Membrane Stress 1

[MadDog88]

I realize that this is a simple question for this forum but could somebody please explain the term 'membrane stress' for me. I am looking through some old vessel design calculations and, being quite new to this area, I am having trouble understanding a few of the terms. Also, if anyone has a suitable website explaining the basics it would be appreciated. Thanks

 

[prex]

The terms membrane stress, bending stress, primary stress, secondary stress need be understood by users of codes like ASME VIII Div.2, and only very accasionally when using Div.1.
In a few words membrane stress is defined as the constant through thickness portion of stress components in a pressure component wall. This might be not very meaningful to you, but it is a concept with very important consequences on pressure vessel analysis, that are too complex to be explained here.
However, I repeat, you don't necessarily need to know about these matters to use the commonest pressure vessel and piping design codes. prex

http://www.xcalcs.com

Online tools for structural design

 

[jte]

mrichards-

Membrane stress is the basic stress which tends to govern the thickness of vessel shells. Occasionally seismic, wind, or other considerations may govern. Bending stresses tend to appear at changes in geometry such as nozzles or changes in diameter, etc.

Membrane stress is analogous to the tensile stress in a rod: force/area. Slice a cylinder of radius "r" and thickness "t" into a ring one unit "u" high. Take a 180° section of that ring and look at it as a free body. The circumferential stress is easily derived: The load due to pressure "p" is p(projected area) = p(2r*u) or 2pru. The area resisting this force on each end is 2(t*u) or 2tu. So force/area is 2pru/2tu = pr/t. This is the basic formula for circumferential membrane stress. The formula for longitudinal stress is also derived this way (force exerted on the head/area of shell resisting this load). Longitudinal stress winds up being half as much as the circumferential membrane stress due to pressure, and the various codes use a variation of pr/t to determine the stress. Division 1 adds the joint efficiency factor "E" and the "nuisance factor" "0.6P" which becomes significant only at unusually high pressures where the vessel is on the verge of not being considered "thin wall".

Hope this helps, and keep in mind that there is much more to vessel design than pr/t!

jt

 

[JohnBreen]

Hello,

Yours is a very good question that is not asked enough either from the discussion boards or in class rooms.

I think the best explanation of this subject and othe ASME B&PV Code terminology that I have ever seen is provided by David Burgreen in his EXCELLENT books (unfortunately, now all out of print).

But go here:

http://www.addall.com/Browse/Info/TECHNOLOGY--44/Power_Resources--2235/POWER-PLANTS--45393

26. Design Methods for Power Plant Structures
ISBN: 0916877035 - Hardcover - List Price: $48.00
Author: David Burgreen

Anyone who has a copy of this book values it highly.

Best regards, John.