Need Help Finding a Movie

$0 - B
Ok, I saw this movie sometime last year, not in theaters. Don't remember if I rented it or saw it only, but, I can't remember the name of it and it's been driving me nuts!

Basically, the movie is set after an apocalypse and basically a worldwide breakdown of government. Theres a city left somewhere in the mid west us, I think either Detroit or Chicago. Inside the city there's a huge skyscraper with a mall and is where all the rich people people live. Outside the skyscraper there are the huddled masses of the poor. The main character basically takes over the tower somehow.

Unfortunately that's all I remember. It seemed to be pretty recent from the special effects. If anyone knows what I'm talking about, please let me know! I've been searching for weeks trying to find this movie.