Kingpin
I actually thought that, myself. By all rights, and a very basic idea of structual engineering, the Firehouse should've collapsed from damaged foundations.
Not necessarily. All “containment units” (cells, cages, whatever you want to call them) have one side that opens, and five that don't. It's entirely possible that the “laser containment grid” was just what kept them from going out the in door, as it were. Kinda like those mouse traps where the floor tips over when the mouse enters so it can't get out. When the grid was shut off, it was like someone threw the door open, and all the ghosts ran out at once. They exploded outward, but all in the same direction, because THAT'S where the opening was. Now why they went straight up afterwards is beyond me…but it IS a movie, after all. As for the structural intregrity, it seems to me that in a firehouse, where most of the bottom floor is open garage space, going straight up through the center isn't such a big deal, as there isn't much in the way of support structure in the way. It would probably put a wicked big hole in the floor, but as long as it shot straight through and didn't buckle the floor itself, the rest of the building should remain intact. (buildings with open area at the base often take their support from playing the sides off of the middle…as long as the sides are intact to brace off each other. I'm no architect, so I can't really explain it very well…sorry)