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.

American River Bridges: Union Pacific Bridge (2) by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.

American River Bridges: Union Pacific Bridge

We're going to spend a couple of days studying a long viaduct that carries Amtrak and Union Pacific trains across the American River, just downstream from the Business 80 Expressway Bridge. This bridge includes a quarter mile long viaduct north of the river and a highly-skewed, five span steel Warren truss across the river.

I hadn't really thought about skewed truss bridges before. Because the trusses rest on skewed pier walls, the middle of each truss is built normal but the ends have triangular segments to match the skew.  Only the spans over the river are skewed. The rest of the bridge, including the trestle, have perpendicular supports.

A couple of years ago (on March 15, 2007) the timber trestle on the north side of the truss spans caught on fire. Sacramento had a couple of fire boats that raced to the truss bridge and sprayed it continuously while other firefighters concentrated on putting out the trestle fire. I guess the Sacramento Fire Department wasn't used to having fire boats because there were so many safety violations that the boats were taken out of service for three weeks after the fire. Apparently there was no oxygen tanks on the boat and so the firefighters were breathing toxic fumes for several hours. Also, the firefighters couldn't get their life vests on over their other equipment.  Fuel cans for the boat were sitting on the deck while they fought the fire. Also, the water pump for one boat was broken and the starter motor on the other boat was broken. Still, quick action by the firefighters probably saved the steel trusses and allowed Union Pacific to reopen the bridge eleven days after the fire.

We'll look at the timber trestle tomorrow.

American River Bridges: Union Pacific Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.