c.
Eccentric Braced Steel Frames (EBF).
7-22. When the in-plane brace connection is welded
as shown in Figure 7-21, the appropriate K value for
(1)
Definition.
An EBF is a steel-braced
restrained end conditions should be used, and the
frame designed in accordance with Section 15, Part I,
welds and gusset plate should be designed for the
of the AISC Seismic Provisions. At least one end of
plastic moment capacity of the brace. For the gusset
each brace intersects a beam at a point offset from the
plate, a section, normal to the brace, or the midpoint
beam intersection with the column or with the
of the connection, should have the necessary capacity
opposing brace (see Figure 7-23). The short section
to resist the above moment.
of the beam between opposing braces, or between a
brace and the beam-column intersection, is called the
(9) Acceptance criteria.
"link beam," and is the element of the frame intended
(a) Response modification factors, R, for
to provide inelastic cyclic yielding.
Performance Objective 1A are provided in Table 7-1.
(2)
Purpose.
The intent of the eccentric
K-braced frames shall be classified as ordinary
braced frame design is to provide a ductile link that
concentric braced frames and are subject to the
will yield in lieu of buckling of its braces when the
limitations of Paragraph (6) above.
frame experiences dynamic loads in excess of its
(b) Modification factors, m, for enhanced
elastic strength. Although they are usually easier to
detail, they are more complex to design than CBFs,
performance objective are provided in Table 7-10.
and they are most useful in areas with SDS $ 0.75.
(c)
Modeling parameters and numerical
acceptance criteria for nonlinear procedures are
(3) Characteristics. To take advantage of the
provided on Table 7-11.
ductility of the link, it is important that all related
framing elements be strong enough to force the link
(d) The expected strength of deformation-
to yield, and that they maintain their integrity through
controlled
components
or
elements
shall
be
the range of forces and displacements developed
determined using the expected yield strength, Fye, as
during the yielding of the link. The braces are the
defined in the AISC Seismic provisions.
most vulnerable of the framing elements because
seismic forces are by far the dominant forces in their
(e)
The
lower-bound
strength
of
design.
Other elements, such as columns and
connections and other force-controlled components
collector beams, are less vulnerable, since their
shall be taken as the nominal strength multiplied by
seismic loads constitute a smaller percentage of their
the appropriate resistance factor, N, determined from
total loads, and since there are frequently redundant
load paths for portions of the forces they carry. The
the provisions of the AISC LRFD Specifications.
rotation demand on the link beam is a multiple of the
lateral drift of the frame as a whole, a multiple that is
7-56
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