The first trunnion bascule bridge was built in Chicago at Cortland Street in 1902. This is the bridge type Chicago engineers are perhaps most famous for. The trunnion bascule has two bridge leaves hinged on opposing riverbanks. The bridge is drawn up by giant trunnion bearings. Bascule is French for seesaw; the bridge is named for the counterweights which balance the great weight of the bridge leaves. Each of these bridges must be built for a specific location so no two are exactly alike. This is an aerial shot of the Cortland Street Bridge looking north in 1953.

Photographer: Unknown
Source: Chicago Historical Society (ICHi-37372)