The bridge is a single arch span in wrought iron, 80m long. It sagged suddenly in 1888, leading to strengthening, apparently by adding "two additional trusses".
I wonder if this accounts for the peculiar elevations of the bridge. In my photos you can see that there are reasonably stiff arch ribs, connected to the deck by two superimposed truss systems. There is a larger "W" pattern truss, and then additional triangulation is present in the upper half of the panels only. I'm thinking that this additional metalwork was a later addition, which would have served a dual function, of bracing the main diagonals against buckling, and of introducing more frequent points of support to the main deck elements.
However, this may just be a peculiar feature of the original design. The Planete TP website offers this explanation of the strengthening, which I can't make great sense of:
"In 1889, intermediate arches were added under each footpath and the longitudinal girder anchors were removed from the abutments as they had been responsible for structural defects."As can be seen from the photos, the bridge looks very good lit up at night (although one panel of balustrade lighting was out of action). It gives a very crisp profile to the ornate balustrades. You might think that a triangulated truss bridge would look less attractive than its neighbours, but I don't think this is the case.