The
Steel Bridge is the only double-deck vertical lift bridge of its
type in the world (The Portland Bridge Book). Five counterweights
in each of the massive towers lift both central spans independently:
the lower deck retracts (telescopes) 45 feet up into the upper
deck (in only 10 seconds); then both decks can rise for additional
clearance. The lower deck is only 26 feet above low water level—the
lowest of the downtown bridges—so it is lifted often, especially
in summer. Most river traffic clears without needing to raise
both decks. The total moving load of that second lift is nearly
9 million pounds and moves only one foot per second.
The
lower deck was built for passenger and freight trains, the upper
deck for pedestrians, gas- and horse-powered vehicles and the
city's first electric trolley. Although those trolley rails were
later removed, in 1986 rails for the MAX line to Gresham were
installed. In 2001 Portland cantilevered a pedestrian/bike sidewalk
onto the outside south face of the lower deck, thus completing
a non-vehicular loop around a downtown stretch of the Willamette
River.
Designed
and engineered by Waddell & Harrington (Kansas City, MO),
this innovative structure was completed in 1912. It kept the acquired
name of the 1888 swingspan bridge it replaced, a structure which
had been built of steel at a time when wrought iron was the common
bridge material.
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