It took a while to identify this bridge because the sign in front made me worry the bridge name began with 'CA.' However, everything in the photo was in the Google Earth view of Puente Montalvo (see map below). Also I was able to follow my photos from Arequipa to Ilo and this was one of the few river valleys that was crossed between the two cities.
Puente Montalvo is an interesting single span, two lane, through truss bridge. It is another odd truss bridge, which apparently delights Peruvian engineers. There is a big compression chord on top and seven tall triangular frames, each with a vertical post that holds up a floor beam and the bridge deck. However, there are no posts between the triangular frames. I found a similar bridge (built in 1871) in the Model Railroader's Guide to Bridges, Trestles, and Tunnels. These are some of the best books on old bridges because of their compulsion to realism in their models.
Peru's Bridges: Puente Montalvo sobre el Rio Moquegua by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.
I think this variant on the Warren truss is more rational than the version which also has verticals in the "downward-pointing" triangles. Here, the vertical ties are mainly there to reduce bending in the bottom chord. They also help provide u-frame restraint to the top chord, although the diagonals can be designed to do that on their own. Verticals within the downward-pointing triangles can also add to the u-frame restraint, but don't do anything to carry axial load. For a bridge like this, with ample plan bracing at the level of the top chord, the requirement for u-frame restraint is largely eliminated, hence the omission of those verticals.
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