Cortland Street Bridge: Chicago's Engineering Prototype

Spanning the Chicago River on the city's North Side, the Cortland Street Drawbridge doesn't look like a revolutionary piece of engineering, but it is. Built between nineteen oh one and nineteen oh two, it was the first Chicago-style fixed-trunnion bascule bridge ever constructed. The design works by suspending the bridge's two leaves on axles called trunnions, with massive concrete counterweights buried in pits below the riverbanks, allowing the bridge to rise with remarkable ease.

City engineers had studied movable bridge designs across the United States and Europe and concluded this approach was the most efficient. Their conclusion was vindicated: the Cortland Street Bridge became the prototype for more than fifty of Chicago's movable bridges. The American Society of Civil Engineers designated it a landmark in nineteen eighty-one, and Chicago landmarked it in nineteen ninety-one.