Re: [PSUBS-MAILIST] ABS calculator

["F. Marc de Piolenc" <piolenc@archivale.com>]

abyssalobsession@gmail.com wrote:
> Why does having stiffeners too beefy make their
> max pressure go down?  And last but most important if I choose to use
> smaller stiffeners than what is most efficient for the hull
> thickness, are their any significant risks as long as I am operating
> within the maximums.  I know these would probably require a degree in
> engineering, but I am still a few semesters away from this kind of
> stuff though.  I appreciate any insights. 

The purpose of stiffeners is not to resist the external load themselves, 
but to stabilize the circular cross-section of the hull. Under external 
pressure, any distortion of the cross-section shape results in forces 
that tend to increase the distortion. In other words, a cylinder under 
external pressure is unstable, and if not stiffened will fail long 
before the material that it is made of reaches its compressive stress limit.

The ideal stiffener, then, would prevent any change in the hull 
cross-section's shape while allowing the diameter to decrease as the 
hull plating takes on a compressive strain to resist the external 
pressure. Of course, a real stiffener is going to resist changes both in 
cross-section shape and in cross-section diameter, and the latter 
property is going to increase shear and bending stresses locally at the 
junction of the stiffener and the hull shell plating. This can produce 
the result you see in the calculator, namely a net reduction in 
strength. Basically, there has to be an optimum relationship between the 
thickness and elastic modulus of the plating and the gauge and spacing 
of the ribs. Presumably, there are empirical or semi-empirical relations 
built into that calculator that take this into account.

Best,
Marc de Piolenc



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