Combined Stress

[Trouser]

I am analyzing an HSS Round section to determine combined stresses for evaluation of fatigue. The AASHTO Standards for Signs, Luminaires, and Signals 2009 is the governing code.

In several of the critical location the section experiences bending (multiple axes), axial compression, and shear (multiple axes). Could someone provide clarification on how these stresses can be combined? Is it simply addition or are square roots of the sum of the squares required? We recently had an office fire, which took my strengths of materials literature, so I could really use the help.

i.e.

fa + fvx + fvy + fbx + fby

fa + SQRT(fvx^2 + fvy^2) + SQRT(fbx^2 + fby^2)

or some other variation?

 

[frv]

I'm unfamiliar with AASHTO, but a conservative approach would be to sum the usage ratios of each (i.e., fa_x/Fallowable_x+fa_y/Fallowable_y+..<=1).

The steel manual covers this pretty well. See chapter H.

 

[ldrm2]

See section 5.12 of AASHTO LTS-5 (Standards for Signs, Luminaires, etc.). It provides the equations for combined stresses.

 

[rb1957]

if you've got bending about x- and y- axis, isn't it more correct to consider bending about the principal axes ?

if you've got bending about two axes then the resultant normal stress is the sum of the normal stress from each moment.

 

[Trouser]

Thanks for the input, but let me clarify my question a bit.

The question originated from an attempt to meet constant amplitude fatigue limits (CAFL) set by AASHTO in the LTS Section 11.9. This section simply states a stress limit for a given detail category. In my case, I have a Category E Detail and the CAFL is 4.5 ksi.

Do fatigue stresses include shear and torsion stresses, or do they only include bending stress? If shear and torsion are included, how are these stresses to be combined?

Hope this clears things up a bit.

 

[rb1957]

i don't know your code, nor your industry's comon practice, but i'd prefer to use principal stresses for fatigue. particularly if the principal stress is significantly higher than the combined normal stress.

just my 2c