Page 365 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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LATERAL-FORCE DESIGN

LATERAL-FORCE DESIGN 8.19

The reader is referred to “State of the Art Report on Connection Performance,” FEMA 355D, Federal
Emergency Management Agency, Washington, D.C., September 2000, for a summary of the con-
nection analytical and experimental research. In addition, “Recommended Seismic Design Criteria
for New Steel Moment-Frame Buildings,” FEMA 350, Federal Emergency Management Agency,
Washington, D.C., July 2000, summarizes the design recommendations for these moment-frame con-
nections. Many of these recommendations are being incorporated into the AISC Seismic Design
Provisions, but the work has demonstrated several connections, which provide good seismic perfor-
mance but are not yet included in the seismic provisions. These reports provide information on these
alternate connections. The SAC Steel Project clearly showed that steel moment frames are capable
of achieving superior ductility and inelastic seismic performance. However, the work also shows that
the engineer must exercise great care in the selection and design of members and connections. The
requirements for special moment frames are summarized briefly in Art. 8.7.1.
Concentric braced frames, defined in Art. 8.4, economically provide much larger strength and
stiffness than moment-resisting frames with the same amount of steel. There are a wide range of
bracing configurations, and considerable variations in structural performance may result from these
different configurations. Figure 8.6 shows some concentric bracing configurations. The braces,
which provide the bulk of the stiffness in concentrically braced frames, attract very large compres-
sive and tensile forces during an earthquake. As a result, compressive buckling of the braces often dom-
inates the behavior of these frames. The pinched cyclic force-deflection behavior shown in Fig. 8.7b
commonly results, and failure of braces may be quite dramatic. Therefore, concentrically braced frames
are regarded as stiffer, stronger but less ductile than steel moment-resisting frames. In recent years,
research has shown that concentrically braced frames can sustain relatively large inelastic deforma-
tion without failure if greater care is used in the design and selection of the braces and the brace con-
nections. However, continuing research work is in progress in establishing the design criteria for the
brace and the connections. The work shows that concentrically braced frames that are designed and
detailed to the higher ductility standards can be designed for smaller seismic design forces. The
AISC Seismic Design Provisions define “special concentrically braced frames” with requirements
for details aimed at achieving the higher ductility. Current detailing provisions are summarized in
Art. 8.7.2.
Eccentric braced frames, defined in Art. 8.4, can combine the strength and stiffness of concen-
trically braced frames with the good ductility of moment-resisting frames. Eccentric braced frames
incorporate a deliberately controlled eccentricity in the brace connections (Fig. 8.5). The eccentric-
ity and the link beams are carefully chosen to prevent buckling of the brace, and provide a ductile
mechanism for energy dissipation. If they are properly designed, eccentric braced frames lead to
good inelastic performance as depicted in Fig. 8.7c, but they require yet another set of design provi-
sions, which are summarized in Art. 8.7.3.
Buckling-restrained concentrically braced frames are a new option for concentrically braced
frames that are not yet fully incorporated in the design specifications. However, because this system
offers the potential for superior seismic performance, buckling-restrained braces are expected be
included in future seismic design specifications. Buckling-restrained braces employ patented braces
in which the axial member yields in tension and compression without brace buckling, as depicted
in Fig. 8.11. This is accomplished by encasing the brace bar so as to prevent lateral deformation
and buckling without bonding the slender bar to the encasing element. This assures that the slender
bar yields in both axial tension and compression, and deterioration in stiffness and resistance due to
buckling is avoided. It increases the inelastic energy dissipation, improves axial yield performance,
and permits development of large inelastic axial deformations. The seismic performance of buckling-
restrained braces depends on both the brace and the connection design. Research is now developing
improved design procedures for the brace and the connection. However, a recent guideline proposes
design criteria and testing and acceptance criteria that can be used to verify that the buckling-
restrained brace is appropriate for the proposed seismic design application. The reader is referred to
“Recommended Provisions for Buckling Restrained Braced Frames,” Structural Engineering
Association of Northern California, Seismology and Structural Standards Committee, San Francisco,
Calif., October 2001, for current thinking and design information on this system. The buckling-
restrained brace is included in 2005 AISC Seismic Design Provisions (see Art. 8.7.4).

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