Tuesday, April 2, 2013

A Bridge Over Sometimes Troubled Waters

The original Big Mack, the Mackinac Bridge is a suspension bridge spanning the Straits of Mackinaw and links the 2 peninsulas (the unimaginatively named “Lower” and “Upper” Peninsulas) comprising the state of Michigan in the United States. The local Native Americans, the Algonquins, called these straits and the surrounding area "Michilimackinac", meaning "the jumping-off place" or "great road of departure".

Over time, the straits became an important area for trade and commerce. The clean air, abundant fish and beautiful views brought many visitors to the straits. Even though the area grew in popularity, ferries remained the only way to cross the straits.

A typical fleet of nine ferries could carry as many as 9,000 vehicles per day; cars would line up for as much as 16 miles (26 kilometers) to wait to cross on the ferry boats.

After more than 3 years in the making (and after some 70+ years of calls for its construction), the Mackinac Bridge was completed in 1957. Sadly, five workers died during its construction. The sheer size of the bridge is difficult to comprehend unless one sees it in person.

With a total length of 26,372 feet (8,038 meters), the bridge is currently the third longest suspension bridge in the world and is the longest suspension bridge in the western hemisphere. Including its anchorages, the length of the suspension bridge is 8,614 ft (2,626 m). The length of the main span (between towers) is 3,800 ft (1,158 m).

The height of the roadway at mid-span is approximately 200 feet (61 m) above water level, which allows the bridge to accommodate the passage of ore boats and other large seagoing freighter underneath it. The bridge spans an underwater chasm some 295 ft (90 m) deep, which forced the builders to scrap the original plans calling for a tunnel and to build a suspension bridge instead. The geology underlying the bridge is detailed in a report linked below in the Resources section.

All suspension bridges are designed to move to accommodate wind, change in temperature, and weight. It is possible that the deck of the Mackinac Bridge at center span could move as much as 35 ft/10 m (east or west) due to high winds. This would only happen under severe wind conditions (which happen during that time of the year called winter by locals). The deck would not swing or "sway" but rather move slowly in one direction based on the force and direction of the wind. After the wind subsides, the weight of the vehicles crossing would slowly move it back into center position. The design allows the bridge to withstand winds of up to 150 miles per hour (240 km/h).

The 1,024,500 tons (929,410,766 kg) of structural steel and concrete are held together with 4,851,700 bolts. If stretched end-to-end, the wire used in its support cabling would reach around the Earth more than 1 1/2 times, for a total length of 42,000 miles (67,592 km).

The Mackinac Bridge was featured in episode 59 ("Bridge Painter") of the show "Dirty Jobs with Mike Rowe" in 2007. Interested parties can cross by foot on the annual (yearly since 1959) Labor Day bridge walk (one span is shut down to motorized traffic, allowing pedestrian usage).

Visitors crossing the Mackinac Bridge from the Lower Peninsula use it as their jumping off point to explore the magnificent beauty of the Upper Peninsula. More than 100 million vehicles have made the crossing since its opening. You are welcome to follow them and depart on an exploration of your own…

Photo courtesy CSM Macker


Alternate photo (Wikimedia Commons user Dehk):

Satellite photo of icebreaker paths (the bridge is oriented vertically in the photo; photo by ISS Crew Earth Observations/NASA/Wikimedia Commons):

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