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LATERAL-FORCE DESIGN

8.30 CHAPTER EIGHT

transferred to the brace. This requirement assures that the energy dissipation occurs in the members
rather than the connections.

Selection of R. Once concentric bracing is selected for seismic design, the force reduction factor,
R, must be chosen. The discussion to this point has focused on special braced frames which have R = 6.
This R value is somewhat smaller than that permitted for special steel moment-resisting frames,
because concentrically braced frames are known to be dominated by brace buckling. As a result, their
resistance may deteriorate and the brace may fracture under seismic loading. Further improvements
to the behavior of concentrically braced frames can be achieved if the special concentrically braced
frame is combined with a special-moment frame to form a dual system. This dual system will per-
mit smaller seismic design forces, with an R = 8. With this system, the moment frame must be able
to resist loads which are at least 25% of the total seismic-design base shear. In addition, both the
braced frame and the moment frame must be able to resist their appropriate portion of the loading in
accordance with their relative stiffness. The braced frame is usually much stiffer than the moment
frame, and so this requirement effectively means that the dual system has a greater total resistance
than required by the basic design equations.
Beams in V- or Inverted V-Brace Systems. As noted earlier, brace buckling in the V-braced system
results in an unbalanced force on the beam. The consequence of this is significant inelastic defor-
mation of the beam during severe earthquakes and concentration of inelastic damage to a single story
of the structural system. The plastic deformation of the beam contributes some energy dissipation,
but the negative consequences are quite severe. As a consequence, the beams of special concentri-
cally braced frames with V-bracing or inverted V-bracing must be designed so that the beam has ade-
quate bending resistance to resist the unbalanced brace forces after buckling occurs. This requires a
significant increase in the beam size, and options such as the zipper column illustrated in Fig. 8.6 are
used to overcome this restriction.

Ordinary Concentrically Braced Frames. Special concentrically braced frames require that the
braced frame satisfy the requirements summarized previously. Historically, designers have had the
option of designing ordinary concentrically braced frames for larger seismic design forces (smaller
R values) and reduced ductility and detailing requirements. The option of ordinary concentrically
braced frames is still available to the structural engineer. This bracing system can be designed for R = 5.
However, the benefits in reduced detailing requirements are modest. Detailing requirements for ordi-
nary concentrically braced frames have become increasingly closer to the requirements for special
concentrically braced frames. As a result, the potential economic benefit of ordinary concentrically
braced frames has been reduced, and no further discussion of this option will be provided.

8.7.3 Eccentric Braced Frames
Eccentric braced frames combine the strength and stiffness of a concentric braced frame with the
inelastic performance of a special moment-resisting frame (Fig. 8.7c). An R value of 8 is permitted
for an eccentric braced frame. This results in seismic-design forces comparable to those required for
special moment-resisting frames if the fundamental period of vibration is the same. However, braced
frames are invariably stiffer than moment-resisting frames of similar geometry and have a shorter
period. This results in a somewhat larger design load than for special moment-resisting frames under
comparable conditions. (C. W. Roeder and E. P. Popov, “Eccentrically Braced Steel Frames for
Earthquakes,” Journal of Structural Division, March 1978, American Society of Civil Engineers.)

General Requirements for Ductility. There are a number of special design provisions that must be
satisfied by eccentric braced frames. As defined in Art 8.4, a link must be provided at least at one
end of each brace. The link beam should be designed so that it is the weak link of the structure under
severe seismic loading. This is done by selecting the size of the steel section and the length of the
link beam to match seismic-load design requirements. The weak link is assured by the requirement
that the brace be designed for a force at least 1.25 times the brace force necessary to yield the link

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