Spanning urban riverways, Chicago-style trunnion bascule bridges represent a pinnacle of early 20th-century mechanical engineering. The Milwaukee Avenue Bridge utilizes this distinctive system, where each leaf of the drawbridge pivots on a central axle known as a trunnion. Unlike traditional drawbridges that rely on massive external winches, the counterweighted bascule design relies on delicate gravitational balance.
Massive concrete counterweights are hidden below the roadway inside watertight concrete pits known as tail pits. These weights exactly match the weight of the bridge deck extending over the river. Because the deck and the counterweight perfectly balance around the fixed trunnion axle, the bridge achieves near weightlessness in its rested state.
Consequently, opening the thousands of tons of steel requires remarkably small electric motors, which only need to overcome mechanical friction and wind resistance rather than the actual weight of the span. As the drive machinery operates internal rack-and-pinion gears, the tail section drops into the pit while the roadway leaf rises gracefully into the air. This mechanical efficiency allows for rapid opening cycles without placing excessive stress on structural components.