If you try this
Graphics Walk-Through link you can see screenshots of the stairs you need to negotiate and the bridge.
Yes, the direct bridge to Scabban is in pieces but if you continue along the upper walkway and then look down at the corner you can see a steep set of stairs leading down. Once at the bottom follow the only possible path to the intact bridge and the zombies who guard it.
Good luck with the rest of the Asylum!
Xignal