ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

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42 | ASPIRE , Spring 2012 S TAT E C oncrete structures are an important part of the bridge inventory of the Michigan Department of Transportation (MDOT). Several reinforced concrete arch structures built prior to the 1900s are still in service today. One example is I-69 BR over the Battle Creek River in Eaton County, an earth-filled spandrel arch constructed in 1921, which remains in fair condition 90 years later. By the 1920s, simple-span reinforced concrete T-beams were the most common concrete structure type being constructed. In the 1950s, a significant portion of the reinforced concrete T-beams built were continuous spans, and in the mid 1950s, precast, prestressed concrete side- by-side box beams were common in Michigan. In 1959, precast, prestressed concrete I-beam bridges were introduced and, within a few years, prestressed concrete box beam and I-beam bridges dominated new concrete construction. The Zilwaukee Bridge When discussing concrete bridges in Michigan, one that must be mentioned is the Zilwaukee Bridge. MDOT made the decision to replace the existing I-75 bascule bridge over the Saginaw River in 1970, after it became apparent that a bascule bridge in an interstate freeway did not fit the purpose and need of the system. The decision to replace the drawbridge led to consideration of many alternatives. A tunnel under the Saginaw River would have cost about two and one-half times as much as a high- level bridge. Major alternatives for relocation of the freeway were also studied, but rejected as too costly and impractical. Closing the river to navigation at Zilwaukee also was considered but was opposed by the city of Saginaw and others with vested interests in maintaining port activity and its associated economic impacts upstream from I-75. Rerouting the traffic onto I-675 Business Loop through downtown Saginaw would have required extensive reconstruction well above the cost of a new bridge. A high-level bridge was identified as the preferred option. Due to the Saginaw River being a strategic navigable waterway, the minimum height required over the waterway was 125 ft as dictated by the United States Coast Guard. The maximum highway longitudinal slope allowed in Michigan is 3%. These geometric factors contribute to the structure's grand size and length. The twin precast, segmental concrete bridges are approximately 8085 ft long and the segments are 73 ft 6 in. wide between the tips of their Concrete Bridges in Michigan— by Rebecca Curtis, Michigan Department of Transportation Battle Creek River Bridge is an earth-filled spandrel arch constructed in 1921 and remains in fair condition 90 years later. All photos and drawing: Michigan Department of Transportation except where shown. Following the 1950s, adjacent precast, prestressed concrete box beam bridges were very common in Michigan as shown in these elevation and underside photos. Pa s t, Present, Future A typical parabolic reinforced concrete T-beam bridge built commonly in the 1950s. Book_Spr12.indb 42 4/3/12 9:18 AM

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