Return to index: [Subject] [Thread] [Date] [Author]

RE: ASD biaxial bending allowable stress

[Subject Prev][Subject Next][Thread Prev][Thread Next]
The way I read it is that per the code when fa/Fa <= 0.15 you may opt to use
H1-3 as you are using... with Fa per Chapter E and Fbx/Fby per Chapter F
(taking into account any requirements from Chaper G if a built-up section is
used, and Appendix B if slender elements exist in the beam/column)  if Fa/Fa
> 0.15 then you need to also check H1-1 and H1-2 (possibly H2-1 if a tension
axial load exists)

Looking through the 9th and 8th editions Spec & Commentary, I don't see a
reference to the 0.6Fy maximum.  This might be a recommendation in an
Engineering Journal that just has not made it to the spec (ASD) yet.  I
don't particularly use the 0.6FY maximum and considering that H1-3 allows
the use of axial, X-axis, and Y-axis stresses and allowables I think you
could infer that the equation as written does take into account biaxial
bending...  Salmon and Johnson also state in their last sentence that the
use of the 0.6FY maximum may be conservative and refer to LRFD for a more
complete biaxial bending check.  

I think per the code you are alright but it would be good to hear some
background on this recommendation.

HTH,
Greg Effland
KC MO

-----Original Message-----
From: Charpentier Joe [mailto:JCharpentier(--nospam--at)tectonicengineering.com]
Sent: Monday, August 27, 2001 9:34 AM
To: 'seaint(--nospam--at)seaint.org'
Subject: ASD biaxial bending allowable stress


Hi-

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.

Joe


* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org 

* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org