Chicago, Milwaukee & St. Paul Railway, Bridge No. Z-6, Spanning North Branch of Chicago River, South of Cortland Street, Chicago, Cook County, IL

Significance: The Milwaukee Road's Bridge No. Z-6 may be the only one of its kind ever built, defying the conventional categories of center-bearing and rim-bearing swing bridges. This ingenious design answers the constraints of a small site on a bend in the river. The 175'-2"-long plate-girder span bears on a short segment of roller nest turning at one-half the span's rate. This is accomplished by an unusual drive mechanism with two parallel racks. As a result, the roller nest need only turn through an arc equivalent to half of the span's rotation, reducing the bridge's footprint accordingly.
Unprocessed Field note material exists for this structure: N652
Unprocessed Field note material exists for this structure: N967
Survey number: HAER IL-162
Building/structure dates: 1898-1899 Initial Construction