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Re: Effective Width for Steel Plates

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A very good point and example.  Yet this is exactly what I wrote - "Such plate will buckle both on the compression and on the tension sides, so no matter what your effective width, I would still somehow stiffen/restrain the plate edges."
I made a little FEA model of that structure as a beam consisting of plates.   The maximum stresses, obviously, are at midspan.  What is interesting - the distribution of these stresses is pretty uniform across the section - from the very edge of the plates to the "weld" at the HSS.  Does that mean that HSS is too flimsy to have any notable effect on the performance of the unit?
In any case, and with any code/criterion, that appears to prove our point - some restraints at the free edges are necessary, otherwise, the plates buckle quite dramatically.
Steve Gordin SE
Irvine CA
----- Original Message -----
Sent: Wednesday, September 14, 2005 8:18 PM
Subject: Re: Effective Width for Steel Plates

> From: "Chance, Acie" <AChance(--nospam--at)>

>      I am wondering what others use for the effective width of plates, =
> used as flanges when the plates are very wide in comparison to there =
> thickness..  I have  3/16 plates that are 24 inches wide, welded to the =
> top and bottom faces of a 3 inch HSS.  This gives a 10.5 inch overhang =
> on each side of the HSS.  I would appreciate any references for the =
> effective width used in stiffness and stress calculations.

> From: "S. Gordin" <mailbox(--nospam--at)>

> I usually limit the effective width based upon Tbl. B5.1, that gives =
> about 3" for 3/16" A36 steel plate.  Even visually, this looks about =
> right, while 10.5" looks excessive.  IMO, such plate will buckle both on =
> the compression and on the tension sides, so no matter what your =
> effective width, I would still somehow stiffen/restrain the plate edges.

Agreed. However, it may not be conservative to assume an "effective
width" since the entire width will be initiated into local buckling when
it occurs at the tips of the flanges. The effective width concept is
based on post-buckling response of the entire width.

A quick check: The effective width limit is based on the element
achieving yield (Fy) or some proportion of yield stress. Replace Fy with
the actual stress and check the effective width. If your element still
exceeds the required ratios, it may still be susceptible to local
buckling failure, despite "effective" element adequacy based on AISC

b/t = 10.5/(3/16) = 56
AISI limits unbraced element b/t to 60.
You might consider running it through the AISI analysis.

R. Paul Ransom, P. Eng.
Burlington, Ontario, Canada
<mailto:ado26(--nospam--at)> <>

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