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# RE: Question about brace design forces for columns

• To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
• Subject: RE: Question about brace design forces for columns
• From: "Carter, Charlie" <carter(--nospam--at)aisc.org>
• Date: Wed, 24 Sep 2003 11:02:52 -0500

```>Could someone please explain to me where the
>rule-of-thumb that column braces be designed
>for 2% of the column vertical load comes from?

The 4th Edition of the Guide to Stability Design Criteria for Metal
Structures (page 55)indicates that the required bracing strength is based
upon the initial out-of plumbness of the column, the ideal and actual
stiffnesses of the member, and the number of points of bracing along the
length of the member. If the initial lean is 1 in 500, the ratio of ideal
stiffness to actual stiffness is 0.5, and the number of braces is 1, the
bracing force is 0.8 percent of P_critical. If the number of braces
approaches infinity, the bracing force is 1.6 percent of P_critical. To
cover the full range, and to account for the uncertainties in this
derivation, the 2 percent value was born as a usual conservative practice.

However! Today's structures look very little like those that were considered
in establishing the above derivation, which was done by Winter circa 1960.
As a result, one should return to the basic assumption behind it -- that
sufficent strength and stiffness are BOTH needed to provide proper bracing.
In old-style construction, the stiffness was usually there and one could
simply assess the bracing by the 2 percent strength rule. If you know what
you are doing, you can still probably do this. But you have to know what you
are doing.

Modern approaches to stability bracing design provide for assessment of both
strength and stiffness. These provisions are included in detail in the AISC
Specification, Chapter C3. It's a free download at www.aisc.org if you don't

Charlie

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