If I was investigating the cause of the floor collapse the information i would need is:
- the legal capacity of the room
- has the structure been changed?
- what are the original plans or specification?
- what was the live load of the room where the floor collapsed
I believe the person could be held responsible is the owner for not being aware of what was happening in his building. A party was being held in a room that designed to be a meeting room. however the owner could decided whether change the purpose of that room. There the architect and the structural engineer would be held responsible for not building a safe steady building. basically for not thinking what else the room could be used for.