Thursday, October 29, 2009

American River Bridges: Union Pacific Bridge (2)

This timber trestle acted as an approach structure for the steel truss railroad bridge over the American River we saw yesterday. I have often wondered whether an embankment or a bridge makes a less expensive approach for a river crossing. At this location however, a bike trail had to pass under the railroad tracks.

This timber trestle structure must be as cheap a bridge as it's possible to build. Lateral (and longitudinal) stability is provided by a pair of diagonal braces. The axial members are slightly battered to provide a little more stability. The axial members are anchored to the top and bottom beams by metal plates with narrow rims. Still, it must have been a remarkably strong structure to carry the dozens of freight and passenger trains that rumbled over it every day.

I wasn't aware of the difference between timber preserved with resin from the creosote bush and timber preserved with resin made from petroleum products, until I did some research for this article. It must have petroleum-based creosote that made this timber structure so combustible. Someone set fire to the bridge and it made a tremendous blaze with a dense cloud of smoke that could be seen from 50 miles away. In hindsight, an embankment supported by retaining walls seems like a cheaper investment, but Union Pacific replaced this structure with a steel trestle bridge in less than two weeks. We'll look at this new structure tomorrow.
Creative Commons License
American River Bridges: Union Pacific Bridge (2) by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.

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