I-35W Mississippi River Bridge - Design and Construction

Design and Construction

The bridge, officially designated "Bridge 9340", was designed by Sverdrup & Parcel to 1961 AASHO (American Association of State Highway Officials, now American Association of State Highway and Transportation Officials) standard specifications. The construction contracts, worth in total more than US$5.2 million at the time, went to Hurcon Inc. and Industrial Construction Company, which built the steel trusses and deck. Construction began in 1964 and the bridge opened to traffic in 1967.

The bridge's fourteen spans extended 1,907 feet (580 m) long. The three main spans were of deck truss construction while all but two of the eleven approach spans were steel multi-girder construction, the two exceptions being concrete slab construction. The piers were not built in the navigation channel; instead, the center span of the bridge consisted of a single 458-foot (140 m) steel arched truss over the 390-foot (119 m) channel. The two support piers for the main trusses, each with two load-bearing concrete pylons at either side of the center main span, were located on opposite banks of the river. The center span was connected to the north and south approaches by shorter spans formed by the same main trusses. Each was 266 feet (81 m) in length, and was connected to the approach spans by a 38 foot (11.6 m) cantilever. The two main trusses, one on either side, ranged in depth from 60 feet (18.3 m) above their pier and concrete pylon supports, to 36 feet (11 m) at midspan on the central span and 30 feet (9 m) deep at the outer ends of the adjoining spans. At the top of the main trusses were the deck trusses, 12 feet (3.6 m) in depth and integral with the main trusses. The transverse deck beams, part of the deck truss, rested on top of the main trusses. These deck beams supported longitudinal deck stringers 27 inches (69 cm) in depth, and reinforced-concrete pavement. The deck was 113 ft 4 in (34.5 m) in breadth and was split longitudinally. It had transverse expansion joints at the centers and ends of each of the three main spans. The roadway deck was approximately 115 feet (35 m) above the water level.

Construction on the bridge began in 1964 and the structure was completed in 1967 during an era of large-scale projects related to building the Twin Cities freeway system. When the bridge fell, it was still the most recent river crossing built on a new site in Minneapolis. After the building boom ebbed during the 1970s, infrastructure management shifted toward inspection and maintenance.