Originally Posted by jestes
Jcarlilesiu, I guess there's a curve by which you can exceed the saftey margin? Meaning that, to a point, you can exceed the saftey margin to a larger degree in a small area than you could in a larger area?
I wouldn't count on the safety margin at all. The safety margin is completely discretionary by the designer. The minimum pound/sq ft rating is established by the building code. I can tell you that when I design houses, I usually include a safety factor, but I certainly wouldn't count on it.
As far as being able to exceed the loading design based on room size. I think you might have slightly misunderstood me. Lets use some real world examples for conversation purposes.
Lets assume that we have a room that is 5'x5' and the perimeter of the room is supported to the foundation at all sides. Lets assume that the floor is framed with 2x6s spaced at 12" and that this floor can hold (actual) 80 pounds per square foot without deflecting (sagging) more than 1/2" at the center when the aquarium is placed in the center of the room.
Alternatively, there is a second room that is 20'x20' and is framed with 2x10s and spaced at 16". This floor is designed to hold (actual) 65 pounds per foot. The same aquarium is placed in the center of this room, and due to the longer span, and the less rating, this floor will deflect 1 full inch.
The third room is 40'x40', framed with 2x12s at 12" o.c. and has a beam running down the middle. This floor is designed for 70 pounds per square foot. When the aquarium is placed in the center of the room, the floor doesn't deflect at all (due to the beam).
Thus, the room size doesn't matter, but the components used to frame that room do.
Typically, due to the smaller span in smaller rooms, the floor will deflect LESS than in larger rooms. This is simply becuase the span is less but there are alot of factors to consider.
It comes down to how the rooms floor transfers the load to the earth or structural components. A very small room may not have ANY structural members at the perimeter and will deflect more than a larger room with a beam. Thus, room size doesn't matter.
To carry on slightly further, think about a 2x4 spanning between 2 concrete blocks on the ground. The farther apart those blocks are, the more the 2x4 will bounce. It always will bend the most in the center of the span.
Due to framing standards, and the fact that most houses maintain the biggest floor framing required for the biggest span, most small areas of floor have really really over sized floor framing. Thus, typically, yes smaller span is gong to be the strongest.
Take our blocks and 2x4 again. Place another block on top of the 2x4, but very close to one of the blocks at the end. It won't cause the 2x4 to sag that badly as compared to placing the block ontop of the 2x4 in the center of the span.
For this reason, typically smaller framed areas, with the aquarium placed closest to the structural bearing point are optimum.
I hope this wasn't too confusing. Hard to do on a forum and with so many variables to consider.