The picture above is a train crossing the Pecos Viaduct in Texas, shortly after its 1892 completion. It was a record-breaker for height and, as can be seen, rather remarkably light. It had to be reinforced twice to increased its load capacity, which is one of the downsides of building such a light structure. The viaduct design itself is not all that interesting, consisting of braced towers, two cantilever trusses, and a number of simple-span trusses, but the construction was fascinating. This bridge was built in an isolated location with a deep river gorge, making the use of ordinary falsework slow and expensive. Instead, most of the viaduct was built from above. Here’s the work beginning at one end:
That’s a temporary crane (the complicated guyed truss-like structure) above the beginning of the trackbed and the first two towers below. The distinction between moving crane and the structure below is clearer once they’d moved two spans further:
The bridge members are all built-up lattices, except for the simple rod diagonal braces, and are being lowered down by ropes from the crane. I count at least seven separate block-and-tackle rigs, four of which are holding tower legs in place until the beams can be installed. This doesn’t mean no one had to work down in the valley, since the tower legs had to rest on foundations, but the steel that makes up the viaduct was all delivered from above, from the crane. Here’s a good view of the work:
And progress, showing the masonry foundation:
The beginning of work on the main cantilever spans:
One half completed and the crane removed:
And closing in from the other side:
Congratulations are owed to the Phoenix Bridge Company, which designed and built the viaduct, and to the photographer who hauled one of the not-pocket-sized cameras of the 1890s up and down that valley.
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