The 116-year-old bridge has been closed to traffic for years and is being turned into a recreational pier.
The Rock Island Swing Bridge, saved from demolition in recent years and being renovated as a public pier, caught fire on Thursday, dealing a blow to Inver Grove Heights' $2.4 million project.
Nobody was injured in the blaze, which may have been started by welding sparks as contractors worked on the project, said Eric Carlson, the city's parks and recreation director.
Firefighters were called about 2 p.m. as flames spit from the underside of the bridge, where a wooden support structure holds up the decking. They extinguished the fire by 5:30 p.m., Carlson said.
Construction was to have been finished in about two weeks, but that timeline will be pushed back, though it was not clear by how much. A November opening already had been delayed until spring because of recent storms.
The fire did "significant damage" to the deck on the remaining two spans of the original bridge, built in 1894, Carlson said. Structural engineers will have to examine the bridge to determine whether the spans can be reused, he said.
The former double-decker railroad and auto bridge over the Mississippi River was called a swing bridge because it swung open in the middle to let boat traffic pass through. It was closed to rail use in 1980 and to road traffic in 1999.
The Washington County side of the bridge began crumbling a few years ago, but Inver Grove Heights worked to save the Dakota County side and use it as a pier. It was seen by some as the future jewel of the city's park system. Construction has been underway since April.