This depends on the load (s) being applied to the bridge. Once an Engineer has determined the forces and loads that will act upon a bridge then he can start the design process selecting the proper...
That all depends on the construction of the floor, do it badly enough and it won't even support itself.
That depends upon the size of the joists & grade/species of wood
14lbs. try it.
It depends on the density (the mass per unit volume) not just the weight. That is why a one ton anchor sinks while a 100,000 ton ship floats.