Interstate 75 in Michigan - Monumental Bridges

Monumental Bridges

Along its route in the state, I-75 utilizes three of Michigan's monumental bridges. The first of them is the Zilwaukee Bridge near Saginaw. The original bridge across the Saginaw River at Zilwaukee was built in 1960 as a bascule bridge to allow shipping traffic to use the river. Opening the drawbridge would back traffic up on I-75/US 10/US 23 for upwards of four hours on holiday weekends. Approved in 1974, construction on the replacement bridge started in October 1979. A major construction accident in August 1982 delayed completion of the new Zilwaukee Bridge; a bridge pier partially collapsed when contractors overloaded a section under construction. The affected 300-foot (91 m) deck segment tilted to rest three feet (0.91 m) higher on one end and five feet (1.5 m) lower on the other. The structure was originally supposed to cost $76.8 million with a 1983 completion date; in the end it cost $131.3 million when the southbound span finally opened on September 19, 1988. The structure is the largest concrete segmental bridge in the United States.

The second is the Mackinac Bridge that connects Michigan's two peninsulas at the Straits of Mackinac. A structure was first proposed in 1888 by one of the directors of the Grand Hotel on Mackinac Island. Carferry service was started in 1923 to cross the straits, and a bridge authority was first created in 1934 to investigate the possibility of building a permanent connection across the straits. They started with a 1921 proposal for a series of bridges that would have connected Cheboygan to St. Ignace by way of Bois Blanc, Round, and Mackinac islands. The federal Public Works Administration rejected loan and grant requests for that project. A second, direct crossing was then proposed based on designs used for the Tacoma Narrows Bridge. The collapse of that bridge and World War II delayed any further work on a structure beyond tests of the lake bottom and the construction of the 4,200-foot-long (1,300 m) causeway on the St. Ignace side; the first bridge authority was abolished in 1947. The current agency was created on June 6, 1950.

The Mackinac Bridge Authority was authorized in 1952 to sell bonds to finance construction, which were sold on December 17, 1953, to finance the $99.8 million cost of the bridge. The structure was designed by David B. Steinman and built by Merritt-Chapman & Scott for the substructure and the American Bridge Company division of U.S. Steel Corporation for the superstructure. Construction started in 1954 and the Mackinac Bridge opened to traffic on November 1, 1957. Final work on the bridge was completed in September 1958. Overall, the structure has a 3,800-foot (1,200 m) central suspension span flanked by two 1,800-foot (550 m) side spans. With the two backstay spans, the Mackinac Bridge is 8,614 feet (2,626 m) long between cable anchorages, the longest in the world at the time it opened. The total length of the structure is 26,444 feet (8,060 m) with two 555-foot-tall (169 m) towers and 155 feet (47 m) of clearance for passing ships under the main span. In 2010, the structure was named a National Historic Civil Engineering Landmark by the American Society of Civil Engineers.

The northernmost of the three monumental bridges along I-75 is the International Bridge, linking the twin cities of Sault Ste. Marie in Michigan and Ontario. The governments on each side of the international border formed a bridge authority to build a highway bridge in 1935. Construction started on the structure September 16, 1960. The International Bridge is nearly three miles (4.8 km) long encompassing spans over the American and Canadian navigation channels for the Soo Locks and the St. Mary's River. The American approach is 2,471 feet (753 m), and the Canadian approach is 2,942 feet (897 m). The center span over the river is 9,280 feet (2,830 m), flanked by 1,260-foot (380 m) and 830-foot (250 m) spans over the American and Canadian shipping channels, respectively. The bridge was designed by the same firm that handled the Mackinac Bridge for a cost of $20 million. It opened to traffic on October 31, 1962.