Duluth Aerial Ferry Lift Bridge

The Duluth Aerial Ferry/Lift Bridge was the first transfer bridge built in the United States and the only stiff-trussed transfer bridge ever constructed. The structure was subsequently converted into a vertical lift span, retaining the top span from the original transporter bridge.

A strip of land about nine miles long separating Lake Superior from Duluth Harbor Basin and Superior Bay, Minnesota Point is is the world’s longest freshwater sandbar. Duluth’s Aerial Ferry/Lift Bridge was built to fulfill a promise to the community of Park Point, which was isolated after the Duluth Ship Canal was cut in 1871 and the southern half of Minnesota Point became an island. In 1891, a vertical lift bridge was initially designed to reconnect Park Point, but the design was rejected then because the US Army Corps of Engineers feared that the liftspan could fail in its lowered position, blocking shipping traffic. 

Considered Duluth’s iconic landmark, the Aerial Lift Bridge was originally built from 1904 to 1905 as an aerial transporter bridge (also referred to as a “transfer bridge” or “ferry bridge”).  A gondola was slung from the top span and was ferried back and forth as needed to carry passengers and cargo across the ship canal. Its original design was inspired by an 1889 transporter bridge over the River Seine at Rouen, France. It is one of only two transporter bridges ever constructed in the United States (the other was built in Chicago for the 1933-34 World’s Fair and was taken down 18 months later.) The transporter bridge’s gondola had a capacity of 60 tons and could carry 350 people plus several wagons or automobiles. 
The Point’s popularity and the rise of the automobile led to the bridge’s obsolescence and its later conversion to a vertical lift bridge in 1929-30, an idea put forth by the residents of Park Point. In order to ensure that tall ships could still pass under the bridge, the top span had to be raised. The bridge spans 390 feet and clears 135 feet above the water level. The truss at the center is 51 feet, making the total height of the highest part of the bridge above the water 186 feet. The total weight to be lifted is almost 900 tons. 

The tower foundations consist of reinforced concrete on top of pipe piles cut off below mean low water. The piers were enclosed in steel sheetpiles and tied back with earth anchors in 1987. During the busy shipping season, the bridge is opened approximately 25 times per day. Duluth is the world’s largest inland port. Repurposed to meet changing needs, the bridge has been a tourist attraction in Duluth since it was first built. The bridge was designated by ASCE as a National Historic Civil Engineering Landmark in 2017.

REFERENCES: 
Brown, J.L. (2013), A Tale of Two Cities: Chicago, Duluth and Birth of Modern Vertical Lift Bridge, History Lesson, Civil Engineering, ASCE, June.
Young, F.A. (1977), Duluth’s Ship Channel and Aerial Bridge, How They Came To Be, In History of Minnesota Point, Stewart-Taylor Co.