This large bridge is one of the most visually unique structures in Minnesota. It consists of a large rainbow arch main span over the navigation channel. An extensive approach system is also present. Next to the rainbow arch span are deck arch spans. North of the rainbow arch spans are three open spandrel deck arch spans which are unusual because they are not ribbed arches, like most open spandrel arches are, and instead have a solid concrete arch barrel. South of the rainbow arch span is four open spandrel deck arch spans, but unlike the spans north of the rainbow arch, these spans are ribbed. South of the southern arch approach spans are also a series of final spans that have been replaced with modern pre-stressed concrete spans. The railings on this bridge have been replaced, but they follow the original design. Otherwise, this bridge appears to retain good historic integrity.
With room for four traffic lanes between the rainbow arch ribs, this bridge is wide, and to accommodate such a large deck with only two arch ribs required that the arch ribs of the rainbow arch be very massive. Another interesting note about the rainbow arch is that despite the fact that the arches soar quite high above the roadway, there is no overhead bracing between the arch ribs. What is most unusual about the rainbow arch span however is its reinforcement. It is not reinforced with reinforcing rods as was common by the time this bridge was built. Instead, the arch rib has a built-up steel arch encased within the concrete to strengthen the arch rib. The hangers are also steel beams encased in concrete. This is similar to Marsh arch bridges of the early 20th Century, and could also be compared to Melan deck arch bridges which are also similarly reinforced. However, these methods were largely abandoned by the time the Robert Street Bridge was built. It may be due to the length of the span and the width of the roadway, that a solid steel arch for reinforcement was seen as a better way to accommodate the size of the span. To add to the unusual design of the rainbow arch span, the floor beams for the bridge are also steel, but they are not encased in concrete and function as all-steel floor beams. There also is some steel bracing between the arches under the deck that is also not encased.
This is a bridge that was designed to look beautiful, and in true opposition to modern bridges of today it achieves its beauty through a careful design of the actual load-bearing structures. The shape of the arches, and the detailing in the arches was carefully designed to look nice. The way in which the bridge would cast shadows was even considered. Superficial decorations are limited to a couple nice looking bas relief sculptures on the rainbow arch pier. The reason this bridge looks so nice is because architects and engineers worked together closely through all aspects of design. The bridge was also challenging to design, since it was designed to accommodate street cars, and had to be built over a navigable waterway, while the approach spans had to work around railroad lines under the bridge.
This bridge is a fixed bridge that achieves enough clearance to not require a movable span for boats. The same cannot be said for the adjacent railroad bridge which is a vertical lift bridge. Interestingly, the railroad bridge's approach spans pass under this bridge.
Be sure to view the Historic Bridge Management Plan for this bridge which provides additional details about the history and design of this bridge.
The June 1923 issue of Iowa Engineer had a biography of Donald B. Fegles who was vice president of Fegles Construction Company at the time. The text is as follows:
FEGLES SPEGIALIZES IN MILLS AND ELEVATORS
Donald B. Fegles, C. E. '05, who is now engaged in the construction business has made a success of his engineering career through the medium of agriculture as well as engineering by building grain elevators. Ever since his graduation trom Ames in 1905, Mr. Fegles has been connected with this specialized form of engineering construction and he is now vice-president and general manager of the Fegles Construction Co. of Minneapolis, Minn. At the present time this company has under construction a million dollar mill for the Pillsbury Flour Mills at Buffalo, N. Y. and within the past few weeks they have completed a flour mill and elevator for the same company at Atchison, Kan. Other recent projects that this company has constructed are a terminal elevator at Brooklyn for the New York State Barge canal, a terminal elevator for the State of North Dakota at Grand Forks and an elevator for the Western Terminal Elevator Co. at Fort William, Ont. Mr. Fegles in a letter to Dean Marston states, "It is very reassuring to note the number of people we are building for a second or third time. We have noticed recently a considerable slackening off of interest in building due to sharp advances in construction costs." Besides grain elevators this company builds warehouses and other heavy engineering structures.
This bridge is tagged with the following special condition(s): Marsh
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