PERMANENT REPAIR work has started on the world's longest floating bridge, the Evergreen Point crossing near Seattle, in the US, which was damaged by a barge on 29 July.
Local marine contractor Hurlen Construction is constructing a 1,200mm diameter concrete column from the bed of Lake Washington to the deck of the bridge's approach viaduct 14m above.
This will replace the temporary steel props installed after the collision to allow state highway officials to partially reopen the vital link between Seattle and Bellevue after 11 days.
The impact, which shattered the original hollow precast concrete column, is the latest in a series of disasters which have dogged the 1.16km long crossing since it opened in 1963.
Concrete pontoons supporting the central floating section developed major cracks after being battered by storms in 1993 and 1995. These were strengthened in an £11M post-tensioning operation in 1999, which used nearly 50km of post-tensioning cables.
But other problems continue.
The mid-crossing drawspan, where the roadway retracts to allow boats through, has malfunctioned several times, killing at least one person.
And in March last year the drawspan mechanism was badly damaged in another storm, closing the bridge for several days and leading to severe restrictions on traffic.
Since 1994 more than £37M has been spent on repairs and strengthening. It is estimated this latest damage will cost £350,000 to put right. All of the crossing's 228 columns are known to need seismic upgrading, but the necessary funding is unlikely to be made available for many years, local engineers believe.
Lake Washington is crossed by several floating bridges. The first to be built, to the south of Evergreen Point, sank in a storm in 1990 and was reopened three years later. Highway 520 across the Evergreen Point bridge carries more than 100,000 vehicles daily, and is said to be one of the area's worst bottlenecks.