Re: [PSUBS-MAILIST] inertia calculation for stiffeners

["Cliff Redus" <dr_redus@devtex.net>]

Gordon

This section of the ABS rules is poorly written.  The purpose, given your
design for the stiffener and your spacing between the stiffeners, is to
determine, what is the maximum pressure, and thus the maximum depth, that
your stiffener design is good for. They look at a number of different
failure modes, such as reaching yield stress at a mid point between two
stiffeners, local buckling of the stiffener and tripping of the stiffener.
For example you could propose a design of the cylindrical pressure hull with
a thick beefy wall thickness and a very light,  weak T stiffener.  In this
case, the calculations would come back with a large allowable pressure for
stress at the mid bay between the rings but a low allowable pressure for
local buckling of the weak stiffener ring.  The process is then to vary
different aspects of the design such as the shell wall thickness, the
stiffener web thickness, the stiffener depth and the flange width to see how
these impact the predicted maximum allowable working pressure. For each of
these calculations, the maximum allowable working pressure is just the
hydrostatic pressure of the water column that is trying to fail the
stiffener with a rather larger factor of safety.  Your calculations for this
section should come up with a Maximum Allowable Working Pressure based on
stiffener stress, a pass/fail test on Stiffener Tripping, a pass/fail test
on Web Depth to web Thickness and the Maximum Allowable Pressure associated
with this stiffener from the minimum moment of inertia constraint.  With all
these calculations for Maximum Allowable Working Pressure, the minimum valve
will set the maximum water pressure your design can be exposed to and meet
ABS classing rules. So the reason "P" is in all these calculations is
because the external water pressure is what sets up the stress loading in
the shell and in the stiffener.

Cliff






----- Original Message -----
From: "gordon alexander" <gordon@curio.com>
To: <personal_submersibles@psubs.org>
Sent: Monday, June 21, 2004 6:15 PM
Subject: [PSUBS-MAILIST] inertia calculation for stiffeners


> I have been trying to use the formulas in section 6 (Metallic Pressure
> Boundary Components) of the ABS "Rules for Building and Classing
Underwater
> Vehicles, Systems and Hyperbaric Facilities". I have a question for any of
> you fellows out there who have a grasp of the inertia calculations for
> stiffeners in pressure boundaries. I note that the inertia formulas for
both
> heavy and non-heavy stiffeners (19.15.1 - pg 67 and 19.15.2 - pg 68)
contain
> "P". This is the external applied pressure, no? What is it doing in a
> formula used to determine the failure pressures of pressure barriers? It
> seems a bit circular to me at this point. My head hurts:-(
> Can anyone tell me what is going on here?
>
> Gordon Alexander
> Shakopee, MN
>
>
>
>
>
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