How Do You Move An Entire Bridge?

When the Sellwood Bridge in Portland, Oregon, was built in 1925, it wasn't designed to carry 30,000 vehicles a day. But by the 1980s, cracks were forming in the bridge's supports, leading inspectors to ban heavy trucks, buses, and fire engines.

So county engineers decided it was time for a new bridge, and the least expensive option was to move the existing structure over to serve as a detour while a new one was built in its place. But the bridge's rare design - a one-piece, 1,100-foot, 3,400-ton truss - posed an unusual problem. How do you move a whole bridge at once?