The Mackinac Bridge, link between the Upper Peninsula and the Lower Peninsula of Michigan.

1956 photo of the building of the Mackinac Bridge.

Thanks to Mark Olsen for sharing this photo taken by his father!

MACKINAC BRIDGE

"The Mighty Mac" There is nothing quite as magnificent as the Mackinac Bridge on a clear sunny day. Crossing the bridge, you can see Mackinac Island in the distance and you may be able to see the famous Grand Hotel from your lofty position on the bridge. Other familiar sights might include a Great Lakes freighter, ferries carrying passengers to and from Mackinac Island, a parasailor, pleasure yachts, and young and old testing the waters of Lake Huron or Lake Michigan. What a place to be, this glorious UP!.

The Mackinac Bridge is one of the longest and strongest suspension bridges in the world, spanning the Mackinac Straits from the Upper to the Lower Peninsula of Michigan. The bridge measures 8,344 feet (2,543 m)between the main anchorages. Heavy pier foundations, the deepest 210 feet were necessary to resist the ice masses that accumulate every winter in the Mackinac Straits. The bridge was open to vehicle traffic in 1957.

The five-mile bridge, including approaches, and the world's longest suspension bridge between cable anchorages was designed by Dr. David B. Steinman.

Suspension bridges are designed to work with nature rather than against it. The Bridge might appear static but it is very much alive. It is a delicate instrument that carefully balances rigidity and flexibility. It is constantly reacting to the wind, the temperature, and the load it bears.

The bridge has four metal "wind tongues" on it's underside that rotate on pins. These allow the roadway to bow as much as 30 feet off-center to either side. In a heavy crosswind, this downward curvature of the bridge is easy for an approaching driver to see. In addition, the thick main cables and the thin cables that reach down and support the roadway, "suspenders", stretch in the heat, lowering the roadway about one inch per degree Fehrenheit. Likewise, the cables contract as the temperature grows colder, raising the bridge.

The middle of the roadway rises and sinks about 120 inches dependent on the temperature, using -20 degrees and 100 degrees. At the same time, the center of the roadway flattens out with weight. If you add together the effects of load and temperature, the center of the bridge moves up and down within a range of about 20 feet.

As the bridge gains weight and sinks, the main cables are drawn down, pulling the tops of the towers toward one another. The top of each 552 foot tower tilts up to 15 feet toward the center under a heavy load. Finger joints embedded in the roadway allow the roadway to expand and contract up to 27 feet along it's entire 5 mile length.