accordingly, it is subject to the requirements of
(c) Design considerations. There are two
Section 14.4b, Part I, of the AISC Seismic Provision,
concerns with KBFs.
The first concern involves
and permitted only in buildings in Seismic Design
gravity load: any change in the load on the beam after
Categories A and B.
the brace is connected induces forces in all the
components of the frame; moreover, the brace has a
(6)
Low buildings.
The AISC Seismic
prying effect that can produce surprisingly large
Provisions provide special provisions for concentric
forces in the beam-column joint. The sequence of
bracing in metal buildings not over two stories, and
erection and the further application of superimposed
for light roof structures such as penthouses.
loads must be carefully controlled.
The second
Manufactured metal buildings are intended to be
concern involves seismic loads: another set of loads
included in this category.
In planning the use of
is applied, and while the brace does stiffen the frame,
manufactured metal buildings, the designer is
its as-built ultimate capacity may cause bending in
cautioned that these buildings can perform well only
the column of sufficient magnitude to cause collapse.
when they are kept light and simple, as they are
intended to be; they may have poor performance if
(d) Design criteria. KBFs shall be designed
extra weight, such as masonry veneer, is added, or if
in accordance with Section 9.4 of the AISC Seismic
they are used as elements of a more complex system.
Provisions, and the use of KBFs shall be restricted to
roof structures or to unoccupied storage or other
(7) Knee-braced frames (KBF).
utilitarian buildings with Performance Objective 1A,
not over two stories in height.
(a) Definition. A KBF is an assembly of a
beam, a column, and a brace whose ends are
(8)
Connections.
The AISC Seismic
significantly offset from the beam-column joints.
Provisions provide the requirements for design of
The braces in CBFs are either truly concentric, or
connections.
Figure 7-22 illustrates the design of
have small eccentricities with the beam-column
gusset plates with welded connections.
Note that
joints; accordingly, they induce forces that are
most steel braces are designed as pin-ended members
primarily axial, while the braces in KBFs have
(K=1.0) for compressive forces.
As the braces
substantial eccentricities, and induce significant
deflect out-of-plane in compression, the gusset must
shearing, and flexural, as well as axial, stresses in the
be able to accommodate the end-rotation. The AISC
columns and beams.
LRFD Specifications prescribe that the brace
connection should provide a minimum length of
(b) Function. Knee braces were often used
gusset plate, a, equal to twice the plate thickness, t, to
in the past to stiffen beams and to provide a measure
permit end-rotation of the brace as shown in Figure
of lateral stability. Their popularity in recent years
has decreased markedly, particularly in zones of high
seismicity, because their seismic behavior has
become recognized as potentially dangerous.
7-54
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