Page 241 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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Brockenbrough_Ch05.qxd 9/29/05 5:12 PM Page 5.21

CRITERIA FOR BUILDING DESIGN

CRITERIA FOR BUILDING DESIGN 5.21

For noncompact ﬂange sections,
  λλ pf  
−
F =  F − (.03 F ) − λ   (5.65)
y 
y
cr
   λ rf pf   
For slender ﬂange sections,
F = 09 . Ek c (5.66)
cr
b 2 )
(/ t f 2
f
where
k = 4 and 0.35 ≤ k ≤ 076 (5.67)
.
c
c
/
ht w
and the λ slenderness terms are as deﬁned with Eq. (5.54).
For tension ﬂange yielding, when S xt ≥ S xc , the limit state of tension ﬂange yielding does not
apply. When S xt < S xc ,
(5.68)
M n = F y S xt
where S xc is the section modulus about the x axis referred to the compression ﬂange and S xt is the
section modulus about the x axis referred to the tension ﬂange.
5.5.4 I-Shaped Members and Channels—Minor Axis Bending
The nominal ﬂexural strength M n for I-shaped members and channels subjected to bending about the
minor axis is the lower of two limit states: yielding (plastic moment) and ﬂange local buckling.
For yielding (plastic moment),
(5.69)
M n = M p = F y Z x ≤ 1.6F y S y
For ﬂange local buckling, three cases may arise. When the ﬂanges are compact, this limit state
does not apply. When the ﬂanges are noncompact,
  λλ pf  
−
M =  M − ( M − .07 F S )   (5.70)
y y 
p
p
n
   λ rf − λ pf   
When the ﬂanges are classiﬁed as slender,
(5.71)
M n = F cr S y
.
F = 069 E (5.72)
cr
b 2 )
(/ t f 2
f
where λ= b/t = ﬂange width-to-thickness ratio
λ pf = limiting slenderness for a compact ﬂange (Art. 5.1.5)
λ rf = limiting slenderness for a noncompact ﬂange (Art. 5.1.5)
S y = minimum section modulus about y axis
5.5.5 Square and Rectangular HSS and Box-Shaped Members
This article applies to square and rectangular hollow structural sections (HSS), and doubly symmet-
ric box-shaped members bent about either axis. The sections may have compact or noncompact webs,
and compact, noncompact, or slender ﬂanges (see Art. 5.1.5). The nominal ﬂexural strength, M n , is the
lower of three limit states: yielding (plastic moment), ﬂange local buckling, and web local buckling.

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