The GBW's crossing of the Trempealeau River at Dodge, Wisconsin used an
unusual skewed two-span pony truss bridge with a curved alignment to cross the
muddy green waters of the Trempealeau River.
Bridge at Dodge
What makes this bridge unusual?
- Skewed: The supports of the bridge were not perpendicular to the track.
- Two-span: There was a center pier, so that the overall bridge actually
consisted of two shorter bridges end-to-end.
- Pony truss: The track is between the trusses but the trusses are not high
enough to be cross-braced at the top.
- Curved alignment: Although the bridge spans themselves are built straight,
the track alignment is curved as it passes over the bridge.
Charles Tomashek's photo of this bridge captures the details of the superstructure.
Note how the track is supported by floor beams which connect the trusses on the
left and right sides of the track. These beams are spaced about ten feet
apart -- there are beams under the ties which span from floor beam to floor beam
and support the ties and rail.
Because the track is curved, the bridge has a "superelevation" -
the outside rail is higher than the inside rail. Note too how the truss is
made up of a series of overlapping plates -- more and more plates are riveted
together as you get to the middle of the span.
There are four rails on the bridge because the two inner rails are guard
rails placed to help keep a derailed car in place so it didn't damage the
bridge. The guard rails were a little lower than the main running rails on
the outside -- notice that the running rails sit on tie plates on each wooden
tie, while the guard rails are just spiked directly to every third tie.
The timbers which run the length of the bridge at the outside of each tie are
used to help keep the ties in their proper location. When a train brakes
or and engine accelerates while on the bridge, it tries to push the rails and
ties in the opposite direction, and these timbers keep the ties from bunching up
or turning over from the train forces.
At some point in the past, additional piers were added in the middle of each
span, making it a four-span bridge. This was done to increase the
load-carrying capacity of the bridge as locomotive and car weights increased.