Etsusa Bridge by Ryohgo Narita

Translated from Japanese Completed Action Supernatural
Set on a giant, almost finished bridge between Sadogashima and Niigata. Abandoned by the government, the bridge, and the giant man-made floating island at the center, has become a city of criminals and societal drop-outs.
Set in the same world as Durarara, but 15-20 years later. There is almost no overlap between Etsusa Bridge and the other novels.
From translation site: “The series is completed with 4 volumes and 1 collection of short stories. The author has been talking about a second short story collection for years, but currently there is no sign of the second collection being released.”

Volumes 1 - 5