Mackinac Trail Bridge

Information and Findings From Michigan Historic Sites Online

Narrative Description

Main span number is 1. Main span length is 60 feet. Structure length is 60 feet. Roadway width is 22.5 feet. Structure width is 26 feet.

Located some eleven miles north of St. Ignace, this graceful concrete arch bridge spans the Carp River on the Mackinac Trail. The crossing is situated immediately west of I-75, about a mile from the river's mouth at St. Martin Bay. The structure is comprised of a 60-foot, filled spandrel arch, with an elliptical profile. The arch springs from massive concrete abutments, which feature battered backwalls and angled wingwalls. Cobblestone retaining walls are built above the wingwalls at the approaches. The tapered arch ring of the Carp River Bridge is corbelled from the spandrel on each side. This, and the MSHD standard guardrail, originally provided the bridge's architectural expression. The original guardrails have since been removed, however, and the deck widened by cantilevering slightly over the spandrel walls to form concrete stringcourses. The existing guardrails are also MSHD standard, this time with ornamental steel rails and concrete bulkheads. This alteration marks the most serious loss of physical integrity for the bridge. The only other change is minor: the bridge plate that was bolted to the guardrail when the bridge was inventoried in 1982 has since been stolen.

Statement of Significance

Soon after the legislature passed the State Trunk Line Act in 1913, authorizing the formation of a trunk line system of roads, a route began to form through Mackinac and Chippewa counties that would link the straits of Mackinac at the southern tip of the Upper Peninsula with Sault Ste. Marie on the Canadian border. Improvement work began first in 1913-1914 on the southernmost segment, from St. Ignace north to a point just beyond Rogers Park, south fo the Carp River Crossing. A major Upper Peninsula artery, the Mackinac Trail was extended to Sault Ste. Marie in the 1910s. Its construction entailed both roadway grading and bridge construction. One of these latter structures spanned the Carp River Mackinac County about midway between the Straits and the Chippewa County line. For this crossing, engineers of the Michigan State Highway Department delineated a 60-foot concrete arch in 1919. MSHD hired Flint contractor A.S. Decker to build the bridge. "The 60 ft. span Arch over the Carp River in St. Ignace Township of Mackinac County has just been completed, "MSHD reported in the summer of 1920. Total cost for the bridge and adjacent roadway grading: $41,223.53. Later incorporated into US-2, the structure carried increasingly heavy traffic, prompting MSHD to widen its deck by five feet and install new guardrails in the 1929-1930 biennium. Decorative cobblestone retaining walls were built at the approaches at that time. This segment of the Mackinac Trail is now a county road, superseded by I-75. Still, the Carp River Bridge carries local vehicular traffic without further alteration.

State highway department engineers began designing concrete arch bridges as early as 1908, but, unlike its practice on other structural types, the agency did not develop a standard concrete arch design. The bearing and superstructural conditions were too site specific, MSHD rationalized, making standardization of concrete arches impractical. Instead, the highway department used special-design concrete arches up to 80-feet in length "wherever it is possible to secure sufficiently hard foundations, and also where there is clearance enough for water to flow freely without the arch choking the stream too much", according to its Seventh Biennial Report. In 1914 MSHD built its first state arch for Trunk Line Bridge NO. 3 - a 50-foot span over Paint Creek in Kent County. As the trunk line system developed in the 1910s, MSHD built several more arches. By 1918 the agency had constructed twelve trunk line arches, ranging in span from 35 to 85 feet. Built in 1919-1920, the Carp River Bridge falls within this statewide construction trend. It is today distinguished as one of the few remaining arches from MSHD's earliest period: a handsomely proportioned exercise in highway bridge design. Although altered by the installation of a new guardrail, this alteration occurred within the bridge's historic period and has thus become a part of the structure's history. Built as an integral, formative part of one of the Upper Peninsula's most important routes, the Carp River Bridge is a significant transportation-related resource.