of-plane length). Braces may be designed for tension
eccentricities must be provided for in the design.
only, but the use of such braces is discouraged
Such eccentricities do not mean that the frame is an
because they tend to stretch under earthquake
EBF: the EBF has unique properties and design
tension, go slack during the load reversal, then snap
methods.
when tension is applied in a subsequent cycle.
Diagonal cable bracing is permitted only for
(2) Concentric braced frame types. Braced
utilitarian one-story Seismic Use Group I buildings in
frames are usually of steel and may be of various
areas with SDS < 0.50g, and where the system is not
forms. Figure 7-19 illustrates some of the common
configurations for concentric braced frames. Trussed
required to provided lateral support for concrete or
portal bracing and K-bracing sometimes used in the
masonry walls.
older industrial buildings are still occasionally used
in bridge design, but have been replaced in buildings
(4) Effect of bracing on columns. The vertical
by one or more of the configurations shown in Figure
component of brace force is transferred into the
7-19. Braced frames with single diagonal members
column, and adds to or subtracts from the gravity
capable of resisting compression as well as tension
load on the column.
When braces are few and
are used to permit flexibility in the location of
heavily loaded, their vertical components may govern
openings. Chevron bracing also permits openings in
the design of the columns. The concern with braces
the middle of the braced bay, but the horizontal beam
of this type is that their true, as-built ultimate
at bracing intersection must be capable of resisting an
capacity may be greater than is assumed in design,
additional load equal to the vertical component of the
and therefore, that such braces could overload the
tensile brace when the compressive brace buckles.
column to the point of collapse.
For all of the bracing configurations in Figure 7-19,
the deflection of the braced frame is readily
(5) Configurations. Diagonal X-bracing is the
computed using recognized methods.
preferred configuration in that the tension brace can
provide in-plane lateral support to the compression
(3) Direction of brace force. Braces that are
brace. The orientation of single braces should be
designed for compression will, of course, act also in
alternated so that not all of the braces are in tension
tension. Diagonal members designed to resist both
or compression at the same time. Chevron bracing
compression and tension forces are preferred because
may have an interaction with gravity-load-carrying
they provide greater system redundancy. X-braced
beams;
accordingly,
special
requirements
are
panels are the most effective bracing configurations
provided in the AISC Seismic Provisions. K-bracing
as the tension diagonal provides direct in-plane
has a potentially dangerous effect on columns;
lateral support to the compression diagonal and also
provides out-of-plane resistance to compression
buckling (as indicated in Figure 7-21 an unbraced
length equal to two-thirds of the total length of the
compression brace may be used for the effective out-
7-51
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