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This bridge was constructed in 1969 and it contains built-up end posts and top chords which use rivets for fasteners. All primary truss connections are bolted. Diagonal and vertical members are rolled. With a 1969 construction date, and no innovative or distinctive details, a strong argument could be made that this bridge has no historic value, and also that its lack of attractive v-lacing or lattice that was found in truss bridges of decades past gives it little aesthetic value. However, this bridge is far more attractive than even today's modern truss bridges which are even more simple and plain in appearance. And certainly, this bridge is far more attractive than a typical modern pre-stressed concrete bridge. While indeed, the 1960s truss bridges found on the Calumet Sag Channel do not have a large amount of historic or technological significance, the group of bridges does offer one of the best looks available at an important event in bridge history, which is the end of the use of the rivets in bridge construction. The use of rivets rapidly faded in the 1960s as bolts and welds quickly became the dominant and preferred fastener type, and rolled beams became preferred over built-up beams as well. By 1970, rivets were rarely if ever used. The Francisco Avenue Bridge, built in 1969 typifies this trend, with only a limited use of rivets and a significant use of rolled beams. Rivets had been used since the early years of metal bridge construction in the 19th Century, and so the abandonment of rivets is a major event in the overall history of bridge construction. Further, the abandonment of rivets is perhaps the most recent major change in bridge construction to date, especially with metal truss bridges. The lack of development in the design of metal truss bridges since 1970 also means that even as these post-1970 bridges age, they will not gain any historic value since they offer nothing different from modern bridges. For these reasons, HistoricBridges.org established 1970 as the cutoff date for including bridges in the Bridge Browser. A bridge like the 1969 Francisco Avenue Bridge thus is important to include on HistoricBridges.org because it brings to a conclusion the fascinating story of the historic metal truss bridge. To see one of the best examples of where this story began, take a look at the Reading-Halls Station Bridge which was built in 1846 and is so old it predates the common use of rivets in bridge construction.
Like other Calumet Sag Channel bridges, this fixed truss bridge was designed such that, as indicated in the original plans for the bridge, the structure could be converted to a vertical lift bridge by the addition of towers in the future if increased clearance for boats was ever needed in future years. This need has never presented itself however, so these changes have never taken place. The plans for this bridge also highlight how narrow the Calumet Sag Channel used to be in terms of width, and how much wider the channel is today. Both the original and current dimensions of the channel are shown in the plans. The construction of this bridge most likely was required to accommodate the construction of this much wider channel.
Alfred Benesch was previously an engineer of grade separations for the Illinois Division of Highways.
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Main Plaque FRANCISCO AVENUE BRIDGECALUMET SAG CHANNEL BUILT 1969 BY COUNTY OF COOK SEC. 095 - 0101 - M. F. T. LOADING HS 20 - 44 AND ALT. |
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