Combined Bending stress checking
Combined Bending stress checking
(OP)
Hello everyone,
I am checking stresses on a rectangular plate (SA-516-70) results from an external load. below is a summary of what I have:
1) Maximum strong axis bending stress is fbx = 15,964 psi, allowable strong axis bending stress= Fbx =0.6 x Fy = 22,800 psi.
2) maximum weak axis bending stress is fby = 5,745 psi, allowable weak axis bending stress = Fby = 0.75 x Fy = 28,500 psi ( the section is compact per AISC 9th edition).
3) Tensile stress = fa= 740 psi (allowable tension = Ft = 0.6 x Fy = 22,800)
now can I say the design is acceptable because:
1) fbx < Fbx
2) fby < Fby
3) fa < Ft
4) fa/ Ft + fbx/Fbx + fby/Fby= 0.93 ≤ 1.0
or I have to find the combined bending stress to check that too separately. if yes, what will be the combined stress and its allowable, will it be √(fbx2 + fby2)= 16,966 or just by adding up fbx + fby = 21,709
Thanks in advance,
Regards,
I am checking stresses on a rectangular plate (SA-516-70) results from an external load. below is a summary of what I have:
1) Maximum strong axis bending stress is fbx = 15,964 psi, allowable strong axis bending stress= Fbx =0.6 x Fy = 22,800 psi.
2) maximum weak axis bending stress is fby = 5,745 psi, allowable weak axis bending stress = Fby = 0.75 x Fy = 28,500 psi ( the section is compact per AISC 9th edition).
3) Tensile stress = fa= 740 psi (allowable tension = Ft = 0.6 x Fy = 22,800)
now can I say the design is acceptable because:
1) fbx < Fbx
2) fby < Fby
3) fa < Ft
4) fa/ Ft + fbx/Fbx + fby/Fby= 0.93 ≤ 1.0
or I have to find the combined bending stress to check that too separately. if yes, what will be the combined stress and its allowable, will it be √(fbx2 + fby2)= 16,966 or just by adding up fbx + fby = 21,709
Thanks in advance,
Regards,






RE: Combined Bending stress checking
another day in paradise, or is paradise one day closer ?
RE: Combined Bending stress checking
There are numerous theories of failure out there - we tend to use von Mises for steel subjected to multiple stresses.
You don't simply add fbx and fby together.
A short summary found here (under item 8): http://www.learneasy.info/MDME/MEMmods/MEM09155A-C...
Here's the snippet:
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RE: Combined Bending stress checking
if you're combining the three loads just to envelop all possible conditions then I'd go with 4).
combined bending stresses should simply add (not vector sum).
another day in paradise, or is paradise one day closer ?
RE: Combined Bending stress checking
yes the three stresses are imposed at the same time but there is only one load applied on the plate. (load has lateral angle so produces weak axis bending too)
I am checking stresses per AISC 9th edition chapter H, combined stresses, (H2. AXIAL TENSION AND BENDING) and equation H2-1 which is fa/Ft + fbx/Fbx + fby/Fby ≤ 1.0
RE: Combined Bending stress checking
another day in paradise, or is paradise one day closer ?
RE: Combined Bending stress checking
1) fbx < Fbx
2) fby < Fby
3) fa < Ft
4) fa/ Ft + fbx/Fbx + fby/Fby ≤ 1.0
Thanks
RE: Combined Bending stress checking
There's no vector summation or von mises excitement (assuming shear is minimal), because your stresses are all in one direction (along the length of the plate). Your max stress is just the corner where the tensile load and two bending maximums overlap, so you basically just need to add them together.
You've effectively done that, but you've accounted for the variance in allowables by summing proportion of capacity instead of pure stresses. If the allowables were the same, your summation would be mathematically equivalent to (fa+fbx+fby)/allowable.
RE: Combined Bending stress checking
Thanks
RE: Combined Bending stress checking
But if you have one fb stress in one direction and the other fb stress orthogonal to it, you don't simply add them together.
I was envisioning your situation as a plate in "two-way bending" where a flat plate deflects into a bowl-like shape. Thus the bending stresses are perpendicular and there are shear stresses within the plate as well...thus von Mises stress considerations.
However, if you mean, by the terms "strong axis" and "weak axis" something more like a beam being bent in two ways, and you are looking at stresses that are both parallel to each other then add away or use the interaction equation you've suggested above.
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RE: Combined Bending stress checking
RE: Combined Bending stress checking
Is this, perchance, a padeye design?
RE: Combined Bending stress checking
@ winelandv, Regarding allowable bending stress at strong axis, per AISC should be (0.6 to 0.75) x Fy, so I am using the minimum allowable. am I not correct?
I am a pressure vessel engineer (mechanical engineer) and not expert in AISC standards. this is a part of lifting lug design
RE: Combined Bending stress checking
fa/ Ft + fbx/Fbx + fby/Fby ≤ 1.0
However, AISC currently has a different interaction equation:
The whole AISC spec can be found (free) here: https://www.aisc.org/globalassets/aisc/publication...
Look in Chapter H in section H1.
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RE: Combined Bending stress checking
- tensile strength through the pinhole accounting for a pin/hole diameter ratios less than .9 (as usually happens with a shackle in the hole)
- single plane fracture strength
- double plane shear strength
At any rate, for the combined stress check, JAE has set you on the right path (though AISC has changed to a strength basis instead of stress for all their design equations).
Have fun designing your lug!
RE: Combined Bending stress checking
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RE: Combined Bending stress checking
@ winelandv, Thanks for your input, I am aware of other design requirements of shear, bearing stresses at pin hole and the rest and I am checking them too.
RE: Combined Bending stress checking
another day in paradise, or is paradise one day closer ?
RE: Combined Bending stress checking
Check out Eng-Tips Forum's Policies here:
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RE: Combined Bending stress checking