One And A Half Decks

The vast majority of road bridges have a single deck; a few are double decked. The 1895 Falls Bridge, across the Schuylkill River in Fairmont Park in Philadelphia is an interesting exception. It has the structure needed for two decks, but the upper deck was never built, leaving a bridge that looks curiously top-heavy. Fairmont Park is a National Register historic district and the bridge was the subject of a HAER survey, so its peculiarities are well documented and preserved.

The bridge has three spans, each 180 feet long, and is 43 feet wide. The trusses are 30 feet deep and are subdivided parallel-chord Pratt trusses, or Baltimore trusses. The compression members are built-up, riveted boxes and the tension members are eyebars with pin connections. The roadway, located at the bottom chords (in a through configuration) has two traffic lanes and two sidewalks. All of this is standard stuff for the era and does nothing to distinguish the bridge. The oddity comes in with the transverse beams connecting the upper chords of the two trusses. They were designed to carry two more road lanes and a pair of streetcar tracks and so are far heavier than the wind-bracing that is at the top of most through trusses. Here’s a view of the deck showing those heavy girders crossing above:

Because the upper deck was never used. the ramps that would be necessary at either end to get traffic up top were never built, leaving the end portals looking odd:

More than anything else, that looks like the end of an elevated rail line, which it sort of is. The large girders don’t particularly contribute to the strength of the bridge – much smaller pieces of steel would be sufficient for wind bracing, and there’s no way for the excess capacity of the upper girders to carry load on the lower deck – but they certainly give it a unique appearance.

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