Specials / The Mighty Mac
The Mighty Mac: A Sublime Engineering Feat
By: Pete Sigmund
(Editor’s Note: This article is the eighth in an occasional series on iconic United States construction projects.)
Observing its 50th anniversary this year, the “Mighty Mac,” the strong, graceful, bridge, which spans the Straits of Mackinac in Michigan, is one of the nation’s greatest bridge-building achievements of the 20th century, uniquely conquering wind, waves and ice over its 5-mi. (8 km) span.
The Mackinac (pronounced mak-uh-naw) Bridge includes an 8,614-ft.-long (2,625.5 m) suspension bridge. This is the longest suspension bridge in the Western Hemisphere and the third longest in the world (after the 12,826-ft. [3,909.4 m] Akashi Kaikyo Bridge in Japan and the 8,921-ft. [2,719.1 m] Great Belt Bridge in Halsskov-Sprogoe, Denmark.)
The length of the entire Mighty Mac, including 30 truss span bridges joining the suspension bridge to land, is 26,242.7 ft. (7,999 m).
Designed to withstand an “infinite” amount of wind, plus other forces of nature, the Mighty Mac also has vastly improved commerce between Michigan’s Upper Peninsula and Lower Peninsula, which had previously been almost two separate states economically, politically and culturally.
“Linking the two peninsulas together has been by far the greatest accomplishment of the Mackinac Bridge,” said Bob Sweeney, administrator of the Mackinac Bridge Authority (MBA), the state authority in St. Ignace, Mich., which operates the stately structure. “It really opened up the Upper Peninsula to much greater tourism.
The logging industry has benefited greatly from it as well.”
As of Jan. 16, 2007, 137-million vehicles have made the crossing in fair weather and foul — and the bridge has remained open more than 99 percent of the time, averaging approximately one hour of closure per year.
Dubbed the Mighty Mac since its early days, the bridge also has drawn many more visitors to the Straits area, which became a premiere resort destination for the rich in the 1880s. It was long a dream of entrepreneurs including railroad tycoon Commodore Cornelius Vanderbilt who remarked as he opened the famous Grand Hotel on Mackinac Island in 1887, “We now have the largest well-equipped hotel of its kind in the world for a short-season business. Now what we need is a bridge across the Straits.”
The Algonquin Indians called the Straits and their surrounding area “Michilimackinac,” meaning “the jumping-off place” or “great road of departure.” Their trails met the shore and went no further. Because of the straits, the Indians moved primarily in an east-west, rather than north-south, direction.
French missionaries and fur traders made the area one of the most important locations in the emerging west, establishing St. Ignace and Fort de Buade on the Upper Peninsula where the land jutted into the Straits. In 1715, the French established a new stronghold, Fort Mackinac, on the south shore (present-day Mackinaw City).
The British occupied the area in 1761, surrendering it, and the rest of Michigan, to the United States in 1796. Mackinac Island, in the Straits, became the center of the fur trade with the northwest for the next 40 years, and the operations center for John Jacob Astor and his American Fur Company.
Americans gradually realized the “sublime views,” wonderful air, abundant fish and valuable iron ore and lumber in the straits area. However, even as the railroads came in the 1880s, followed by cars and better roads in the 1900s, everything still dead-ended at the Straits of Mackinac.
Water, Wind and Ice
Travel between the Upper Peninsula and Lower Peninsula was difficult, sometimes impossible, without a bridge. The Straits were often locked in ice during the winter, so one walked, rode by horse, or took a sleigh or dog team. Some travelers froze to death trying.
The state of Michigan first tried to handle the increasing demand with eight ferries. Sometimes as many as 800 waiting cars were lined up for 5 mi.
As the delays got worse, people got serious about a bridge. Completion of the Golden Gate Bridge in San Francisco in 1937 gave planners greater incentive.
The obstacles, however, were intimidating. In 1820, Henry Schoolcraft, a member of a scientific expedition, which the War Department sent to study the Straits area, noted cavities in the rock. Some geologists in the 1930s said this rock couldn’t support the weight of a bridge.
The north-south bridge also would have to stand broadside to winter gales, which swept in from the Great Lakes as if in a wind tunnel. (Wind speeds actually reached 124 mi. per hour in May 2003.) Not least, the bridge would have to withstand swirling currents and tremendous pressure from ice, which could be eight or more feet thick in the shipping channels.
The entire bridge, including truss spans, also would have to be 5 mi. (8 km) long.
Quite a few engineers said it couldn’t be built.
Then there was the cost.
The first bridge authority, created in 1934, made a bid for depression-era federal financing. It estimated a total cost of $35 million. The government rejected the bid.
Former governor Chase Osborn, who had once opposed the bridge idea, vehemently supported it by 1935.
“Suppose on a trunk line — and the Straits road [bridge] is that — there was a mud hole or chasm or abyss or sink-hole eight miles wide that every car had to be pulled over or across or through,” he wrote that year. “Something would be done about that at once.”
Osborn articulated a vision of the economic development, which would result from joining the Upper Peninsula with the northern part of the Lower Peninsula.
“Michigan is unifying itself,” he wrote, “and a magnificent new route through Michigan to Lake Superior and the Northwest United States is developing, via the Straits of Mackinac. It cannot continue to grow as it ought with clumsy and inadequate ferries for any portion of the year.”
Many also recognized that a bridge would spur tourism, Michigan’s second most important industry. W. Stewart Woodfill, manager of the Grand Hotel, closed the hotel at the end of the 1949 season and lobbied for the bridge during the winter, 1950, session of the state legislature in Lansing.
It paid off. In 1950, the legislature created a new publicly owned Mackinac Bridge Authority and appropriated funds for preliminary studies and surveys.
Results showed it wouldn’t be cheap. On Dec. 17, 1953, bids were accepted from a group of underwriters for the sale of $99,800,000 in bridge authority bonds, which investors throughout the United States subsequently purchased.
The first construction contracts were awarded in February 1954.
Major Construction Feat
Construction of the Mackinac Bridge officially began on May 7 and 8, 1954, and was completed in just three years, opening to traffic Nov. 1, 1957.
Designed by the noted Engineer David B. Steinman, who was once a newsboy near the Brooklyn Bridge, the Mighty Mac graces a beautiful region. Its 8,614-ft.-long suspension span between anchorages surpasses The Golden Gate Bridge’s 6,450-ft.-long (1,966 m) suspension span, though the Golden Gate’s central span is longer — 4,200 ft. (1,280.2 m) compared with Mackinac’s 3,800 ft. (1,158.2 m).
Merritt-Chapman & Scott won a $25.7 million low-bid contract for 33 marine foundations. It mobilized the largest marine equipment fleet in peacetime history for the job.
The American Bridge Division of United States Steel Corp. won a $44.5-million contract to build the superstructure.
A total of 3,500 bridge workers, 350 engineers, and 7,500 others in quarries, mills and shops worked through good and bad weather to accomplish the feat in three years.
How They Did It
“The way the bridge is built, with a North-South orientation, means the wind comes straight through the Straits and hits it sideways,” Sweeney said. “The wind speed, current in the Straits, and the massive ice, which builds up were three of the most challenging features in building the bridge. The bridge can withstand an infinite amount of wind in the summer. During the winter, we assume that several features of the bridge are plugged with ice and snow, reducing the wind-failure speed down to just more than 600 miles per hour.
“Ice in the channel, and just outside the channel, builds up to about eight feet thick. That was one of the reasons why massive caissons around the two main tower piers are approximately 10 feet below the surface, allowing the ice to move back and forth without putting pressure on the caissons.”
To produce a titanic bridge withstanding these forces, Steinman designed special features in suspension bridge aerodynamics. The bridge is absolutely windproof in terms of stability against all types of oscillations. (see A Critical Wind Velocity of Infinity page 40.)
For strength, the bridge protects and strengthens support piers with caissons and cofferdams. (Extensive geological studies, and load tests on the rock under water at the construction site had shown that the rock was not weak because of cavities. Results showed, in fact, that even the weakest rock could support more than 60 tons [54 t] per sq. in., more than four times the greatest possible load from the bridge structure.)
“The two anchorages [holding the cables at each end of the bridge] and the two main towers were most critical to meeting the bridge’s timeline, so the contractor focused on completing these piers first,” Sweeney told Construction Equipment Guide.
Merritt-Chapman and Scott used stacks of double-walled circular caissons for these foundations, which went down to solid rock 210 ft. (64 m) below the surface. Each watertight caisson section, 44 ft. (13.4 m) high and 116 ft. (35.4 m) in diameter, grout tubes were placed 20 ft. (6 m) apart from the surface of the water to the bottom. Aggregate was then placed around the pipes to a height of 40 ft. (12.2 m) on each section so that the interior of the caisson was solid, like concrete. The caissons, with tapered leading sections, could cut into the overburden at the bottom.
Most piers, however, used a variation of caisson called a cofferdam. Steel sheets and pilings (sheetpiles), 2 to 4 ft. (.6 to 1.2 m) wide, were driven down to bedrock and connected together. After digging out the overburden with clambuckets, workers put in reinforcement steel, placed grout tubes 20 ft. apart, and surrounded the tubes with aggregate, likewise to a height of 40 ft. for each section.
The two foundations in which the cables are anchored measure 115 by 135 ft. (35 by 41.1 m), approximately one-third the size of a football field.
Indicating the massive underwater construction — 750,000 lbs. (340,194 kg) of the 1 million tons of concrete and steel in the “Mac” are underwater.
Construction of the towers themselves began on July 2, 1955.
Merritt-Chapman and Scott built a 50-ft. high (15.2 m) trestle on top of each anchor foundation and then raised a crane to the top of the trestle to handle support frames and other material. Each of the two towers, composed of 40-ton (36.3 t), 16-ft. high (4.9 m) steel sections fabricated in Ambridge, Pa., is 552 ft. (168.2 m) above the water. Sections were joined with a total of 6 million rivets and bolts.
Spinning the Cables
Once the towers were completed, contractors built a catwalk for the cable spinners by unreeling 2.25-in. (5.7 cm) wire rope from a barge, pulling the ropes over the tops of the towers, and covering them with a cyclone wire fence.
A spinning wheel, working from 16-ton drums at the anchorages containing 320,000 ft. (97,536 m) of wire, then laid down four wires at a time to form a strand of 340 wires. Thirty-seven of these strands formed a 24.25-in. (61.6 cm) cable for the suspension bridge. Total length of the wire in the main cables is 42,000 mi. (67,592 km).
Besides the suspension span, the Mac includes 30 truss span bridges, some approximately 600 ft. (182.9 m) long. Once all these spans were built and joined, deckwork was completed, with 6 in. (15.2 cm) of concrete topped with 2 in. (5 cm) of blacktop. The 54-ft.-wide (16.5 m) roadway provides two lanes in each direction.
This year, 50 years after the Mighty Mac opened to traffic, is a time to celebrate this construction achievement. The celebration on July 27 to 28 (in the summer tourist season) will include parades in St. Ignace and Mackinaw City, plus fireworks. Celebrations also will mark the actual anniversary on Nov. l.
For more information visit www.mackinacbridge.org.