the link beam by permitting doublers or allowing
(5) Link beam rotation. The link beam rotation, at a
frame drift 0.4R times the drift calculated from
holes within it.
prescribed seismic forces, is limited to the values
(e) Brace sizing. Once the link beam size
given in Paragraph 15.2g of the AISC Seismic
has been selected, the brace size is determined by the
The procedure for calculating the
requirement given in the AISC Seismic Provisions
rotations is as follows (refer to Figure 7-24):
that its compressive strength be at least 1.5 times the
axial force corresponding to the controlling strength
(a) Perform an elastic analysis of the frame
of the link beam. The controlling strength is either
for the prescribed seismic forces, being certain that
the shear strength Vs or the reduced flexural strength
the analysis includes the contribution of the elastic
MRS described above, whichever results in the lesser
shear deformation of the link beam.
force in the brace. Note that once the link beam is
selected, the brace forces are determined from its
Calculate 0.4R times the drift angle
strength, and the brace forces calculated in the elastic
obtained from the analysis in (1).
This angle is
analysis will not govern, and will not be used in the
denoted as " in Figure 7-24.
(c) Calculate the rotation angle 2 , as shown
(f) Brace-to-beam connection. The AISC
in Figure 7-24, for the appropriate configuration.
Seismic Provisions require that the brace-to-beam
This simplified procedure is slightly conservative,
connection develop the compressive strength of the
since the elastic curvature of the beam segments
brace, and that no part of the brace-to-beam
between hinges and of the brace deformations have
connection extend into the web area of the link. The
been ignored, and would contribute a minor amount
required development may be at the strength level of
of the required deformation. It should be noted that
the connection. The prohibition of the extension of
calculation of the rotation by multiplying the elastic
the brace-to-beam connection into the link beam is
deflections of the link beam by 0.4R would be
intended to prevent physical attachments that might
unconservative, since these deflections include elastic
alter the strength and deflection characteristics of the
effects, such as the axial deformation of the braces,
link beam. It is not intended to prevent the centerline
that would not increase proportionally after the link
intersection of brace and link beam from intersecting
begins to yield.
within the link.
(g) Column sizing. FEMA 302 requires
doubler plates are prohibited in AISC Seismic
that the columns remain elastic at 1.25 times the
Provisions because tests have shown that they are not
forces causing yield of the link beam.
fully effective. The performance of eccentric braced
elastic at" means the same as "have the strength to
frames relies on the predictability of the strength and
resist." The strength, including bending moments,
strain characteristics of the link beam.
It is not
considered advisable to complicate the behavior of