Chicago is world-renowned for its moveable bridges, but the Milwaukee Avenue Bridge stands as a classic triumph of trunnion bascule engineering. Built to replace inefficient center-pier swing bridges that choked river traffic, this double-leaf bascule bridge operates like a giant, perfectly balanced playground seesaw. Each leaf rotates around a central axle called a trunnion.
The magic, however, happens entirely out of sight inside large watertight concrete pits beneath the riverbanks. Massive concrete and iron counterweights are attached to the short tail ends of the bridge leaves. Because these counterweights perfectly match the weight of the long roadway spans extending over the water, the structure's center of gravity rests almost exactly on the trunnion axis.
As a result, lifting the massive steel structure requires shockingly little motor power. Instead of hoisting thousands of deadweight tons upward, small electric gear motors merely need to overcome mechanical friction and wind resistance to rotate the balanced assembly. When activated, an internal rack-and-pinion gear system smoothly rolls the counterweighted tail down into the sub-street pit while the long roadway leaf tilts skyward.
This under-deck counterweight design was a revolutionary aesthetic and structural choice, as it eliminated ugly overhead gantries and tower trusses while preserving clear city sightlines. The result is an enduring marvel of civil engineering that has safely carried heavy urban traffic for over a century while gracefully opening for river vessels at a moment's notice.