Code Criteria - von Misess Stress

[hungrydinosaur]

Hi All,

I am designing and analyzing a container frame 20' x 8' x 8'. As per DNV 2.7-1 guidelines, the equivalent von Mises stress is not to exceed 0.85 of the yield stress of the material. Now as I am doing a beam/column analysis in my software the output stress is only in axial, shear, or bending+axial stresses. The software does not output von Mises stress.
Can someone advice by what method can I check/show that the stresses developed in the frame is not more than the allowed von Mises stress? Thanks in advance.

Regards,

HD

 

[CELinOttawa]

So this is a bit of a fundamental. Von Mises is uses as a short hand for combined stresses. Most combined stress checks are either linear or elliptical (referring to whether or not the terms being combined are squared).

Common checks:

(Factored Moment/Factored M Resistance) +(Factored Shear/Factored V Resistance) +(Factored Compression/Factored C Resistance) <= X

Where in your case the 'X' appears to be set as 0.85. That's very strict... Normally you see something equal to 1.0 or up to 1.25 on the right hand side of that equation.

It is also important to note that you only need to check this combination where the stresses actually are coincident. So you'd never have this problem for say a simply supported UDL loaded beam. Peak moment at centre, peak shear at ends, and no compression to speak of.

Does that help?

 

[glass99]

I have never seen an FEA program output vM streses for beam elements. I have also never seen a beam designed with vM stresses, perhaps for the same reason. vM stress makes loads of sense with shell and solid elements, but its clunky with beam elements.

My FEA (Strand7) outputs average and peak shear stresses. My first approach would be to try to show that shear stresses are low where axial stresses are high, so can be neglected. Otherwise, you are going to have to either build a shell model, or make a spreadsheet to combine stresses. One complexity will be that peak shear stress does not occur in the same location in the cross section as peak axial stress. You could conservatively assume that peak shear stress is co-located with peak axial stress and write a simple combination formula. Happily you only have axial stresses in one direction to worry about.

 

[KootK]

If you need to satisfy a reviewer then the best approach will be the least labour intensive one that the reviewer will accept. If there is no reviewer, then you've got free reign to do what makes sense to you. Some options:

1) The standard beam-column checks that CEL mentioned are obviously the easiest as you likely already have those results. They are also the most representative of the true safety margin in my opinion. Many steel beams and columns are governed by buckling of some sort which Von Mises doesn't capture.

2) Run Von Mises on the max shear and axial stress even if they are not coincident as Glass suggested. Chances are that shear stresses are low enough that they won't make much difference.

3) With low shear stresses and uniaxial loading, perhaps you can make an argument for using the principal plane stress formulation. Then it's just axial stress < 0.85 Fy.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.