How can I hang a heavy bag from this beam in my garage?
Since this is a rental unit I would have concerns with drilling holes large enough to hang a heavy bag. It could cost you the security deposit when you leave. If you check the website of the bag maker they may offer a free standing frame or you could fabricate an "A" frame out of wood. The advantage of the freestanding unit is it will isolate the house from the vibration hitting the bag generates. I have a bag hanging in my basement. The load is spread over three floor joists and it is very obvious when someone is hitting the bag as the vibration resonates through the entire house.