I just checked the numbers, devitg, and I'm not coming up with what you got. At 13.75psi, an 18' x 6' chamber on the floor comes in at 13.75psi * 144 in^2/ft^2 * 18' * 6' = 214000 lb. If you add in the plate and members, it only adds a few thousand more pounds. I don't know where you get the 258000 lbs. Secondly, if the load really was 258000 lbs and the 28000 lb weight put inside was evenly distributed, how could it go from 13.75psi to 0.55psi? The weight is over 10% of the load you calculated, so the pressure would have to come out as over 10% of what you calculated. The pressure from the 28000lb load would be: 28000lb / (18' * 6') / 144 = 1.8psi, which is not insignificant. Even so, the load is in the opposite direction as the vacuum, so I couldn't understand how the floor would be the worst problem if it was bending in.
From the previous post, we see that it was actually bending out and was from forklift wheels - which makes sense. He also stated that the walls are bending in more, which is precisely what you said and I agree with it. When I get a chance, I'll do a few calcs on this one and see what I come up with.