BCO rule of thumb (which corresponded very closely to the span tables in my day) was that floor joist permissible span in feet was twice depth in inches - 2, which gives 14' or 4.2m.
Flat roof joists 2D -1
Ceiling joists 2D
A job load of expanding foam canisters?
IRTA exploding foam...
Hah!, Remember a real party at an old house in South London many years ago, first floor flat was heaving!! the floor must have been going up and down about a couple of inches and the floor upstairs seemed the same, everywhere someone was bonking away like there was no tomorrow!..
Ah!, ye good olde days;)...
if the bounciness is because the joists are flexing, one will eventually lead to the other
tim
According to my structural engineer, inadequate joists will shear (I think that was the fail mode) with absolutely no warning at all
tim
For normal joists deflection determines sizing and failure will only occur after excessive deflection.
Quick check assuming BS permissible stresses give a factor of safety of 1.8, 4m span 50x200 C16 joists
Normal deflection, 9.4mm
Bending failure occurs when deflection = 21.6mm
Shear do, 52.4mm
+1.
At a pure guess you might need a deflection of a couple of feet before it starts to fail, and you might not judge that to be an acceptable amount of "bounce".
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