The Sixth Street Viaduct project is located through the Menomonee Valley in Milwaukee, Wisconsin. This was a $50 million Design-Build project awarded to the joint venture group Milwaukee Gateway Partners, composed of Lunda Construction Company, Zenith Tech, and HNTB. The previous 2,780-foot viaduct traversed the North Canal of the Menomonee River, South Menomonee Canal, five local streets, and nine Railroad tracks located at three separate locations. An intermediate connecting bridge for a major post office terminal was also attached to the existing structure. The new structures were required to have a 75-year design life. The project was allowed a 24-month construction period with impact to traffic being limited to 18 months. Within the 18-month traffic impact period, access was to be maintained to the post office terminal.

The new structures consist of pairs of two-way post-tensioned cable-stayed structures and bascule bridges separated by an at grade street intersection near the middle of the previous structure. The cable stay structures have overall lengths of 574 and 665 feet. A pair of 142-foot high inclined concrete pylons supports the mainspans, with lengths exceeding 190 feet. These structures have transverse widths varying from 81 to 96 feet. The structures are supported on over 50,000 feet of piling with a large portion being high capacity 12-inch diameter cast-in-place piles, some reaching 185 ton bearing. The post-tensioned spans are cast-in-place decks with a transverse concave-like bottom profile reaching thicknesses of up to five feet at some haunch locations. A high performance concrete mix was designed and used in all superstructures. All post-tensioned decks are also surfaced with sacrificial microsilica overlays to allow removal and resurfacing in the future.

Lunda Construction Company was responsible for bridge removal and the construction of the two cable stay structures, a 270’ two-way post-tensioned approach structure, and a new slab deck connection bridge for the post office terminal. The bridge decks required extensive falsework arrangements due to unfavorable soil conditions, railroad and vehicular clearance constraints. Adding to the falsework complexity of the irregular shaped bottom of deck, was the just-in-time delivery of plans due to the design build process. Over 2 million pounds of structural steel beams and 12,000 feet of 12-inch diameter high capacity cast-in-place piling were required in Lunda Construction Company’s falsework design.