7th Street Bridge

Information and Findings From MDOT

The Seventh Street Bridge is the only single-leaf bascule bridge in Michigan. Although it has experienced some alterations, the bridge's historical integrity remains essentially intact. The bridge qualifies for the National Register as significant engineering work.

In December 1928, the city commission of Port Huron began planning for a new bascule bridge over the Black River at Seventh Street. The structure at this crossing, which was at least fifty years old, was obsolete, according to Mayor John J. Bell: "The bridge is obviously too narrow. . . . Two automobiles can barely pass on the bridge without touching. We must remember that the old horse-and-buggy days, when this bridge was built, have passed." The vice president of the Strauss Engineering Company, a Chicago firm that specialized in bascule design, volunteered concept plans for a new bridge at the commission's December meeting. The estimated cost of the structure was $240,000. The commission anticipated that it would be necessary to issue bonds to raise that amount. Two significant problems delayed the project. The Depression roiled the city's finances, diminishing the potential for a successful bond sale. On top of that, an unfavorable interpretation of a state law called the legality of the bond sale into question. By June 1931, however, the legal issues had been straightened out and the city's treasury was on sounder footing. The city rushed ahead with the project, which was supervised by city engineer Earle R. Whitmore.

Despite the efforts of the Strauss Engineering Company to woo the city commission, the contract to design the bridge went to rival John Alexander Low Waddell, another prominent movable bridge engineer. Born in Ontario in 1854, Waddell received a degree in civil engineering from Rensselaer Polytechnic Institute in New York in 1875. After working as a draftsman in Canada and serving on the faculty at Rensselaer and the Imperial University of Tokyo, Waddell founded his own bridge engineering firm in Kansas City, Missouri, in 1886. Around 1920, he moved his headquarters to New York City. He had several business partners during the course of his practice. From 1927 until his death in 1938, Waddell worked with Shortridge Hardesty, who continued the practice of Waddell and Hardesty thereafter.

Waddell discussed the development and varieties of bascule design in his comprehensive two-volume work Bridge Engineering, published in 1916. He described three types of bascule structures: trunnion, rolling lift and roller bearing. In a trunnion design, the end of the leaf pivots around an axis, the trunnion, that essentially remains in a fixed position. In rolling-lift bascules, by contrast, "the centre of rotation continually changes and the centre of gravity of the rotating part moves in a horizontal line." The movement of roller-bearing bascules is similar to trunnion spans, but there is no trunnion to serve as an axis. Instead, the load moves on rollers on a curved track. Waddell noted that "more bascules of the trunnion type have been built than of the other types." Trunnions could be installed in a variety of soil conditions, whereas the movement of a rolling lift required a bedrock foundation.

Waddell's trunnion design for the new Seventh Street Bridge was apparently completed by the summer of 1931. The city closed the old bridge to traffic on 22 July, and awarded construction contracts three days later. Three days after that, the demolition of the old bridge was underway. By November, the coffer dam was in place. Willits Brothers, the contractor for the substructure, completed its part by the beginning of the new year. The Wisconsin Bridge and Iron Company then set to work on the superstructure. Most shop drawings had been prepared in August and September 1931, and fabrication apparently followed in the fall and early winter. The drawings indicate that the trunnions had a forged nickel steel finish. Over the years, Waddell had experimented with steel alloys, particularly nickel steel. In Bridge Engineering, he explained that "the addition of the nickel increases the strength and the elastic limit of the metal."

In July 1932, the city officially accepted the completed structure. A large plaque on the operator's house lists the members of the city commission at the time that the bridge was dedicated: Mayor Fred J. Kemp, Commissioner of Streets and Public Improvements Charles D. Rettie, Commissioner of Public Safety Colonel C. Lincoln Boynton, Commissioner of Finance Thomas H. Molloy, and Commissioner of Parks and Public Property William Robertson.

The engineering firm Hazelet and Erdal, successor to the Scherzer Rolling Lift Bridge Company, drew up plans to replace the bridge deck with an open-grid steel floor in 1961. The company also drafted plans for some repair work in 1963, and for a major renovation in 1970. Contractor K. G. Marks won the contract for the latter project with a bid of $310,230. The project included cutting a pedestrian tunnel through the north approach to connect an existing walkway in the park to the east with a planned walkway and mooring area to the west. At the same time, new aluminum railings were installed and the operator's house was significantly remodeled. In 1974, the bridge was out of service while workers repaired the upper end of the rack support, erected a new anchorage at the top end of the rack, and completed other repairs. Work in 1993 included replacing deteriorating concrete at the bridge seat and along the north wall.

An engineering report in February 1996 recommended replacement of the sidewalk and the brackets that support it. The study also called for installation of a new electrical control system and other machinery, and the addition of a bathroom that would be more convenient for bridge tenders than the existing facility, which is beneath the roadway. The engineers also advised removal of the exterior mortar coat, which is cracking.

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