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Re: Width/Thickness Ratios

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The normal compact section limit (lambda-sub-p equals 65/SQRT Fy) ensures
that the entire cross-section can reach yield before flange local buckling.
The plastic compact section limit ( lambda-sub-pd equals 52/SQRT Fy)
ensures sufficient rotational capacity to develop the necessary plastic
hinge for the expected seismic behavior.

See Chapter 10 of "Ductile Design of Steel Structures" by Bruneau, Uang,
and Whittaker (1998) for a more complete discussion.

Rick Drake, SE
Fluor Daniel, Aliso Viejo, CA


"Dan Goodrich" <dang(--nospam--at)> on 01/24/2001 01:47:38 PM

Please respond to seaint(--nospam--at)

To:   <seaint(--nospam--at)>

Subject:  Width/Thickness Ratios

UBC 2211.8.4.b requires that beams and columns in  special moment
frames have a b/t ratio less than  52/sqrt(fy).  I have a beam that was
designed with A36 steel which meets this  requirement.  The supplier
substituted A572 steel which does not meet the  requirement.

What's the  background on this requirement?
If the frame is significantly stronger than  strength levels require
of drift limitations, would it be okay to not meet  the requirement?

b/t = 8.1
52/sqrt(fy) = 7.35

Dan Goodrich, P.E.

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