Re: [PSUBS-MAILIST] inertia calculation for stiffeners

["gordon alexander" <gordon@curio.com>]

Okay that makes sense. Forgive me for restating/asking what should now be
clear - I use my design depth for this P then?
I will share my spreadsheet, which is of course for conical sections, for
peer review when I get it closer to finished. I have spent at least 18 hours
on it so far but it is still not organized very well. I hope this will only
be a few more days.

Thanks Cliff,
Gordon Alexander



----- Original Message ----- 
From: "Cliff Redus" <dr_redus@devtex.net>
To: <personal_submersibles@psubs.org>
Sent: Tuesday, June 22, 2004 10:27 AM
Subject: Re: [PSUBS-MAILIST] inertia calculation for stiffeners


> 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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