Hmmm. I'm speculating here...
But my guess is the upper truss is a height addition to the lower Pratt truss. There is a joint in the end post where the middle chord (???) is. This would make the lower truss a stand-alone truss - but likely with only about 10 ft clearance.
So was the upper truss put there to add height to overhead clearance? Or was it put there to add strength to the primary truss?
My vote is height. Why? While the vertical members line up, the diagonals do not. And the diagonals don't pass the full height. If the goal was to add span length or weight capacity, not having diagonals from the top to the bottom is a weakness.
The clearance is posted as 13 ft 5 in but nearly every bridge has a bit more clearance than posted. Standard height for a full height truck is 13 ft 6 in. Thus a truck will try to find another route - or procede with great caution. Most likely it will fit - but a prudent driver would proceed very carefully. Rough calculations suggest the clearance _without_ the upper truss would be about 10 ft. It's clear a bridge with 13.5 ft clearance would be more useful than a bridge with 10 ft clearance.
Just a reminder, I don't _know_ why there is a double-stack truss - but I can twist some logic to support these possible explanations.
very strange indeed. I would like to see a side view before confirming this as a Pratt. Could be some type of offset Lattice as well.
Very interesting bridge. Perhaps a Pratt through truss that has had its height increased? It looks like a Pratt on top of another Pratt.
What kind of truss is this????!!? I'm starting to get a little freaked out. I just discovered this bridge last night and I imagine this is some kind or variation of a Pratt Truss or just a Pratt on meth or something. I need help here since I have never seen a truss configuration of this kind.