Monday, March 30, 2015

The Mighty Mackinac

This is a short article I read on Facebook about the Mackinac Bridge. It's a direct copy from the posting. (Thanks Midget Robin!)

For those who don't know, the Mighty Mackinac is the span of I75 that crosses the Straights of Mackinac, which passes water from Lake Michigan to Lake Huron. The bridge leaves Lower Michigan from Mackinac City, and lands on the Upper Peninsula of Michigan in St. Ignace.

Why the metal grating on inside lanes of The Mackinac Bridge? Simple - it wouldn't survive harmonic wind stresses without it.

Mackinac Bridge January 1953, Steinman was appointed as the design engineer for the Mackinac Bridge, and his recommendations were incorporated into its design. Representing a new level of aerodynamic stability in suspension bridges for its time, the Mackinac Bridge was the first long-span suspension bridge to incorporate specific design features, including a porous deck, to manage the forces imposed on it by winds. Construction of "Mighty Mac" took over three years in a demanding climate, and the structure's completion made all-weather travel between Michigan's two regions possible.

Claim to Fame: Representing a new level of aerodynamic stability in suspension bridges for its time, the Mackinac Bridge was the first suspension bridge to incorporate specific design features to manage the forces imposed on it by winds.

The design of the Mackinac Bridge was directly influenced by the lessons of the first Tacoma Narrows Bridge, which failed on November 7, 1940 due to instability under wind stresses. Three years after the Tacoma Narrows disaster, engineer David B. Steinman published a theoretical analysis of suspension-bridge stability problems. Among his recommendations were that future bridge designs include deep trusses to stiffen the bridge deck and an open-grid roadway to reduce its wind resistance.

Prior to the construction of the bridge, a fleet of nine ferries would carry as many as 9,000 vehicles per day, with traffic backups stretching as long as 16 miles.

In New York the Verazzano-Narrows Bridge, designed by Othmar Amman, was 10 years in the making and finally opened in November 1964.

Both of these monumental spans directly benefited from the legacies of the failed 1940 and the successful 1950 Tacoma Narrows Bridges.


And there folks, is the history lesson for today that most people don't know...

Enjoy, and Thanks for Sharing!! - - U.P. Michigan