From: Antoine Delafargue <antoine.delafargue@gmail.com>
To: personal_submersibles@psubs.org
Sent: Monday, April 20, 2009 4:26:30 PM
Subject: Re: [PSUBS-MAILIST] ABS Hull Calculator concerns.
Hello all,
I have been looking at the spreadsheet recently, I had doubts on these two lines too:
-line 35, moment of stiffener alone (in4) about the mid web axis: it seems that the function Iz is calculating the sum of the moments of both the flange and web about their own centroids, not around the mid web axis.
to correct for that there should be an additional term in the Iz _expression_: (Lw/2+tf/2)*tf*Lf
This plays in the stiffener tripping criteria. using the original _expression_ gives a conservative view on tripping.
-line 67, distance from outer surface of the stiffener flange to the neutral axis of the combined stiffener and effective shell section.
It seems that the formula calculates the distance between the neutral axis of the section to the outer surface of the shell instead.
the ABS is not clear on where exactly on the flange the distance is calculated (mid point, inner, outer surface). In doubt the conservative view would use
Centroid_of_Section(...)+line19/2+line20 or Centroid_of_Section(...)+Lw/2+tf
instead of Centroid_of_Section(...)+line19/2+line26 or Centroid_of_Section(...)+Lw/2+t
For this cell, using the original formulation is not conservative in general so it may lead to lower revised stiffener stress limits
And as to what Hugh said:
-I concur, the yield strength should be used in line 8. it is defined precisely in the ABS rules, min 37,700 psi for A516-GR70
or 260 MPa in SI units that I have been using
-Line 88 the formula column C should be used. what is in column D works too but for the heavy stiffeners.
Regards
Antoine
On Sun, Apr 19, 2009 at 5:42 PM, Jon Wallace
<jonw@psubs.org> wrote:
Antoine, Cliff,
Any comments?
Jon
Hugh Fulton wrote:
Just another find is that Line 88 in Cylindrical Shell calcs the formulas between Column C and column D differ.
These should be the same. Affects I min. Had me going for a bit there with oversized stiffeners required.
Hugh
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