Page 373 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch08.qxd 9/29/05 5:21 PM Page 8.27

LATERAL-FORCE DESIGN

LATERAL-FORCE DESIGN 8.27

illustrated in Fig. 8.9. As a result, the AISC seismic provisions require a balance of the relative plastic
capacity of the beam and the column considering the full expected plastic moments (M p = R y F y Z) in
the beam and the nominal moments in the column.
Column splices also require some special consideration with moment-resisting frames. Inelastic
analysis shows that significant bending moments may develop in the columns despite the balancing
requirements to assure strong-column, weak-beam behavior. The distribution of plastic deformation
varies widely in moment frames during severe earthquake shaking. The consequences of this are that
the columns may sustain limited plastic deformation and may temporarily be in single curvature
rather than the double curvature assumed in design. Column splices are usually made near midheight
of a story, where the bending moment is relatively small. However, the AISC seismic provisions rec-
ognize that plastic strains may occur in this region, and the column splice is required to have a min-
imum resistance in flexure and shear. If groove welds are employed at this splice, the welds must be
complete-joint-penetration welds unless a smaller splice-resistance requirement can be shown by
inelastic analysis. If bolts or other splice-connection methods are used, the splice-connection flexur-
al resistance must exceed the expected plastic-moment capacity of the column.
Ordinary and Intermediate-Moment Frames. Some steel moment-resisting frames are not
designed to satisfy the preceding conditions. In many cases, these frames are used in less seismically
active zones. Sometimes, however, they are used in seismically active zones with larger seismic
design forces; that is, they are designed with R = 3.5. As a result, the design forces would be more
than twice as large as required for special-moment frames. The seismic ductility demands will be
significantly smaller, but the detailing requirements are also reduced. These are known as ordinary
moment frames. Ordinary moment-resisting frames must satisfy some of the requirements noted
above but not all, depending on the seismic zone and the design forces in the structure.
Intermediate-moment frames are intermediate alternatives to ordinary and special-moment-resisting
frames. They have intermediate ductility demands and detailing requirements, and permit interme-
diate seismic design-force levels.

8.7.2 Limitations on Concentric Braced Frames
Concentric braced steel frames are much stiffer and stronger than moment-resisting frames, and
they frequently lead to economical structures. However, their inelastic behavior is usually inferior to
that of special moment-resisting steel frames (Art. 8.6). One reason is that the behavior of concentric
braced frames under large seismic forces is dominated by buckling. Furthermore, the columns must
be designed for tensile loads and foundation uplift as well as for compression. As with moment-
resisting frames, concentric braced frames may be designed to different seismic-design standards.
Special concentrically braced frames are designed for the largest R values and the smallest seismic-
design forces. Special concentrically braced frames also have more detailed design requirements
because of the necessity of achieving greater ductility from the braced frame system. Ordinary
concentrically braced frames may also be designed. These latter braced frames use larger seismic-
design forces and have less reliance on inelastic deformation capacity and buckling from the braced
frame system. As a result, design requirements are somewhat more liberal. Ordinary concentrically
braced frames are less commonly used today for demanding seismic applications. Unless otherwise
noted, the discussion in this section will focus primarily on the special concentrically braced frame
system.
Figure 8.6 shows some of the common bracing configurations for concentric braced frames.
Seismic design requirements vary with bracing configurations.
X-bracing has historically used very slender braces designed as tension-only bracing or bracing
with only limited compressive buckling capacity. The resulting braces had high slenderness ratios,
KL/r. This historic practice lead to economical designs but poor seismic performance. As a result,
many past seismic provisions discouraged or disallowed X-bracing. Today a very different practice
has evolved with X-braced frames. In many cases the X-bracing extends over multiple stories to
effectively combine V- and inverted V-bracing. Second, design requirements for special concentri-
cally braced frames require a balance of the shear resistance provided by braces in tension with

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.