A Look at History
Bright Lights on Mackinac Bridge Were Design Innovation
BY FRANK STRAUS

This summer, August 2006, marks the 50th anniversary of the spanning of the first cable from anchorage to anchorage across the towers of the Mackinac Bridge. This first wire rope was a little thicker than a rope in size and weight; from this strand of steel was slowly spun, in 1956-57, the thick cable that we see today. The bridge deck was, in turn, hung from these cables. The 50th anniversary of the bridge itself will not be celebrated until next year, 2007.

The work of "spinning" (actually unreeling and re-spinning) the bridge cables was so slow, and so labor-intensive, that it had to be carried out 24 hours a day. One of the first things electricians did after the first cables were hung was to string lights from the preliminary

cables, so that the workers could see what they were doing at night. From the start, the Mackinac Bridge's cables had to bear their own weight. They swooped toward the surface of the Straits of Mackinac in a curve called a "catenary" - familiar to almost all of us from seeing high-voltage electric cables carrying power across the countryside.

Larry Rubin, the bridge project's chief operating officer, later recalled that during the final week of July 1956, bridge designer David B. Steinman was visiting bridge authority chairman Senator Prentiss Brown at Brown's getaway cabin on Marquette Island, northeast of Mackinac Island. It was a clear night, and from this site the lights of the construction cables showed clearly over the water. The two men apparently realized that the installation of permanent lighting would enhance the appearance of the massive structure. Rubin was asked to scrape together $100,000 from the construction project for this purpose, which he did. The powerful cables could easily bear the weight of a few additional electric lights.

Cable lights on the Mackinac Bridge were supplemented with special Michigan sesquicentennial lights in 1987, marking 150 years of statehood. (Photograph by David Latva)

In 1956, most of the older suspension bridges of the United States were dark at night, except for traffic lights and road lighting. For example, the vermilion-red towers of the Golden Gate Bridge could not be seen at night. The decision to light the Mackinac Bridge was an innovation in bridge design, and it was made while construction was still taking place.

As the cables were slowly spun, corrosion proofing was applied to lengthen the bridge's lifespan, as the builders knew the Mackinac Bridge could not last longer than the working life of the cables that it would hang from. (The Silver Bridge, a suspension bridge over the Ohio River at Point Pleasant, West Virginia, would come down in December 1967 after the builders failed to galvanize the bridge's cables.) This was one of the slowest segments of the entire project. Each massive cable contained 12,580 separate, pencil-thick wires. The wires were grouped into bundles and wrapped; then the bundles were grouped together into the cylindrical cables, and wrapped again.

The two boxy concrete "anchorages," which are one of the most obvious features of the bridge to a Mackinac Islander looking at the structure from the side, are the locations where these cables are anchored to the lakebed below the water's surface. As with the tower bases, huge cofferdams had to be built in the open lake so as to enable shovelers to excavate these anchorage sites down to bedrock.

With each of the thousands of individual steel wires required to span a distance of more than 8,600 feet between concrete anchorages (the longest anchorage-to-anchorage distance in the world in 1957), the bridge's main cables as spun and wrapped contained 42,000 miles of wire - a distance long enough to circle the entire planet more than one and a half times.

With the cables completed by early 1957, a different series of workers began their job. Segments of bridge deck seemed to extrude themselves from each anchorage and tower, hanging from the cables and reaching toward each other. The weight of the bridge deck changed the curves of the cables from catenary curves into parabolic curves. The beauty of curves derived from conic sections has been familiar to every mathematician since the time of the ancient Greeks, but most of us are familiar with parabolic curves from homely tasks of our own, such as stringing a rope from tree to tree and hanging laundry from it.

During this final construction period, other construction lights floodlighted the bridge's two main towers, each rising 552 feet above the water's surface. While the wrapped main cables were painted dark green, the towers were painted a slightly creamy white, called "ivory." The ivory-white towers shone in the nighttime floodlights, as they continue to do to this day.

Following the bridge's actual opening to traffic on November 1, 1957, and its dedication in the early summer of 1958, the tower and bridge cable lighting program's purpose changed from construction to aesthetic beauty. Prentiss Brown's insistence that the bridge be lighted permanently had paid off.

Since 1958, the bridge's light combinations have changed repeatedly over the years. Rubin, a graduate of the University of Michigan, explained that the cables' original yellow lights were supposed to contrast with the bluish mercuryvapor lights that lighted the bridge deck, the yellow and blue together making a tribute to the Wolverine colors. When red aircraft-warning lights were fixed atop the towers, a further element was added to the potential color combinations. Many Islanders will remember the red, white, and blue lights installed by the Mackinac Bridge Authority in honor of our country's bicentennial in 1976.

For many years the Bridge Authority has chosen a red, blue, and yellow color scheme for the Mackinac Bridge's cable lights. The blue mercuryvapor lights continue to illuminate the deck's driving surface, and for mariners and Mackinac Islanders, a thin line of dark red lights outlines the lower side of the bridge's deck. Interestingly enough, red, blue, and yellow are the primary colors on the State's great seal and flag.

Today, suspension bridges all over the world have permanently lighted cables. With many modern bridges, provisions are made in the bridge's design for permanent cable lights; in some cases, the cables or towers are being relighted some years after the bridge was originally built.

When the Mackinac Bridge was built in 1957, the other two long suspension bridge spans in the world were the George Washington Bridge and the Golden Gate. For decades, both bridges were proud of their distinctness from the Mackinac Bridge. The Golden Gate's bridge towers were lighted in 1987. In February 2006, the authority that operates the George Washington Bridge announced plans to install floodlights and re-light the New York bridge's towers.