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FW: Stresses at Shell Attachments

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-----Original Message-----
From: Ron Hill [mailto:ronhill(--nospam--at)mindspring.com]
Sent: Sunday, February 21, 1999 8:02 PM
To: Ron Hill
Subject: RE: Stresses at Shell Attachments
 
You can use an obround  pad rather than round.  Be sure not to use square pads or pads with pointed corners.  I usually require a 1" radius on corners.

 
I incorrectly stated the last sentence.  It should read .....by analyzing the large cylinder as a sphere and doubling the stresses.
 
(Its been a long time since I did that analysis :} )
Ron Hill
-----Original Message-----
From: Ron Hill [mailto:ronhill(--nospam--at)mindspring.com]
Sent: Sunday, February 21, 1999 5:58 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Stresses at Shell Attachments

Your size clip will not work on those references you cited.  Please be aware of stress concentration factors that must be applied to the vessel as noted in WRC 107.  For permanent clips for platforms and pipe supports I recommend that you weld a round pad to the vessel and weld support clips/members to the pad.  The pad should be constructed from code material.  This will aid you in your analysis and reduce the possibility of brittle fracture/cracking.
 
The use of Bednar's method on a sphere is conservative.  Bednar developed the method from WRC107.
 
If you compare a sphere of radius R, thickness T and radial load P with a cylinder with the same parameters you will notice that the cylinder has much higher stresses.  If you use conservative values from the charts you will see that membrane and bending stresses are as follows:
sphere:(.18+1.8)P/(T*T)
cyl:(.4+2.4)P/(T*T)
 
As you can see, the stresses in the cylinder are close to double the stresses in the sphere.  In the old days before WRC297 and Desktop FEM, advantage of this relationship was taken for large Cylinders above WRC 107 parameters by analyzing the large cylinder as a sphere with radius 2R.
 
Ron Hill

HILL Consulting Engineering
Birmingham, Alabama  USA

-----Original Message-----
From: P.Rajendran [mailto:rajendra(--nospam--at)mmind.net]
Sent: Sunday, February 21, 1999 7:24 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Stresses at Shell Attachments

Please see my inserted comments.

Ron Hill wrote:

WRC 107 addresses spherical shells with rectangular attachments.  This
analysis is rather involved for temporary (and sometimes non code) loads.
The aspect ratio is limited to 1.5, which means that standard clips will be out of scope of the formula.
Bednar does have a good procedure for cylinders that can be used for
spheres.
Here too, the aspect ratio is limited to 1.5.  Also, the example in the book has a reinforcing pad.  Furthermore, what is the justification for using formula, developed for cylinders, on spheres?
I have used the simple method by Blodget for lifting lugs a number of times.
See "Design of Welded Structures",Blodget, section 6.6.3 'Effect of hanger
or support welded to shell'.
I have a copy of the book and, I will take a look at the section you have referred to. Thanks.