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Re: Effective Width for Steel Plates - AISC FAQ and other perspectives

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Here are a few other perspectives:

A. For rings stiffening the shell of oil storage
tanks, API 650 permits an effective width of 16 times
the shell thickness on each side of the stiffener.

B. For longitudinal bulkheads of ships, the IACS
(International Association of Classification
Societies) permits an effective width of 20 times the
hull thickness.

C. For hull stiffeners in aluminum boats, the US Coast
Guard permits 19 times the hull thickness (NVIC

D. A prominent dry dock manufacturer I work with uses
30 times the hull thickness.

E. And our very own AISC has this answer on its FAQ

4.8.1. When a stiffening element is attached to a
plate, what width of plate is effective in resisting
load with the stiffening element?
Answer: A general rule of thumb that is used in
American Petroleum Institute and American Iron and
Steel Institute standards for liquid storage
tanks is that 16 times the plate thickness is
effective on either side of a stiffening element, but
not greater than:

   1. The actual projecting width. 
   2. One-half the distance to the next stiffening
   3. One-quarter of the stiffener span. 
   4. The width based upon the limiting
width-thickness ratios in the 1999 AISC LRFD
Specification Table B5.1 for compression elements.

last modified 26 July 2002

F. I had the following exchange with AISC Solutions
Center regarding item number 4 of the FAQ.  Regarding
the last reply from AISC, my feeling is that while it
may be true that none of the cases in Table B5.1 are
strictly applicable, it is likely that one of them is
"close enough" and appropriate to approximate the
"effective width" of an infinite plate with an
attached stiffener.  Also, the "stem of tee" limit in
B5.1 is probably applicable to the stiffener (although
residual stress from welding the stiffener to the
plate may make the analogy somewhat less than

d a v e e v a n s

-----Original Message-----
From: d a v e e v a n s
To: Solutions Center
Subject: Clarification of FAQ regarding effective

Regarding subitem 4. in the answer to FAQ 4.8.1 

My question is: which of the many limiting
width-thickness ratios in Table B5.1 should be used?  

Some of the ratios are obviously not applicable (such
as those for stems of tees and circular sections), but
there are many others that might be applicable, and
the FAQ provides no guidance.


>>> "Solutions Center" <Solutions(--nospam--at)>

We are in the process of updating, modifying and
adding new FAQs.

Apparently, the FAQ you indicated needs modification,
as the reference to Table B5.1 is confusing, as there
is no particular case in the table that represents a
stiffening element attached to a plate.

The closest case for an unstiffened element may
actually be the stem of a tee (i.e. stem = stiffening
element, tee flange = plate.)   However, we cannot
make conclusive judgments, as this FAQ needs to be

We apologize for the inconvenience.

Sergio Zoruba, Ph.D., P.E.
Senior Engineer
American Institute of Steel Construction, Inc.

-----Original Message-----

Ok.  We'll wait for the FAQ update.  Can you give us
an idea when that will be? 

In the meantime, a comment regarding your reply: 

The FAQ deals with the question of the effective width
of the plate, not the stiffener.

The tablulated limiting width-thickness ratio for
stems of tees deals with the "effective width" of the
stem (stiffener), not the flange (plate).

So it seems that some other case would be more


>>> "Solutions Center" <Solutions(--nospam--at)>

You're right.  However, my answer was based on item
#4.  That is, item #4 appears to be checking the local
buckling width-to-thickness ratio of the stiffener
rather than the plate.  The plate, presumably, is
infinite in width as it can be part of a tank.  All
entries in Table B5.1 have finite plates (i.e.
compression elements) based on a finite width "b".

Hence item #4 is either completely out of context
because it addresses the local buckling of the
stiffener, or if item #4 addresses the plate,
it should be removed from the FAQ because there is no
plate in Table B5.1 with an infinite width.

In other words, there's a serious flaw with item #4
any way you look at it.  We should be posting the
updated FAQs by Spring of 2006.

Sergio Zoruba, Ph.D., P.E.


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