Page 377 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch08.qxd 9/29/05 5:21 PM Page 8.31

LATERAL-FORCE DESIGN

LATERAL-FORCE DESIGN 8.31

beam considering the expected yield strength (R y F y ) of the link beam. Yielding or buckling of the
columns must also be avoided. Therefore, the column must be designed for the combined axial force
of 1.1 times the sum of the expected nominal shear strength of all link beams above the level under
consideration. These brace and column design forces are needed to ensure that the brace and column
do not buckle as the link-beam strain hardens during inelastic deformation.

Link Beam. Eccentrically braced frames develop good inelastic behavior because yielding in the
link beam occurs well before brace buckling or inelastic deformation of the columns, and this yield-
ing permits large inelastic deformations and great energy dissipation during severe earthquakes. The
link beam may yield in shear, flexure or a combination of the two depending on the size of the beam
and the length of the link. The normal yield shear of the link beam is the lesser of V p or 2M p /e. In
this expression, M p is the normal link beam plastic moment (M p = ZF y ) and e is the clear span eccen-
tricity of the link beam. The plastic shear capacity V p of the link beam is
(8.30)
V p = 0.60F y (d − 2t f )t w
where d, t f , and t w are the depth, flange thickness, and web thickness of the link beam. The nominal
yield shear and moment of the link beam may require further reduction if the axial force in the link
beam exceeds 15% of the yield axial force. These reduced capacities are

V = V 1 −   P   2 (8.31a)
u
p
pa
 P 
y
and
 P 
M pa = M 1 − u  (8.31b)
p 
 P 
y
However, it is not very common to design eccentrically braced frames with large axial forces in the
link beams.
In general, link beams yielding in shear are preferred, because they have significantly larger
inelastic deformation capacity. Link beams for which
16 M
.
e ≤ p (8.32a)
V p
are controlled by shear yield behavior, and they have a maximum plastic link rotational angle of
0.08 rad. Link beams for which
26 M
.
e ≥ p (8.32b)
V p
are controlled by flexural yield behavior, and they have a maximum plastic link rotational angle of
0.02 rad. Link beams with lengths between these two limits are intermediate links, and their rota-
tional limit is determined by interpolation. The rotational limit must be compared to the maximum
rotation predicted for the link in the analysis of the system under seismic loading.

Stiffeners and Lateral Support of the Link Beam. The link beam is subject to high bending stress,
high shear stress, and significant inelastic deformation. As a result, it must have lateral support to
both the top and bottom flanges at both ends of the link beam. The lateral supports must have ade-
quate resistance to develop 6% of the expected flange force (R y F y Z/h). The beam must also satisfy
all of the web and flange slenderness requirements previously noted for special moment-resisting

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