The Sixth Street Bridge always pulls me in when either I have found nothing else to photograph or simply when I drive by and notice something else about that keeps my attention. It really seems that it only gets me at bizarre times since I take photos late at night when it's really cold or when it's being painted. I don't know why but it's probably my single most common subject other than one of my dogs.
Your pictures of the 6th street bridge and of the city are amazing. I use the 6th street bridge every day, several times a day and like you I have always found it intriguing. Theres a certain haunting about this old bridge. Your pictures capture this haunting. But its not an evil place, I always have a good feeling when I cross it. I swear it calls me to use it on a daily basis!