Page 243 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch05.qxd 9/29/05 5:12 PM Page 5.23

CRITERIA FOR BUILDING DESIGN

CRITERIA FOR BUILDING DESIGN 5.23

For lateral-torsional buckling, of rectangular bars in major axis bending:
008E Ld 19 . E
.
When < b ≤ ,
F y t 2 F y
  Ld F 
b 
M = C 1 52 0 274 y  M ≤ M (5.79)
b 
.
− .
n
  t 2  E  y p
Ld 19 E
.
When b > ,
t 2 F
y
M =  19 . EC  S ≤ M (5.80)
b
n   x p
 Ld t 
2
/
b
where t = width of rectangular bar parallel to axis of bending, in (mm)
d = depth of rectangular bar, in (mm)
L b = distance between points braced against lateral displacement of the compression region,
or between points braced to prevent twist of the cross section, in (mm)
When rectangular bars are subjected to minor axis bending, and for all cases of round bars, this limit
state does not apply.
5.5.8 Hole Reductions
For flexural members with holes, such as for bolts, the limit state of tensile rupture of the tension
flange must also be considered.
When F u A fn ≥ Y t F y A fg , the limit state of tensile rupture does not apply.
When F u A fn < Y t F y A fg , the nominal flexural strength at the location of the holes in the tension
flange is limited to
M = FA S (5.81)
u fn
n
A fg x
2
2
where A fg = gross tension flange area, in (mm )
2
2
A fn = net tension flange area, in (mm ) (see Art. 5.3.1)
Y t = 1.0 for F y /F u ≤ 0.8
= 1.1 otherwise
5.5.9 Proportioning Limits for I-Shaped Members
Certain proportioning limits also apply for I-members. Singly symmetric I-shaped members must
satisfy the following:
01≤ I yc ≤ 09 (5.82)
.
.
I y
Stiffened, slender web I-shaped members must satisfy the following. When a/h ≤ 1.5,
h E
.
≤11 7 (5.83)
t w F y
When a/h > 1.5,
h 042 E
.
≤ (5.84)
t w F y
where a = clear distance between transverse stiffeners, in (mm).

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