Page 242 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch05.qxd 9/29/05 5:12 PM Page 5.22

CRITERIA FOR BUILDING DESIGN

5.22 CHAPTER FIVE

For yielding (plastic moment), use Eq. (5.40).
For flange local buckling, three cases may arise. When the flanges are compact, this limit state
does not apply. When the flanges are noncompact,
 F 
M = M − ( M − F S ) . 357 b y − . 40  ≤ M (5.73)

p
y eff
p
n
 t E  p
When the flanges are classified as slender,
(5.74)
M n = F y S eff
where S eff is the effective section modulus determined using the effective width b e of the compression
flange of width b, calculated as

.
E 038 E 
b = 192 t  1 −  ≤ b (5.75)
.
e
F y   bt / F y  
For web local buckling of noncompact sections under pure flexure,
 
M = M − ( M − F S ) .0 305 h F y − .0 738  ≤ M p (5.76)
y x 
p
n
p
 t w E 
When the web is compact, the limit state of web local buckling does not apply.
5.5.6 Round HSS
For round HSS that have a diameter-to-thickness ratio D/t less than 0.45E/F y , the nominal flexural
strength M n is the lower of two limit states: yielding (plastic moment) and local buckling.
For yielding (plastic moment), use Eq. (5.40).
For local buckling, three cases may arise. When the section is compact, this limit state does not
apply. When the section is noncompact,
 0 021. E 
M =  + F S (5.77)
y
 Dt / 
n
When the section has slender walls,
 033. E
M = F S =   S (5.78)
cr
n
 Dt 
/
5.5.7 Rectangular and Round Bars

The nominal flexural strength M n of rectangular and round bars subjected to bending about either geo-
metric axis is the lower of two limit states: yielding (plastic moment) and lateral-torsional buckling.
For yielding (plastic moment), when
.
Ld ≤ 008 E
b
t 2 F y
use Eq. (5.69).

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