This is a bridge with a complicated history. In 1868, a new railroad bridge was built by the Keystone Bridge Company at Dubuque over the Mississippi River to replace an existing bridge. This bridge was called the Dunlieth and Dubuque Railroad Bridge. However, the westernmost spans of the previous bridge were reused and not replaced. A few years later in 1872 however, these approach spans were also replaced, again by the Keystone Bridge Company. In the 1890s, the bridge was replaced in a piecemeal fashion, a few spans at a time over a period of years, and at least two of the spans were salvaged and relocated to highways in Dubuque County. One span was relocated to Clay Hill Drive to Cross Little Maquoketa River in Dubuque County. The other span that was known to be relocated from the Dubuque Bridge also survives and is today over Bergfeld Pond. Another portion of the Dubuque Bridge made it all the way down to Vicksburg, Mississippi.
This bridge is an extremely significant historic bridge on multiple levels. Although the main truss members are all wrought iron, the bridge still makes extensive use of cast iron making it an exceedingly rare example of a bridge that uses cast iron for substantial parts of the bridge. Notably, the portal bracing and struts are all cast iron. Cast iron is also used for the rather complex connection assemblies. The bridge is an extremely rare example of a bridge that uses Keystone Columns, a patented design of built-up beam that is similar to the more common Phoenix Column. However, Keystone Columns do not have the curved design of Phoenix Columns, and there are gaps between the sections of the column. The gaps help prevent moisture from being trapped inside like what happens with Phoenix Columns, however, the gaps also allow dirt to build up in the bottom of the columns, which does trap moisture and leads to deterioration. These sorts of problems, as well as the complicated design of the columns in general mean that Phoenix Columns and Keystone Columns were not used for many years and were soon abandoned for more standard designs of built-up beams. However surviving bridges with these columns are significant as they document the creative designs that different companies came up with during the development of the metal truss bridge in the United States. Finally, this bridge is also significant as a surviving span from a long-lost Mississippi River Bridge. No bridges from this era survive on the Mississippi River aside from the Eads Bridge and so these surviving spans are a rare look at what 1870s Mississippi River bridges looked like.
This bridge also has a lot of beauty. The cast iron details add a pleasing complexity to the bridge, and the castings for the portal bracing have a lot of ornamental details.
Both of the spans that were moved to Dubuque County highways from the Mississippi River bridge have today been moved again onto trails where they serve pedestrians today. This bridge has been moved onto the Heritage Trail. It was reportedly moved here in 1992 and apparently the deck was added in 2011. Strictly speaking, it is not used as a bridge on the trail, but as a non-functional exhibit. It has been decked so that visitors can walk on it and admire the beautiful and complex details of this bridge. It is good that this bridge was saved from demolition by being moved here. That is a lot more than can be said for most historic truss bridges in Iowa. However, there are a few problems that were noted with the bridge that should be put on a to-do list. First, when the bridge was set on the ground here, not enough clearance was provided between the floor beams and the ground. When a truss bridge is being placed for an exhibit enough space should always be left to allow air to freely circulate around all parts of the bridge. Dirt should never be in contact with any part of the bridge since this will trap moisture and rapidly cause deterioration. Unfortunately, this bridge's floor beams were noted to be in contact with the ground. Either the bridge need to be raised slightly, or the ground could be dug out a bit from under the bridge. Another problem is with the vertical members of the truss bridge. One of the problems with the design of the Keystone Columns is that the open gaps allow dirt to enter the inside of the column, but do not allow it to escape from the bottom. As such, it piles up inside the column over time. Over the life of this bridge, this dirt trapped moisture inside the bottom of the columns and led to section loss. Today, numerous areas of complete section loss can be found at the bottom of the vertical members. These areas of the bridge should be repaired. Additionally, the inside of these columns should be cleaned out at regular intervals to prevent dirt buildup. Finally, for unknown reasons one of the portal braces for the bridge is not attached and is laying on the ground under the bridge. The portal bracing should be installed on the bridge. And until that happens, the portal bracing should not be lying on the dirt under the bridge.
Aside from the problems noted above, it is great to see that its value has been recognized and it has been preserved. While some might question why it was placed in a location where it is not fully functional as a useful bridge crossing, it is worth nothing that it will never be washed away by flood waters where it is located. Many historic bridges in Iowa have been destroyed by floods. Given how often devastating floods occur on Iowa rivers, perhaps a solution like this where the bridge crosses a tranquil pond is the best outcome for a nationally significant historic bridge in Iowa. If the recommended improvements outlined above can be completed at some point, this bridge will be an outstanding example of a preserved truss bridge in Iowa.
This bridge is tagged with the following special condition(s): Keystone Columns
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