Half A Loaf…

“Phoenix columns” were an inspired piece of nineteenth-century design, first in wrought iron and then in steel. They were built-up riveted columns, like nearly all ductile-metal columns at that time, but rather than using the plates, angles, channels, and zees that went into most such columns, they were made of circular arc segments with projecting flanges for the connections There were four-, six-, and eight-segment circular sections (maybe others, too, I forget) sold by the Phoenix Iron Company. The circular cross-sections were better for compression than a lot of the other built-up sections, and the projecting flanges made attaching beams easy. There were used in a good number of truss bridges as well, for the compression members. That’s the Walnut Street Bridge over the Susquehanna River at Harrisburg, Pennsylvania, and it’s got Baltimore trusses that use Phoenix columns as compression struts.

The bridge was built in 1890 with seven spans (175 and 240 feet each) across the east channel of the river between Harrisburg and City Island, and eight spans (175 feet each) across the west channel from the island to the river’s west shore. The connecting road across the island is less than 800 feet long, so it really does look like one bridge, not two. The pin-connected Baltimore trusses with the Phoenix columns are surprisingly good looking:

To the point of this discussion: I lied above. It used to look like a single bridge. Two of the piers in the west channel failed in a flood in 1996, leading to the failure of three central spans over that channel. The entire west channel structure was removed, leaving the east channel structure as a bridge connecting Harrisburg to City Island.

The presence of City Island made it possible to save half of the bridge as a bicycle and pedestrian route between downtown Harrisburg and the island. In ordinary circumstances, half a bridge is useless and is quickly turned into no bridge at all.

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  1. Pingback: Half A Loaf… — Old Structures Engineering – The Bridgehunter's Chronicles

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