The Meridian Street Bridge is a very unusual bridge that is unique among surviving truss bridges in the United States. Its modified polygonal Warren truss configuration is similar to, but not identical to, a Turner truss of which no examples remain. Details that give the Meridian Street Bridge its distinctive Turner-like appearance include the complete omission of vertical members. Polygonal Warren truss bridges typically include vertical members, but the Turner truss did not. Additionally both the Meridian Street Bridge and true Turner truss bridges included longitudinal members that ran between diagonal members in the middle of the truss. These details make the Meridian Street Bridge look a lot like a Turner truss, an appearance that strongly emphasizes the triangles in the truss.
The Meridian Street Bridge has additional unusual details that give the bridge a distinctive appearance. Typically, sway bracing and struts will be mounted to vertical members on a polygonal Warren through truss, but without any vertical members on the Meridian Street Bridge, sway bracing is instead mounted on the diagonal members. Also unusual is that although the Meridian Street Bridge has sway bracing, it does not have the upper strut component of overhead bracing that runs from top chord connection to top chord connections. In fact, as originally designed, the sway bracing was limited only members that ran at an angle, with no direct transverse members. Today, there is a strut at the bottom of the sway bracing, but this was added when some of the original sway bracing was cut out to increase clearance for trucks. The lack of struts gives the overhead bracing a strong unbroken pattern of "X's" not usually seen in the overhead bracing of a truss bridge.
Meridian Street Bridge had for a number of years been carrying northbound traffic only, while a modern bridge carried southbound traffic. The historic bridge is currently being replaced. As of 2014, the historic bridge was slid over to serve as a temporary crossing while a new bridge is built in the original location of the historic bridge. View the project website. However, the outcome of Section 106 is that the bridge will be placed in storage for potential reuse in a new location, potentially by the Foothills Trail.
The state of North Dakota, which is noted for its striking lack of historic bridges, particularly large and rare ones, once had the only Turner truss in the entire country, Bismarck's Liberty Memorial Bridge. The Turner truss was a special subdivided modification of the Polygonal Warren truss configuration patented by noted engineer Claude Allen Porter Turner. Moreover, this singular example was a large multi-span bridge. It was nationally significant as the only one of its type, and one of the few monumental historic bridges in the state of North Dakota. It was in great physical condition, and wasn't even below a 50% sufficiency rating, yet the state built a new bridge on new alignment and then demolished this priceless bridge even though it was not in the way of anything. The demolition of this bridge in 2008 represents one of the worst atrocities against a historic bridge in the United States in decades. View HAER Documentation for Liberty Memorial Bridge.
Technically speaking, the Meridian Street Bridge is not a Turner truss. It is missing a few specific members that are specifically part of the Turner truss patent. The drawings on this page compare the Meridian Street and Liberty Bridges, and if you look closely you will note some sub-verticals and sub-diagonals are not present on the Meridian Street Bridge. However, the fact remains that the Meridian Street Bridge has striking similarities to a Turner truss. The Meridian Street Bridge is not a typical polygonal Warren truss bridge, and like the now-lost Liberty Memorial Bridge, appears to be the only one of its kind. Moreover, it is today the only bridge in existence that has a truss configuration that looks anything like a Turner truss, even if it isn't a true Turner truss. For all these reasons, it is essential that the Meridian Street Bridge be preserved.
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