This landmark bridge provides the only vehicular and pedestrian access to Belle Isle, which has traditionally been the premier public park of Detroit.
This
crossing has an interesting history. The first bridge was a bridge composed of
pin-connected through truss spans. One of the spans was a through truss
swing span. The bridge was completed in 1889. The photo to the left, from the
Library of Congress, shows the ornate portal this bridge had. In 1915, the
creosote blocks that formed the deck of the bridge caught fire sending the
bridge up into a blaze. The heat was so great it affected not only the metal of
the truss, but the stone piers as well, and caused the bridge to collapse.
Following the collapse, a temporary bridge was completed in July 1916. Primarily a simple timber structure, it also included a steel bascule span to accommodate boats. The Scherzer Rolling Lift Bridge Company of Chicago had presented the city with two options for a temporary bascule (using the company's patented rolling lift design): a single leaf truss bascule, and a double leaf deck girder bascule. It appears the city selected the deck girder bascule.
Detroit
also began planning for a new permanent bridge after the truss bridge collapsed.
The planning process involved a lot of effort, discussion, and debate. Some
proposals were for a traditional concrete arch bridge, and even included a
proposal by Daniel Luten, who was a noted Indiana engineer and patent-holder for
a design of reinforced concrete arch bridge. Architect, city planner, and co-author of the famous Plan of Chicago, Edward Bennett, produced for Detroit an early proposal for a concrete arch bridge to Belle Isle in 1915. Historical articles and reports refer to some drawings and plans that are different from what the final product actually ended
up being showing that the debate was highly publicized.
However, Detroit was uncomfortable with a traditional concrete arch design because of concerns the soil conditions would not support the thrust forces from such a bridge.
In response to the soil concerns, a proposal for an exceedingly rare and unusual type of bridge was proposed that would visually look like a concrete arch bridge, but would in terms of function act as cantilever arms extending from each pier. This type of bridge would transmit forces to the piers in a way that would be compatible with the soil conditions. Even this initial design was not without debate, discussion, and revision, as the initial proposal was so expensive that their were concerns about the cost. The final product itself was designed by a University of Michigan professor, Lewis Merritt Gram. Gram also was involved with the initial design of the Michigan Stadium.
This is an extremely long multi-span concrete arch bridge that
gives people access to Belle Isle, and the structure is the longest arch bridge
in the state of Michigan.
This bridge is extremely significant not only because of its length, but because it is a very early example of a cantilevered concrete arch. A cantilevered concrete arch does not function like a traditional arch. In fact, many engineering articles discussing this bridge are careful to NOT call the bridge a concrete arch, and instead describe the bridge as an arch-shaped cantilever bridge. Traditional arch bridges require the arch to be a complete and connected structure to function and they have forces that thrust into the piers and abutments, something this bridge was specifically designed to avoid. With this bridge, each half of each arch span is a cantilever arm that is structurally independent from the other half of the arch in that span. Standing under a span of this bridge, a clear gap at the center of the span is visible. Indeed, on the outermost part of the arch, a decorative "keystone" was placed for aesthetic reasons to cover up this gap. Above this gap, a short concrete suspended span connects the two cantilever arms. Period texts described this bridge's units of two arms extending from each pier as "umbrellas" and this description might help to visualize how the different parts are arranged.
The internal reinforcing for this bridge is very complex, reflective of the unusual cantilever design. While traditional rebar is a part of the bridge's concrete, at the core of the bridge's cantilevered arch ribs (the cantilever arms) are heavy riveted steel trusses. Even more unusual, an eyebar stay system connects the end of these cantilever arms back to the top of the piers, adding strength to these cantilever arms.
The
ornamental sidewalk railings on this bridge are either original or they
replicate the original design. Today, vehicular traffic is kept at bay via modern New Jersey
barriers, which MDOT mentions were added in 1985. Obviously, this bridge goes
over the side of the Detroit River that freighters do not use, which accounts
for why a low-clearance bridge was built here. The earlier bridges at this
location required a movable bridge for boats, but apparantly by the time this
bridge was built, large boats were only using the channel on the other side of
the island.
A 1918 copy of a book entitled "Concrete engineers' handbook: data for the design and construction of plain and reinforced concrete structures" by George A. Hool et al. lists several bridges using this design, including an additional unidentified example in Wayne County, and the Hopple Street Viaduct in Cincinnati (now demolished). As such, the Belle Isle Bride may not be the second example of its kind as sometimes is claimed, but is still an early example, and is without a doubt the largest and most technologically complex example of a concrete cantilever deck arch bridge. The other large example is the