with the changing temperature, then the piers also need to move. Until now, each pier has sat on two roller bearings, like large rolling pins that allow the bridge to "roll" backwards and forwards - up to 20mm a day between a cold night and warm day and 50-60mm between Summer and Winter.
One of the original bearings
However, over time, these bearings have become rusted and worn. As part of the re-stressing of the Flyover, the 15 pairs of bearings have gradually been replaced. This has meant that the bridge has had to be lifted up and a single temporary bearing installed whilst the old bearings are removed and the new bearings are installed.
The temporary bearing sits at the centre of each pier (between the old and new bearings) and is supported by 4 large hydraulic jacks. To stop the bridge from falling sideways, there are also four jacks, one at each corner, holding the pier in position.
The new bearings (working in pairs) have to be installed to an extremely tight tolerance to allow the piers (and the bridge) to move smoothly. If the bearings were twisted or uneven, this would cause the bridge to jam or lean. However, the vibrations from the heavy traffic passing over the flyover can cause vibrations and effect the accuracy to which the new bearings are installed. Therefore, the bridge has to be closed to vehicles - meaning that the work has to be carried out during the night when the flyover is less busy.