Civil Engineering Reference
In-Depth Information
Table A.3
Comparison between response characteristics of framed and braced systems (
relative measures
).
Frame
Stiffness
Strength
Ductility
MRF
L
H
H
CBF
H
H
L
EBF
M / H
M / H
M / H
KBF
M / H
M / H
M / H
Key
: H = high; L = low; M = moderate; CBF = concentrically braced frame; EBF = eccentrically braced frame;
KBF = knee - braced frame; MRF = moment resisting frame.
μ
Δ
of CBFs is generally small, especially when compared with MRFs. Buckling restrained braces can
be used to enhance the inelastic deformation capacity and energy absorption of braced frames (e.g.
Inoue
et al
., 2001; Bozorgnia and Bertero, 2004, among many others). Improved brace confi guration
layouts have been proposed. Thus, for large bay widths, KBFs are attractive lateral resisting systems
because of the shortening of the length of the braces (Balendra, 1993; Sam
et al
., 1995 ). In KBFs, one
end of the brace is connected to a short knee element instead of beam-column joint. The brace provides
the required lateral stiffness, whereas the ductility is obtained through shear yielding of the knee
element. KBFs are suitable for the seismic retrofi tting of steel, composite and RC moment frames: they
enhance lateral stiffness without endangering the ductility of MRFs.
Eccentrically braced frames employ bracing members with axis offsets to deliberately transmit forces
by combined bending and shear. Adequate lateral stiffness and ductility may be achieved by means of
link beams. For steel and composite structures, experimental and numerical tests have shown that active
links should yield in shear (known as 'short link') rather than in bending (known as ' long link ' ) to
dissipate a larger amount of energy (Hjelmstadt and Popov, 1984; Kasai and Popov, 1986 ; Qi
et al
.,
1997). Comparisons between response characteristics of MRFs, CBFs, EBFs and KBFs are outlined in
Table A.3. Both EBFs and KBFs exhibit enhanced seismic performance. These systems can be reliably
employed in medium- to high - seismicity regions.
EBFs and KBFs can accommodate architectural features such as door and window openings with
less intrusion; this is not the case for CBFs. Viable locations for braces are around cores and elevators,
where frame diagonals may be enclosed within walls. The braces can be joined together, thus behaving
as closed or partially closed spatial cells that may withstand torsional effects. Braced frames are cost-
effective for medium- to high-rise buildings, up to 30 storeys (Di Sarno, 2002). These systems are
widely used for several other types of constructions, such as towers, bridges and tanks, because high
lateral stiffness can be achieved with great economy of materials.
For high-rise building structures, e.g. up to 50-60 storeys, internal braced cores (either CBFs or
EBFs) are often connected to exterior columns of frames through deep and rigid truss beams (termed
'outriggers'). The resulting structural systems are known as ' outrigger - braced frames ' (OBFs). These
systems, which are suitable for steel, composite and RC tall buildings (Di Sarno, 2002), consist of four
components: (braced) core, outriggers, columns and beams. Typical layouts for OBFs are displayed in
Figure A.15. It is observed that outriggers, which are generally truss deep beams, can be as high as
two to three storeys and are generally located at the top and/or mid-height in the structure.
Under horizontal seismic actions, the core behaves in a fl exural mode. It exhibits high uplift forces
and base overturning moments because of its slenderness. Therefore, its effi ciency, as a free- standing
structure, is reduced as its height increases: this reduction can be expressed as a cubic function of the
height (Taranath, 1998). Exterior columns, which are offset with respect to the core (Figure A.15), act
as stays, increasing the lever arm to resist overturning moments. Elongation and shortening of these
columns may be advantageously used to prevent uplift in foundations (Balendra, 1993). Deep cap
trusses (also known as 'roof outriggers'), which connect cores to exterior columns, limit the curvature
in the core, thus reducing lateral deformations of the system (high stiffness) and bending moments (low
