Seven Mile Bridge: Construction, History & Facts

Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

The Seven Mile Bridge is a symbol of perseverance and ingenuity thanks to its location. In this lesson, we'll look at the long history of this bridge and see how people are able to use it today.

The Seven Mile Bridge

The Florida Keys are absolutely spectacular. With the sparkling waters, soft sand, and warm sun there's really just one problem: they're a little hard to get to, or at least, they used to be. The history of transportation construction is really the history of figuring out how to get stuff to places that are hard to reach, and the Florida Keys presented a constant challenge. Today, the legacy of this long story is embodied in the Seven Mile Bridge, a part of the 110-mile long Ocean Highway that connects the remote Keys to the mainland.

The Seven Mile Bridge
Seven Mile Bridge

The Old Bridge

The history of the Seven Mile Bridge is really the story of two bridges. The first begins with railroad tycoon and partial founder of Rockefeller's famous Standard Oil Company, Henry Morrison Flagler. Flagler saw southern Florida, still largely underdeveloped in the late 19th century, as a great business opportunity and sunk his fortune into building up St. Augustine, Miami, and Palm Beach. To connect Florida to other industrial networks, he envisioned a grand railroad, eventually known as the Florida East Coast Railway.

By the early 20th century, the railway had become tremendously successful, and was looking to expand south from mainland Florida and into the remote Florida Keys (then only accessible by boat). The goal was to be able to get American products as far into the Caribbean as possible and then into the newly announced Panama Canal. From 1909 to 1912, Flagler oversaw the construction of the Oversea Railroad, which was mocked as an engineering impossibility in its day. Flagler, of course, had the last laugh after his railroad was completed and heralded as the 8th Wonder of the World.

The most impressive stretch of this railway was known as the Seven Mile Bridge, one of the longest segmented bridges in the world. In 1935, however, it was badly damaged in a hurricane and sold to the United States government. The government rebuilt and widened it for automobile use, and added a section that could open to allow for passing ships. This new road was also heralded as a marvel of modern engineering, built over the constant challenge presented by construction in even the calmest of waters.

The New Bridge

Unfortunately, the old bridge began to erode over time thanks to the constant exposure to salt water and tropical storms. After Hurricane Donna caused major damage in 1960, it became clear that a new bridge would be needed.

The new bridge, as seen from the old
A View from the Old Bridge

Construction began in 1978. The challenge with a bridge like the new Seven Mile Bridge isn't that the structure has to support its own weight over a deep chasm; it's that it stretches for miles with nothing to realistically anchor it to. So, many traditional types of bridges (from arch to suspension) were not viable. The Seven Mile Bridge is, therefore, a very simple box and girder style of bridge, in which a deck of concrete is supported by steel girders that distribute weight across dozens of columns.

It's a simple system, but maritime construction itself is never simple. To mitigate the challenges of building at sea, the bridge was created from precast concrete spans that were built offsite and assembled over the water. New engineers also made a few adjustments to make the new bridge more efficient. The Seven Mile Bridge is higher off the water than the old bridge, protecting it from waves, and features an elevated section tall enough for ships to pass underneath. This innovation eliminates the need for swinging or raising spans that open up, letting ships pass but also stopping the flow of vehicle traffic. It should be noted that there was one other slight change: while the new road is nearly parallel to the old there are some deviations. As a result, the new Seven Mile Bridge is actually only 6.8 miles long. Don't tell anyone.

The new bridge has some new features, like this elevated section
An Elevated Section

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