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(6) Web Crippling. Provisions for web stiffeners, as presented in
Section 1.15.5 of the AISC Specification, should be used in dynamic design.
In applying these provisions, Fy should be taken equal to the specified
static yield strength of the steel.
(7) Lateral Bracing. Members subjected to bending about their
strong axis may be susceptible to lateral torsional buckling in the
direction of the weak axis if their compression flange is not laterally
braced. Therefore, in order for a plastically designed member to reach its
collapse mechanism, lateral supports must be provided at the plastic hinge
locations and at a certain distance from the hinge location. The distance
from the brace at the hinge location to the adjacent braced points should
not be greater than *lcr as determined from either Equation (94a) or (94b)
depending on the value of M/Mp.
EQUATION:
*lcr/ry = l,375/Fdy + 25, 0.5 < M/Mp < /= 1.0
(94a)
EQUATION:
*lcr/ry = 1,375/Fdy,
1.0 < /= M/Mp < /= 0.5(94b)
where,
ry = radius of gyration of the member about its weak axis
M = lesser of the moments at the ends of the unbraced segment
M/Mp = end moment ratio; positive when the segment is bent in
reverse curvature and negative when bent in single
curvature.
(a) Since the last hinge to form in the collapse mechanism is
required to undergo less plastic deformation, the bracing requirements are
somewhat less stringent. For this case, in order to develop the full
plastic design moment, Mp, the following relationship may be used for
members with design ductility ratios less than or equal to 3.
EQUATION:
*l/rT = [(102 x l03Cb)/Fy]1/2
(95)
where,
*l = distance between cross sections braced against twist or
lateral displacement of the compression flange
rT = radius of gyration of a section comprising the compression
flange plus 1/3 of the compression web area taken about an
axis in the plane of the web
Cb = bending coefficient defined in Section 1.5.1.4.6a of the AISC
Specification.
(b) However, in structures designed for ductility ratios
greater than 3, the bracing requirements of Equation (94a) or (94b) must be
met.
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