It's hard to tell from the pictures about how long this bridge is, but if you look at Mike Condren's Fort Smith railroad website, this span looks a lot like one of the four shorter (~160 feet) spans of the original (1885) Frisco railroad bridge, which would have been replaced around 1914 and seem to have been offered for re-use as road bridges. Some spans seem to have been moved to Morrilton, AR. The only difference I see quickly is that this bridge does not have the lattice portal bracing shown in the Frisco pictures, but that could easily have been left off if the bridge was re-erected.
I don't know if this bridge was once a railroad bridge, but it bears a striking resemblance to this Farnsworth and Blodgett bridge in Kansas, which is probably lost.
So, I am guessing that this bridge is probably a Farnsworth and Blodgett structure.