# strong axis bending of a bar

• To: seaint(--nospam--at)seaint.org
• Subject: strong axis bending of a bar
• From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
• Date: Wed, 31 Jan 2001 07:39:15 -0500
```Ken Peoples wrote:

>>I think this is an easy one but I haven't found it yet.  I have written in
my AISC 9th edition that the allowable bending stress of a plate bent about
its strong axis is Fb = 10,000/ (Ld/(t cubed)) <.66Fy. Anybody know where I
got this? Is it correct?  Is it in the AISC spec?  I know that .75Fy is good
for the weak axis bending.  Thanks for your help.<<

and, John Riley responded,

>>I don't know where you got that formula, but I'll take a shot at trying to
figure out the appropriate stress within AISC.  The allowable bending stress
for a plate bent about its strong axis has to be found in:

Chapter F - Beams and Other Flexural Members.

F1 - I-shaped members and channels.
F2 - weak axis bending.
F4 - shear stress.
F5 - stiffeners.
F6 - built-up members.
F7 - web-tapered members.

Clearly none of these sections apply, so it must be:

F3- Allowable Stress:  Bending of Box Members, Rectangular Tubes and Circular
Tubes.

Possibly AISC considers a plate bent about its strong axis as a special case
of box or rectangular tube.  Within F3 the allowable flexural stress will be
either 0.6Fy or 0.66Fy, depending on lateral support.<<

Actually, the allowable stress is controlled by buckling and the requirement
is not in the AISC Specs.  See the Structural Stability Research Council's
"Guide ..." or, Brockenbrough and Johnston's "USS Steel Design Manual" for
critical stress of plates with various edge support and loading conditions.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

```