Definition of bending & membrane stress..?

[ansys1000]

Hi!

I am working with ASME section III NF-3000, mainly with the plate and shell type supports.

When looking at the bending vs membrane stress do you look at the whole cross section? For example, consider a console (H-beam) with a moment load. The flanges will take most load but is this stress considered as bending or membrane? This stress is linearly varying over the whole cross-section but very uniform for the flanges. Should this stress be compared to Sm or 1.5*Sm?

Thanks!

Thomas

 

[prex]

An I beam is not generally treated as plate and shell type, though you can do that.
The important point to account for is that, in the 1.5S_m limit for bending, the factor 1.5 is simply the ratio of the plastic modulus of a rectangular section to the elastic one. As it is a rectangular section, it applies only to unreinforced plates and shells.
To follow your example of an I beam, you can consider the flanges as plates, but then the stress is essentially membrane (=constant through thickness) so the limit is S_m. If you used a linear type approach instead of the plate and shell, then by using plastic collapse analysis, you would gain higher allowables, as the ratio plastic/elastic moduli is generally between 1.1 and 1.2 for I beams.

prex

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