Page 276 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch05.qxd 9/29/05 5:12 PM Page 5.56

CRITERIA FOR BUILDING DESIGN

5.56 CHAPTER FIVE

strength for the limit state of web sidesway buckling is determined using φ= 0.85 (LRFD) and Ω= 1.76
(ASD). The nominal strength, R n , depends on the flange rotational restraint and the (h/t w )/(l/b f ) ratio.
When the compression flange is restrained against rotation:
If (h/t w )/(l/b f ) ≤ 2.3,
Ct t   / 3 
3
ht 
R = rw f 10  w   (5.168)
+ 4.
n
h 2    / f 
lb  
If (h/t w )/(l/b f ) > 2.3, the limit state of web sidesway buckling does not apply.
For the case with flange rotational restraint, if the required strength of the web exceeds the avail-
able strength, provide local lateral bracing at the tension flange, a pair of transverse stiffeners, or a
doubler plate. The stiffeners should extend from the loaded flange for at least half the member depth
and, here, designed to carry the full load.
When the compression flange is not restrained against rotation:
If (h/t w )/(l/b f ) ≤ 1.7,
Ct t   / 3 
3
ht 
R = rw f 04.  w   (5.169)
n
lb  
h 2    / f 
If (h/t w )/(l/b f ) > 1.7, the limit state of web sidesway buckling does not apply.
For the case without flange rotational support, if the required strength of the web exceeds the
available strength, provide local lateral bracing at both flanges at the point of application of the con-
centrated forces.
The following definitions apply in Eqs. (5.168) and (5.169):
l = largest laterally unbraced length along either flange at the point of load, in (mm)
b f = flange width, in (mm)
t f = flange thickness, in (mm)
t w = web thickness, in (mm)
h = clear distance between flanges less fillet or corner radius for rolled shapes; distance between
adjacent lines of fasteners or the clear distance between flanges when welds are used for built-up
shapes, in (mm)
6
C r = 960,000 ksi (6.62 × 10 MPa) when M u < M y (LRFD) or 1.5M a < M y (ASD) at the location
of the force
6
C r = 480,000 ksi (3.31 × 10 MPa) when M u ≥ M y (LRFD) or 1.5M a ≥ M y (ASD) at the location
of the force
M u = required flexural strength (LRFD); M a = required flexural strength (ASD); and M y = yield
moment

5. Local Web Buckling. This limit state applies to a pair of compressive single-concentrated forces
or the compressive components in a pair of double-concentrated forces, applied at both flanges of a
member at the same location. An example is the web of a column where forces are applied by bot-
tom flanges of beams in a moment connection. If the required strength of the web exceeds the avail-
able strength, provide a single transverse stiffener, a pair of transverse stiffeners, or a doubler plate
extending the full depth of the web.
The available strength for the limit state of local web buckling is determined using φ = 0.90
(LRFD) and Ω= 1.67 (ASD). The nominal strength is
24 t 3 EF
R = w yw (5.170)
n
h
where symbols are as previously defined. When the pair of concentrated compressive forces to be
resisted is applied at a distance from the member end that is less than d/2, reduce R n by 50%.

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