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Re: AISC Steel Spec Combine forces question

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H1. DOUBLY AND SINGLY SYMMETRIC MEMBERS SUBJECT
TO FLEXURE AND AXIAL FORCE
1. Doubly and Singly Symmetric Members in Flexure and Compression
The interaction of flexure and compression in doubly symmetric members and singly symmetric members for which 0.1 ≤ (Iyc/Iy ) ≤ 0.9, that are constrained to bend about a geometric axis (x and/or y) shall be limited by Equations H1-1a and H1-1b, where Iyc is the moment of inertia about the y-axis referred to the compression flange, in.4 (mm4).

User Note: Section H2 is permitted to be used in lieu of the provisions of this section.


I do see this Permission to use Section H2 inl lieu of H1 in the 2005 Spec for Struct Steel Bldgs.

But let's examine, if bending is in X direction, perhaps both flanges are in compression, so Iyc would have to include both flanges, like tfb*bf^3/12 * 2 for 2 flanges.  But for Y direction bending, it would refer to tfb*bf*(d/2)^2.  For the idea of warping, this somewhat makes sense.  The ratio would be between 0.1 and 0.9 and one can see that there is a better distribution for X direction bending and warping would be less likely.  In the other direction, the high compression in the flange, would point to more warping effects.  In the X-direction, if the flanges are very big, perhaps the Iyc/Iy will be a bigger ratio than 0.9, and so one would not have to limit to H1.1a and H1.1b.  The warping would not be such a consideration.  We see the same constant used in bending section F, where if the Iyc is less than 0.23 then the Torsional resistance of the member cannot be used at all (ie. J).

H1.1a and H1.1b do not appear to consider warping directly, and perhaps are fairly conservative in their approaches

My shot in the dark, perhaps it is very dark.

On 12/30/05, Garner, Robert <rgarner(--nospam--at)moffattnichol.com> wrote:

I had that same question - what does "referred to ............" mean.  Sorry for the non-answer but you're not alone with this question.

 

Bob Garner

 


From: Jason Christensen [mailto:jason.christensen(--nospam--at)es2eng.com]
Sent: Thursday, December 29, 2005 3:02 PM
To: seaint(--nospam--at)seaint.org
Subject: AISC Steel Spec Combine forces question

 

To all,

 

I have been reading section H of the new steel spec.  This section is for designing members for combined forces and torsion.  My question is on section H1 it reads as follows:

 

H1.          DOUBLY AND SINGLY SYMMETRIC MEMBERS SUBJECT TO FLEXURE AND AXIAL FORCE

 

1.         Doubly and Singly Symmetric Members in Flexure and Compression

The interaction of flexure and compression in doubly symmetric members and singly symmetric members for which 0.1 ≤ (Iyc/Iy ) ≤ 0.9, that are constrained to bend about a geometric axis (x and/or y) shall be limited by Equations H1-1a and H1-1b, where Iyc is the moment of inertia about the y-axis referred to the compression flange, in.4 (mm4).

 

My question is with the definition of Iyc, specifically the use of the word 'referred'.  My first thought was Iy + Ad^2, this cannot be correct in that Iy is taken about the y-axis and transferring it to the compression flange does not make sense if my compression flange is involved in x-axis bending, i.e. transferring a y moment of inertia with an x direction.  My second thought is that Iyc is just the moment of inertia of the compression flange about the y-axis, but I am unsure of this also.  Any thoughts would be appreciated.

 

 

Jason

 

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