Sounds like the engineer and contractor for the recently finished rehabilitation need to rethink how the boards are fastened in place. Perhaps screws would be better than nails in this case.
A recent AP report states that this bridge is now closed temporarily as the nails on the planks are popping out due to the vibrations of heavy farm equipment crossing it.
http://seattletimes.nwsource.com/html/localnews/2015357825_a...
Actually, engineers would do well to study how decks were historically fastened together with little metal plate brackets through which nails would be driven. Holds the deck together very nicely.