Von Mises vs Yield Stress - AISC

[cgstrucg]

Hello,

I have modeled a structure that has a lot of plates in STAAD Pro. STAAD shows Von Mises stress for plates and a general question came to my mind. How do we compare yield stress and von mises, I mean I think there would be a phi factor to show the relation between them but I am not able to find it. Does someone knows or refer me to something related to it, please. I am located in the US.

I can do a hand check but the thing is we have too many small structures with a lot of plates so modeling plate in STAAD helps us decide a typical plate size so that we can check connections at the end. Any help would be really appreciated.

Thanks

 

[GC_Hopi]

You just need to use the ASD methodology and reduce the allowable stress by omega. Luckily steel is an isotropic material so direction of the load is not critical.

 

[STpipe]

Von Mises stress is a yield criterion for metals. Provided that the calculated stresses from your model are below the design yield strength (Fy/Omega for ASD or phi*Fy for lrfd), then the design is satisfactory for that limit state and your plates will not yield.

However, a von Mises check isn’t everything, you would still need to check to ensure other failure modes (like buckling) do not govern the design.

 

[ATSE]

Compare von Mises with material Fy (or phi-Fy if you are using LRFD as a design basis).

One item that can get missed is a triaxial condition that will mask a tensile (fracture) failure. In this rare situation, you can have fracture before yield. Triaxial stress state suppresses plasticity (yield), but does not alter favorably the tensile.
That is, check principal stresses (vs tensile) in addition to von Mises stresses (vs yield) at intersecting plates in 3 planes or very thick intersecting plates in 2 planes.