Crippling of Section Due to Bending

[mjeand]

In the NASA Astronautic Structures Manual (Vol. 1), there is a section B4.4.0 titled Bending-Crippling Failure of Formed Beams. The first paragraph reads as follows:

"This section contains methods of analysis applicable to formed or built-up sections which are critical in the bending-crippling mode of failure. This method is to be used when plastic bending curves are not available, otherwise use Section B4.5" (Section B4.5 is the plastic bending section, akin to Bruhn's method of determining ult and yld strength in bending)

My question based on the above quote is, if I'm using the Bruhn approach (with plastic bending curves) to obtain the ultimate/yield bending strength of the section already, is it even relevant to look at crippling?

Additionally, is the NASA manual the only reference to address Bending-crippling failure? Or does anyone know of another source I can use as well? The vast majority of references deal with uniform compression rather than compression due to bending.

Thanks!

 

[rb1957]

plastic bending assumes stable sections. if you have a thin section with a reasonably low crippling allowable then i'd expect the section would bend elastically untill the compression cap cripples when the section fails.

 

[YoungTurk]

if I'm using the Bruhn approach (with plastic bending curves) to obtain the ultimate/yield bending strength of the section already, is it even relevant to look at crippling?

If you're using the plastic bending curve for the same section profile then you needn't check crippling. If you've got a section such as a hat or zee for which plastic curves are not available/applicable, then you need to check the crippling value for the compressive cap as rb says.

I've often seen people apply plastic factors to, for example, rectangular sections of more complex shapes such as formed zees. This isn't correct, the plastic curves are for the entire sectional shape, not an element of it.