Seismic Design Practice For Eccentrically Braced Frames Based On The 1994 UBC

EBFs address the desire for a laterally stiff framing system
with significant energy dissipation capability to accommodate
large seismic forces (ref. 7). A typical EBF consists of
a beam, one or two braces, and columns. Its configuration
is similar to traditional braced frames with the exception
that at least one end of each brace must be eccentrically
connected to the frame. The eccentric connection introduces
bending and shear forces in the beam adjacent to
the brace. The short segment of the frame where these
forces are concentrated is the link.
EBF lateral stiffness is primarily a function of the ratio of
the link length to the beam length (ref. 8, p. 44). As the link
becomes shorter, the frame becomes stiffer, approaching
the stiffness of a concentric braced frame. As the link
becomes longer, the frame becomes more flexible approaching
the stiffness of a moment frame.
The design of an EBF is based on creating a frame which
will remain essentially elastic outside a well defined link.
During extreme loading it is anticipated that the link will
deform inelastically with significant ductility and energy
dissipation. The code provisions are intended to ensure
that beams, braces, columns and their connections remain
elastic and that links remain stable. In a major earthquake,
permanent deformation and structural damage to the link
should be expected.
There are three major variables in the design of an EBF:
the bracing configuration, the link length, and the link
section properties. Once these have been selected and
validated the remaining aspects of the frame design can
follow with minimal impact on the configuration, link length
or link size.
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