Page 374 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch08.qxd 9/29/05 5:21 PM Page 8.28

LATERAL-FORCE DESIGN

8.28 CHAPTER EIGHT

braces in compression, and this effectively prevents the very slender braces noted in historic prac-
tice. As a result, X-bracing is used more frequently today, since the seismic performance is regarded
as improved with the changes in design practice.
V-bracing or inverted V-bracing has the bracing connection at midspan of the beam. Under lat-
eral load, one brace acts in compression while the other acts in tension. The capacity of the tensile
brace is significantly larger than the compressive capacity of the other brace, and this unbalanced
force at the brace–beam intersection causes beam yielding during severe seismic excitation. Beam
flexural yield with this bracing may provide significant increases in energy dissipation. As a conse-
quence, these bracing configurations were more favorably regarded in years past. However, the flex-
ural yielding causes floor damage after an earthquake, which may be quite severe, and the economic
consequences of this damage are significant. Further, the concentration of damage to a single floor,
which is possible when brace buckling or fracture occurs within a given story level, has resulted in
increased concern with the design of these bracing systems. Today, a special concentrically braced
frame with V- or inverted V-bracing must be designed so that the beam has adequate bending resis-
tance to withstand the unbalanced forces after brace buckling has occurred. This increased beam
resistance results in less damage to the floor system during severe earthquakes, and it also aids in
distributing inelastic deformations and demands to other parts of the structure.
Multistory X-bracing may be thought of as another special combination of the V-brace and the
inverted V-brace systems. Multistory X-bracing prevents unbalanced brace force after brace buckling,
as noted with the V-braced systems. This prevents extensive flexural yielding in the floor beam and
reduces the potential for concentration of damage within a given story of the structural system.
Zipper columns are sometimes used with V- and inverted V-bracing as an alternative to the
strong beam required for special concentrically braced frames. After brace buckling occurs, the zip-
per column transfers the unbalanced force at the brace–beam connection to other bracing levels. This
procedure negates the need for the heavy beam required to resist the unbalanced force by flexure. It
distributes the inelastic deformation to other levels of the steel frame, and prevents the extreme floor
damage noted with V-bracing system.
K-bracing (and knee bracing) has an intersection of a tensile and compressive brace at midheight
of the column. This application has the same unbalanced force problem as described with the V-bracing
systems. However, the inelastic deformation resulting from this unbalanced force occurs within the
column rather than the beam. The column is needed to support the gravity loads of the structure.
Because this inelastic deformation cannot be tolerated in a ductile system, K-bracing is expressly
prohibited for special concentrically braced frames.
Diagonal bracing acts in tension for lateral loads in one direction and in compression for later-
al loads in the other direction. The tensile capacity of the brace is significantly larger than the com-
pressive capacity of the brace. As with other bracing systems, the AISC seismic provisions for
concentrically braced frames require that the direction of inclination of bracing be balanced to assure
appropriate resistance in both directions at all times. Thus, the braces are used in pairs.

Buckling of Bracing. In general, the energy dissipation of concentric braced frames is strongly
influenced by postbuckling brace behavior. This behavior is quite different for slender braces than
for stocky braces. For example, the compressive strength of a slender brace is much smaller in later
cycles of loading than it is in the first cycle. In addition, very slender braces offer less energy dissi-
pation, but are able to sustain more cycles and larger inelastic deformation than stocky braces. In
view of this, the slenderness ratio of bracing in special concentric braced frames is limited to

KL E
< 4 (8.25)
r F y

where K is the effective length coefficient of the brace, L is the brace length, and r is the radius of
gyration. An exception to this limit is permitted for braces with slenderness ratios less than 200, if the
column can be shown to have adequate resistance to support the full expected tensile load (R y F y A g ,
where A g is the gross area of the brace) of the brace elements in the building.

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.