Page 239 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch05.qxd 9/29/05 5:12 PM Page 5.19

CRITERIA FOR BUILDING DESIGN

CRITERIA FOR BUILDING DESIGN 5.19

where

IC
r = yw (5.50)
2
ts
S x
2
For doubly symmetric I-shapes with rectangular flanges, C w = I y h o /4 and thus Eq. (5.50) becomes
Ih
r = yo (5.51)
2
ts
2 S x
For other shapes, approximate r ts as the radius of gyration of a section comprising the compression
flange plus one-sixth of the web, which is
b
r = f (5.52)
ts
(
/
[
12 1 + 1 6)( ht b t )]
/
w
ff
5.5.2 Doubly Symmetric I-Shaped Members with Compact Webs
and Noncompact or Slender Flanges—Major Axis Bending
For doubly symmetric I-shaped members, subjected to bending about their major axis, and having
compact webs but noncompact or slender flanges (see Art. 5.1.5), the effects of local buckling must
also be considered. The nominal flexural strength M n of such members is the lower of two limit
states: lateral-torsional buckling and compression flange local buckling. The nominal flexural
strength for lateral-torsional buckling is the same as given in Art. 5.5.1.
For sections with noncompact flanges, the nominal flexural strength for compression flange local
buckling is given by
−
  λλ pf  
M =  M − ( M − .07 F S ) − λ   (5.53)
y x 
p
n
p
   λ rf pf   
For sections with slender flanges, the nominal flexural strength for compression flange local
buckling is given by
M = 09 . Ek S (5.54)
cx
n
λ 2
where λ pf =λ p = limiting slenderness for a compact flange (Art. 5.1.5)
λ rf =λ r = limiting slenderness for a noncompact flange (Art. 5.1.5)
λ= b f /2t f
/
k c = 4/ ht w and 0.35 ≤ k c ≤ 0.76 for calculation purposes

5.5.3 Doubly Symmetric and Singly Symmetric I-Shaped Members
with Slender Webs—Major Axis Bending
This article applies to doubly symmetric and singly symmetric I-shaped members, subjected to
bending about the major axis, with slender webs (see Art. 5.1.5) attached at the mid-width of the
flanges. For this case, the nominal flexural strength M n is the lower of four limit states: compression
flange yielding, lateral-torsional buckling, compression flange local buckling, and tension flange
yielding.

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