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ASD biaxial bending allowable stress

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In reviewing calculations from another office, we (our office) came across
an interesting difference of practice. When calculating biaxial bending in
beams (wide-flange sections for the most part), Using ASD 9th edition, we
generally use equation H1-3 (light or no axial loading):

fa/FA + fbx/Fbx + fby/Fby = 1.0

which in many cases, allows us to use 0.66Fy as Fbx and 0.75Fy as Fby.

However, we've recently reviewed calculations where Fbx and Fby are 0.60Fy,
no matter what the compact length attributes of the beam are. The only
reference I've been able to find to this is in my Salmon and Johnson
textbook (mine is the 3rd edition, and one of my colleagues has the 4th.)
In section 7.11 (both editions) they say:

"The use of an interaction equation such as used for beam columns is not
believed adequately conservative for cases where there is no axial
compression or tension. The AISC ASD specification has traditionally limited
the combined stress according to eq 7.11.1 to a maximum of 0.6Fy"

Where equation 7.11.1 is:  rho(maximum stress) = Mx/Sx + My/Sy

Now, I understand their contention that the interaction equations were
developed specifically for situations where there is tension or compression
involved (they specifically state that in the 4th edition where they go over
LRFD in section 7.11), but I haven't found anywhere in the code where it
references to 0.6Fy for biaxial bending. 

Can anyone shed some light on this?  Is the 0.6Fy in biaxial standard?

Thanks for your help.


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