Re: Moment calculation

From: <Gang>
Date: Thu Jul 01 2004 - 04:58:00 EDT


Dear Reddek,
Please do not address me as Gang. It happens to be just an abbreviation in my E-mail id. My name is Gangadharan. If you feel it is too difficult, you may cut it short to Ganga. (Ganga is the name of a river in India.) It might be easier for you to handle it. Let us come to the subject! It is a tricky situation: Imagine a three-element piping configuration, with say 10 m pipe towards North, then 10 m pipe towards East and the next 10 m upwards. I call the plane constituted by the first two as H-plane. The other on is V-plane. Now, the 10 m East-West segment is common to H-plane as well as V-plane.
If we consider the effect of “out-of-plane vector” from the H-plane on it; the same vector is “in-plane” for it in the V-plane. It has got its own “in-plane vector” due to the V-plane as well.
Now the next question: Can these vectors be added together? If yes, is the addition algebraic or vectorial? We resolve all the forces and reactions into X, Y and Z components. It is from this standpoint, the possibility of algebraic addition comes into the picture. This is my curiosity. Can anyone give a convincing explanation to the concept of the forces and reactions with reference to the stress analysis softwares?
The effect of combined reactions in a complex pipe run cannot probably be broken down into simplified sectors of pipes where we consider the forces act in the most simplistic way, resolvable by elementary rules of “Strength of Materials”. What the FEM does is precisely this. The results could be taken as the solutions, which justify the many observed phenomena. And to that extent, we have to be content also. The “real” values still remain elusive. We work within the limits “safely” due to the built-in safety margins attributed and accumulated over various successive steps. This avoids catastrophic failures, within the expected plant lives. Luckily, there are not many accidents due to pipe failures! We have to be “happy” about this, and hope for the (still) better. If one takes a set procedure of checks of the checklist, in the context of HOT and CRITICALl lines, the outcome generally turns out to be SAFE.
There is another fallacy in the stress analysis programmes. That is regarding the friction factor. Friction, as we all know, is associated with motion. The pipe (the support point) does move from cold to hot condition, DURING THE WARMING UP period. Once it attains the HOT / OPERATING condition it is STATIONARY or otherwise known as STEADY STATE. Friction no more comes to the scene. Most of the stress analyses are evaluated at the HOT / OPERATING condition. Then the question comes; what is the role of friction factor, 0.3 for steel to steel and 0.1 for steel to PTFE? Probably, it brings down the number of iterations of the loop and comes to a FASTER solution. Also it might result in more acceptable and palatable values of forces and reactions. Can anyone throw some more light on this matter?
Regards.
Gangadharan.


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