When the Sellwood Bridge in Portland, Oregon, was built in 1925, it wasn't designed to carry 30,000 vehicles a day. Or to hold back a slow landslide. But by the 1980s, cracks were forming in the bridge's supports, leading inspectors to rate the bridge a 2 on a 100-point federal safety scale and to eventually ban heavy trucks, buses, and fire engines. So county engineers decided it was time for a new bridge, and the least expensive option ($306 million) 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?