Try your aldermans office, and tell them it's a safety issue.
When you call 311, ask for a Report Number. That will make sure the report is made. Call the alderman's office repeatedly, once a week. Get name of others on the alderman's staff - and include them in your emails. Go to a CAPS meeting and bring the issue up there.
Fill out the form at http://www.cityofchicago.org/city/en/svcs/iwantto.report_file.html also.
Here is a step most people don't know about. After you file a report and get a tracking number (you must ask for a tracking number) give it a couple days and check the tracker system. This will tell you status. If it is in the system but not fixed, ask for a supervisor. Keep doing this weekly until someone fixes the problem.