Plastic bending

[franc11]

Hi,

Here's a question that's been bothering for some times now.

Cozzone showed a method to calculate an ultimate or yield bending moment considering the beam is in pure bending.

The catch is that most of the beams also have axial loads. For example, a tube I'm trying to analyse has a bending moment, a torsional moment and a tension load. I found in a couple of places that the axial load would actually decrease the ultimate plastic bending moment allowable. I found in another reference some graphic which shows a form factor for tube based on their thickness and a ration of the moment and axial load. And guess what, when I use this form factor, it gives me an allowable plastic bending moment lower than the if I use Ftu. So, in this case, plastic bending is not helping me at all.

Any ideas on this? Is there another way to calculate plastic bending with an axial load and bending moment?

Thanks

 

[rb1957]

i would use cozzone to calculate the allowable bending moment on the section, and then combine this with the axial stress ...

(Rt+Rb)^-1 = RF

Rt = axial stress/ftu
Rb = applied moment/cozzone moment