50 Years Strong
This year marks the 50th anniversary of the Mackinac Bridge, a marvel of engineering and construction spanning the Straits of Mackinac to link Michigan’s two peninsulas. Many of the innovations introduced by Chief Engineer David B. Steinman remain the standards in bridge building today.
The Mighty Mac measures 26,372 feet, from Mackinaw City in the Lower Peninsula to St. Ignace in the Upper Peninsula. The roadway of the bridge is suspended from steel cables strung between two concrete-and-steel towers, and anchored at both ends in 170,000-ton concrete blocks that descend to bedrock 100 feet below the water and rise 119 feet above the surface. Each of the two cables is composed of 37 strands of 340 wires that each measure 0.196 inches in diameter, for a total diameter of 24.25 inches.
The piers securing the bridge remain the deepest structures ever sunk in suspension-bridge construction. Nearly 750,000 tons of concrete and steel are submerged in places to a depth of 230 feet.
According to Pat Zacharias, a Detroit News librarian, the young Steinman was a newsboy in the Brooklyn Bridge neighborhood who boasted to his friends that he was "going to build bridges like the famous structure that towers above us. They laughed at me." By the age of 63, Steinman had designed 400 bridges around the world before he tackled the Mackinac.
The span between the two towers measures 3,800 feet. Currently, this central suspended span is the third longest in the United States, behind New York City’s Verrazano-Narrows Bridge (4,260 feet) and San Francisco’s Golden Gate Bridge (4,200 feet).
The total suspended length of the bridge is 8,344 feet — longer than any other in the Western Hemisphere. Worldwide, it is only surpassed by the Akashi-Kaikyo Bridge (12,831 feet) that connects the Japanese cities of Honshu and Shikoku. When taking into account the approaches leading to the suspended span, the Mackinac Bridge is considerably longer, at 5 miles, than the 2.4-mile Akashi-Kaikyo.
Bridge workers toiled without safety harnesses or nets, according to Ms. Zacharias. The dangers claimed five lives during the 42-month construction, between May 7, 1954 and Nov. 1, 1957.
The Mackinac Bridge was the world’s first to incorporate Steinman’s principles of aerodynamic stability. He developed this set of principles several years before the spectacular collapse of the Tacoma Narrows Bridge in 1940 — a failure Steinman predicted after its builders ignored his advice.
Designing a bridge to cross the Mackinac Straits posed three significant challenges: high winds, deep water and the extreme pressures of ice accumulations. To deal with the wind, Steinman utilized open stiffening trusses in place of solid sheets beneath the roadway to increase stability, thus improving the critical wind velocity from 40 mph to 632 mph. Steinman further improved bridge stability by using an open grid road surface to further increase the critical wind velocity to what he described as an "unprecedented level of essentially infinity." His formulae and treatise on aerodynamic stability remain the world’s standard.
To protect against ice pressure, Steinman designed the towers’ concrete piers using a most-secure safety factor of 20 (20 times the maximum ice pressure achieved under laboratory-controlled conditions). The pressure of ice buildup can render a bridge structurally unsound — if not cause it to collapse. The maximum ice pressure ever encountered by bridge engineers is 21,000 pounds per lineal foot of pier width. In the laboratory, the greatest ice pressure achieved was 23,000 pounds per lineal foot. Steinman arrived at his safety factor of 20 by multiplying the maximum possible ice pressure of 23,000 pounds by five and dividing the safe foundation pressure by four. For further safety, each of the concrete piers was sheathed in armor plate above and below areas of potential ice contact.
To sink the concrete piers in bedrock at depths of more than 200 feet, engineers on site designed and built an ingenious pile driver they nicknamed "The Gismo." The project also involved some of the deepest "cofferdams" — submerged air-filled work areas — ever constructed.
Steinman also introduced "Prepakt" concrete pourings to unequaled depths. Developed in the 1950s, Prepakt was better suited for work underwater, where conventional concrete is difficult to use. The use of Prepakt in construction of the Mackinac Bridge established a world record for the underwater consolidation of concrete placement, which was set in May 1955 when 200,000 tons of concrete was poured into the bridge’s foundations.
The innovative design and construction techniques devised for the Mackinac Bridge have been emulated many times since 1957, when the Mighty Mac was completed. Fifty years later, the bridge still stands as an engineering marvel and an emblem of our state.
Henry Fleischer, P.E., CMf.g.E, retired as vice president of Numatics Inc. in Highland, Mich., in 2001 and continues to consult for the company. Mr. Fleisher has authored texts on fluid dynamics as well as numerous articles for technical journals. He was named Michigan’s Outstanding Engineer in Industry by the Michigan Society of Professional Engineers in 1985 and received the group’s Clovis Key Award the following year. He served in the U.S. Army Corps of Engineers during World War II.