The significant engineering feature of this bridge is the steel box girder suspended span. This was the second bridge constructed in the United States with a steel box girder span, and one of the first five or six bridges of-its type built anywhere in the world.

The Portage Canal Bridge was the only structure of this type built in Washington between 1951 and 1960. It was designed in 1950, constructed in 1951, and opened to traffic in January of 1952. Its 140-foot steel box girder suspended span is significantly longer than the similar 100-foot span used on the Patlon Bridge. Use of the lighter steel box girder suspended segment in the central part of the main span allowed the designer to extend the length of that span. This resulted in a three span, rather than a five span bridge, eliminating the need for and cost of two additional piers.

The steel link and hanger connections between the steel span and the end of the concrete box girder cantilevers were a departure from the support seats’ provided on the Patlon Bridge. This bridge design was a second pioneering step by Hadley in demonstrating the potential use of steel box girders for longer span bridges. The bridge is unaltered, except that in 1997, Dwydag earthquake restrainer bars were installed under the roadway deck overhangs across the interfaces between the steel and concrete box girder section.