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# RE: Effective Width for Steel Plates

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: Effective Width for Steel Plates
• From: "Chance, Acie" <AChance(--nospam--at)lacsd.org>
• Date: Thu, 15 Sep 2005 12:13:04 -0700

```Paul

I am sure I am misunderstanding something here when you say to take the actual stress and use it to determine the effective width.  Note that if there is 10.o KSI "actual" stress in the member the effective width is 24 inches and if I use the yield stress of 50 KSI the effective width is 10.75 inches.  For the case where I have 24 inches of plate on each side and 10 KSI I get an actual moment of 25.18 K-FT.  Now assuming I cut off 13.5 inches of plate on each side and use the AISC with .6 FY I get 64.5 K-FT.  I lose capacity with more plate.  What am I missing?

Acie

-----Original Message-----
Sent: Wednesday, September 14, 2005 8:19 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Effective Width for Steel Plates

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

>      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)sgeconsulting.com>

> 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
limits.

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.
Civil/Structural/Project/International

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