a function of the geometry of the frame (see Figure 7-
restrictive flange-width-to-thickness ratio of 52 / Fy
24). Link beams can yield in shear, in bending, or in
are required for the beam portions of eccentric braced
both shear and bending at the same time. Which
frames in order to provide the beams with stable
yield mechanism governs is a function of the
inelastic deformation characteristics.
The same
relationship of link length to the ratio of its bending
requirement is used for the beams of special moment-
strength to shear strength. Where the length of the
resisting space frames.
link beam is less than 1.6 Ms/Vs, the yielding is
almost entirely in shear. Where the length is greater
(b)
Link beam strength.
The basic
than 2.6 Ms/Vs, the yielding is primarily in bending.
requirement for link beam strength is given in the
Where the length is between 1.6 Ms/Vs and 2.6 Ms/Vs,
AISC Seismic Provisions, which states that the shear
both shear and bending yield will occur. Since link
in the link beam web due to prescribed seismic forces
beams that yield in shear are considered to have the
be limited to 0.8 Vs. Paragraph 15.2f of the AISC
most stable energy-dissipating characteristics, most
Seismic Provisions addresses the concern for the
of the EBF research has tested the cyclic inelastic
effect that substantial axial loads in the link beam
capacity of link beams with shear yielding at large
could have on its inelastic deflection performance. It
rotations.
Consequently, most of the design
presumes that in shear links, the web' capacity is
s
provisions are concerned with limiting the link beam
fully utilized in shear, and that flanges provide the
shear yield rotation to less than the maximum cyclic
needed axial and flexural capacity. Shear links with
test rotations, and then requiring details indicated by
a length less than 2.2 Ms/Vs are considered to be
the tests as necessary to ensure that this rotation can
controlled by shear. Substantial axial loads occur in
occur through a number of cycles without failure.
some EBF configurations when the link beam is
required to transmit horizontal forces to or from the
(4)
Design criteria.
The specific criteria
braces.
It is recommended that, insofar as it is
governing the design of eccentrically braced frames
possible, link beams be located so that they are not
are
given
in
the
AISC
Seismic
Provisions.
required to transmit the horizontal force component
Additional detail is provided in the following
of braces or drag struts. Where axial forces in the
paragraphs.
link cannot be avoided, the flexural strength shall be
reduced by the axial stress fa, giving MRS = Z (Fy - fa).
(a) Link beam location and stability. Link
The fa should correspond to the lesser value of the
beams are the fuses of the EBF structural system, and
axial force corresponding to yield of the link beam in
are to be placed at locations that will preclude
shear, or that which, when combined with link
buckling of the braces. A link beam must be located
bending, causes the beam flanges to yield.
in the intersecting beam at least at one end of each
brace.
There are exceptions permitting concentric
bracing at the roof level and/or at the bottom level of
EBF over five stories in the AISC Seismic
Provisions.
Compact sections meeting the more
7-61
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