To me, when you say "VA Tech type situation," it conjures up a number of different possible lock down situations that could have occurred and that may apply in your case.

• Seal the building
  Seal all entrances so no one can enter or leave. For example, you know that the gunmen is outside or maybe shooting up people in the office building next to the hospital, but could be coming your way and you want to prevent them from entering the facility at all, or you want to prevent someone from going outside and possbily becoming a victim outside.
• Isolate the entire hospital into sections
  Lock down all the individual floors/wards/departments so that a gunmen can't wander through the whole facility. People in a locked down area should be safe from anyone trying to enter that area from another area. Useful when you don't know where the intruder is.
• Isolate only a particular section
  Maybe a gunmen holes up somewhere or you know where the person is, so you want to quickly lock down only that area while allowing people in surrounding areas to flee quickly.
• Lifting lock down quickly
  One thing that makes me nervous about any plan that involves locking people into an area involuntarily is what happens if people really need to get out. Say a fire breaks out during a lock down or maybe the gunmen is temporarily "stuck" in one area so that people could escape the immediate area. Any plan that relies on someone running around physically locking doors that can only be opened with a key or putting chains on makes me very nervous. Actually, come to think of it, Cho locked exits himself to keep his victims trapped.

Locking down a facility is a very complex thing to try and plan for since what seems prudent in one situation would be dangerous in others. In some emergencies, you want to keep people out, sometimes you want to keep everyone in (child abduction would be one case), sometimes you want to partition the facility but still allow some people out, etc. Good luck!