Margin of safety for combined loads
Margin of safety for combined loads
(OP)
Here's a question that's been bothering me for some time.
I have a rectangular section in the XY plane. For this section, I have an out of plane tension force, I have one shear force along Y axis, one shear force along Y axis, plus a bending moment around X axis and a bending moment around Y axis.
I need to find the margin of safety for that structure. Normally, I would take the stress ratio for the tension force, then adding the two stress caused by the moment, I would find the stress ratio for bending, then, taking the root square of the summation of the squares of the shear forces, I would calculate de stress ratio. That said, I would have three stress ratio : One for tension (Rt), one for shear(Rs) and one for bending (Rb).
How can I combined those ratios to get a margin of safety? In Bruhn, somewhere, there's a formula giving : MS = (1/((Ra+Rb)^2+Rs^2))-1, but they seem to say that it's only availabe if your axial force is compression and not tension.
Do you guys know if there's another formula for this? Do you think my method is valid to get a margin of safety?
Thank you very much!
I have a rectangular section in the XY plane. For this section, I have an out of plane tension force, I have one shear force along Y axis, one shear force along Y axis, plus a bending moment around X axis and a bending moment around Y axis.
I need to find the margin of safety for that structure. Normally, I would take the stress ratio for the tension force, then adding the two stress caused by the moment, I would find the stress ratio for bending, then, taking the root square of the summation of the squares of the shear forces, I would calculate de stress ratio. That said, I would have three stress ratio : One for tension (Rt), one for shear(Rs) and one for bending (Rb).
How can I combined those ratios to get a margin of safety? In Bruhn, somewhere, there's a formula giving : MS = (1/((Ra+Rb)^2+Rs^2))-1, but they seem to say that it's only availabe if your axial force is compression and not tension.
Do you guys know if there's another formula for this? Do you think my method is valid to get a margin of safety?
Thank you very much!





RE: Margin of safety for combined loads
RE: Margin of safety for combined loads
that being said, you could always calculate the stresses at 8 places on your section (the corners and the mid-sides) and combine your axial stresses (due to axial load and bending) and your shear stresses using your failure criteria of choice (principal stress, von Mises, etc) and compare to the material allowables.
good luck
RE: Margin of safety for combined loads
RE: Margin of safety for combined loads
in the same order of idea, is there a reason why the interaction formula for a tube and a rectangular section is different. I have for a rectangular section MS = 1/(SQRT((Ra+Rb)^2+Rs^2)-1 and for a tube : MS= 1/(RA+SQRT(Rb^2+Rs^2)) ? It's becoming confusing how to combine all those stresses.
Thanks again!