Civil Engineering Reference
In-Depth Information
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Figure A.14
Common brace confi gurations for concentrically (
top
) and eccentrically (
bottom
) braced frames
stiffness and strength of CBFs are well above that of MRFs. Lateral defl ection modes depend upon the
vertical slenderness of braced bays. In low-rise (squat) BFs, shear defl ections are predominant, while
high-rise (slender) BFs displace primarily in fl exural modes. Shear deformations are caused by the
elongation of braces and shortening of beams. Conversely, fl exural deformations are generated by
shortening and elongation in exterior columns.
In CBFs, internal actions are primarily transmitted through axial actions, either compression or
tension. Under horizontal seismic forces, beams are in compression while braces are either in tension
or compression. By reversing the load direction, beams are in tension. Lateral strength of CBFs depends
on the capacity of braces, beams and columns.
Diagonal braces are the dissipative elements in CBFs. They are expected to yield under moderate- to
high-magnitude earthquake ground motions. Alternate stress reversals can cause buckling of com-
pressed braces, thus inhibiting large energy absorption. In steel and composite CBFs and/or RC frames
retrofi tted with steel diagonals, the amount of dissipated seismic energy is signifi cantly reduced by the
onset of buckling, local and global. Braced frames can be employed in areas of high seismicity
provided that they employ high ductile details, especially for braces and their connections with beams
and columns.
Inelastic seismic performance of CBFs is considered fairly poor because of proneness to buckling
of strut components along with softening due to the Bauschinger effect. The global translation ductility
