Padeye calculation acc to DNV 2.7-3

[Qluq]

Hi everyone,

when sizing a padeye using DNV 2.7-3 as a guideline, I come across the requirement that the von Mises equivalent stress may not exceed 85% of Re (yield strength). My problem is: how does one find the von Mises equivalent stress in a simple hand calculation (without using FEA)? Neither principal stresses are known, nor any local tensile and shear stress components.

It is allowed to revert to the AISC ASD method, but that shouldn't be necessary?

Thanks!

 

[TomekZeWschodu]

For sure you can calculate those stress using only theoretical methods. Check In-planed strength:
Calculate Von Mises components:
-shear stress (simply: F/As)
-bending stress calculate beased on curved beams theory
-tension stress


Naval Architect/ Structural Engineer

 

[victor007]

Hi Qlug, You can do a hand calc and come up with VM stress (von Mises stress) yourself. You just need to borrow the equation from any Structural or Mechanical engineering handbook (like Roark's Formulas for Stress and Strain), and go from there.
In some handbooks you can find the equation you need under "von Mises theory" or "Maximum distortion energy theory" titles.

Getting back to second part of your question, "VM stress equation" and AISC ASD "combined stress equation" are two different things, and if you have to follow DNV 2.7-3 per your project requirements, you can not utilize the AISC equations. It does not hurt to check the AISC equations in your design but there is no correlation between these two methods.
Have fun!

 

[Qluq]

Thanks for your pointers, guys!

I am quite new to his stuff (trained in aerospace engineering, which is all about thin-walled assumption) and this forum (in fact this was my first post), and it is very nice to find people so willing to help.

Victor007 , i am aware of the von Mises equation, but unsure how to find the components of stress (shear and normal) using only simple handcalculations. Also, i get that VM criterion and ASD are not reconcilable, but it is stated in the DNV 2.7-3. That is also a reason why i am reluctant to start working according to AISC ASD.

TomekZeWschodu, i'll be back behind my desk next tuesday. Ignoring out-of-plane forces for now, would the following method comply with customary methods?:
1. cut the padeye by a plane through the axis of the pinhole and normal to the sling load vector, and use that section to find tensile stresses
2. cut the padeye by a plane rotated 90 degrees to the former (so that the sling force vector is included in it) and use curved beam theory on both of the cut portions of the padeye to find bend stresses at the pin hole
3. but how to find shear stresses? you mention F/As, but what section to use for the shear area?
4. substitute all stress components in VM equation et voila...
5. verify tear-out and bearing stresses using the DNV.

Wouldn't this method oversee stress concentrations at the pinhole edges?

Laterz, Qluq

 

[TomekZeWschodu]

@Oluk
Approach you listed above should be enough to check accordance with the code requirements.
To find the shear stress I would suggest you to use the smallest cut area within the padeye (including the cheek plates) or this one beeing in-line with the sling force. You can also use stress concentration factors around the pin hole to do the calculations more accurate.


Naval Architect/ Structural Engineer

 

[Qluq]

Big thanks!