AISC ASD allowable stress checks for von mises 1

[Theozz]

Hi All,
What will be the allowable stress for von mises stress checks to AISC ASD 1989 Edition?

Major axis bending Fby = 0.66Fy
Minor axis bending Fbz = 0.75Fy
Tension Ft = 0.6Fy
Shear fv = 0.4Fy

Combined stress check
fa/Fa + fby/Fby + fbz/Fbz < 1.0

v.m = ((fa + fby +fbz)^2 + 3(fvy + fvz)^2)^0.5

Based on the above, can one approximate the allowable stress for von mises to be ((0.66^2 + 0.4^2))^2 = 0.96

Your comments on above will be much appreciated
Thank you.
Theo

 

[DaveAtkins]

First of all, I wouldn't use the AISC ASD 1989 edition. I don't believe any codes reference it anymore.

Second of all, I don't believe the AISC ASD method is meant for comparison to von Mise's stresses. ASD considers bending separately from shear, for example.

DaveAtkins

 

[WARose]

I ran up against this with the 9th edition myself. If you have to use it, the hard part becomes: what SF to use? I've typically used a SF of about 1.3-1.67 for structural steel. My basis for that is an old AISC Journal article (third quarter 1986; 'Combined Shear and Tension Stresses') where interaction curves were developed based on Von Mises for the interaction of shear and tension.

So what does that yield in equation form?

(ft/Ft)² + 4/3(fv/Fv)²<1 [SF of about 1.67; considered a bit too conservative]

(ft/Ft) + (fv/Fv)²<1 [recommended equation by previously mentioned paper]

Obviously bending, tension, compression, etc would be combined in the first term (ft/Ft).

And Ft is taken as 0.6Fy & Fv is taken as 0.4Fy. (In the article.)

 

[Theozz]

Thanks Dave,

I agree the 9th Edition is no more in use, what there is I have been checking structural members on old oil & gas plaforms and there's always confusion when the new codes are used and there's differences in the member capacities.

Almost all the codes are very quiet on the allowable stress for von mises which is a shame, DNV recommends 0.85Fy in some instances which is very conservative.

Thanks WARose,

I'll limit my SF to 1.0Fy just to be on the safe size. The von mises stress being a localised stress means limiting SF to 0.85 is too conservative in my opinion

 

[Celt83]

Think the issue with checking the von mises stresses is that doesn't really capture all the possible failure modes, like buckling.

If you can get the stresses by components you could jump into the checks proposed in AISC Design guide 9 - Design for Torsion.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[WARose]

Think the issue with checking the von mises stresses is that doesn't really capture all the possible failure modes, like buckling.

It isn't intended to check buckling. We are talking a checking a combined state of stress from a failure mode very different than buckling. That's a separate check.

 

[ATSE]

I think your approach is unconventional and not necessary. The AISC code equations for capacity apply to specific actions (i.e. web shear, beam flexure...)
Use von Mises results from FEA plate to compare against yield stress. So, for an LRFD code check, your demand is your vm results, and your capacity is phi*Fy (usually 0.9 * 50 ksi in the US). That's part of the beauty of vm.
Alternatively, you can get peak shears from FEA and compare with phi*0.577*Fy (or Tresca via phi*0.5*Fy).

What exactly are your doing? Most (all?)of code-driven conventional steel construction stresses can be checked without plate FEA.
Keep in mind that using FEA for some typical steel conditions - like a bolted knife plate connection - will show alarming results (closed form elastic solution will show 2x to 3x yield), but the code quietly incorporates the ductility of steel and localized yielding, so we don't care if an FEA shows 70 ksi around a bolt hole for 50 ksi yield material.

 

[Theozz]

Hi All

Many thanks for the different view on the VM.

I presume the VM checks is for localised stress checks, say padeyes, where there's combimation of tension+bending+shear. Obviously the AISC factors are for complete beam sections

I'll resort to using allowable stress to 1.0Fy as there's no clear guidance.

Thanks to ALL, lovely we can share ideas as engineers......love it!!!!!!!!!!!!!!!!

 

[canwesteng]

I don't follow your methodology at all. Are you suggesting to use no safety factor? I would use 1.67, this is consistent with how steel is treated in other parts of the code.