9psi's nothing to sniff at.
The average bag of sugar is 2lbs/1kg, and has a "footprint" of, oh, maybe 3" by 4" (I don't have one handy, so could be off a bit there). So a single bag of sugar is 2lbs/12sq.in.
To achieve 9 psi would require a total of 108lbs over that 12sqin footprint. 54 bags of sugar.
If you were to pile bags of sugar over an entire surface in your house, and pile them 54 layers high, I wouldn't put long odds on that surface lasting too long. There's at least be some creaking going on.
Alternatively, typical feet sizes of a male are probably about 4" wide by 12" long. With two feet, that's a rough footprint of 2*3*12 = 72 sq inches If you weigh just over 200lbs - which is getting a bit chubby in most cases - that's still only 3psi. How many people could you stand (with two others sat on their shoulders) before an airplane structure might start to be a bit upset. Especially if our hypothetical 3-person load were repeated as close as you could get them to stand?
I'm actually quite impressed those thin sheets of metal can take 9psi.