Page 363 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch08.qxd 9/29/05 5:21 PM Page 8.17

LATERAL-FORCE DESIGN

LATERAL-FORCE DESIGN 8.17

8
Approx. 20% of resistance
required to remain elastic
6
4

2
Time (s) −2 0

−4

−6 Frame with approx. 40%
of resistance required to
remain elastic
−8
Elastic structure
−10
0 5 10 15
Relative displacement (in)
FIGURE 8.8 Comparison of the elastic and inelastic response of three frames with identi-
cal mass and stiffness but different resistance.

that structures with smaller design force (larger R) require the structure to have the ability to maintain
its integrity through larger inelastic deformations than if a larger design force (smaller R) were
employed.
While some steel structures are very ductile, not all structures have this great ductility. Fracture
of the connections has a very detrimental effect on the structural performance, since it may cause a
significant loss in both resistance and deformational capacity. Local and global buckling may also
change the hysteretic behavior from that of Fig. 8.7a to Fig. 8.7b. The combined effects of these
potential problems means that the structural engineer must pay particular attention to the design
details in the seismic design of buildings, since those details are essential to ensuring good seismic
performance.
Effects of Inelastic Deformations. The distribution of inelastic deformation is a second factor that
can effect the inelastic seismic performance of a structural system. Some structural systems concen-
trate the inelastic deformation (ductility demand) into a small portion of the structure. This can dra-
matically increase the ductility demand for that portion of the structure. This concentration of
damage is sometimes related to factors that cause pinched hysteretic behavior, since buckling may
change the stiffness distribution as well as affect the energy dissipation.
Ductility demand, however, can also be related to other factors. Figure 8.9 shows the computed
inelastic response of two steel moment-resisting frames that have identical mass and nearly identi-
cal strength and stiffness and are subject to the same acceleration record as that in Fig. 8.8. The
frames differ, however, in that one is designed to yield in the beams while the other is designed to
yield in the columns. This difference in design concept results in a significant difference in seismic
response and ductility demand. Design codes attempt to assure greater ductility from structures
designed for smaller seismic forces, but attaining this objective is complicated by the fact that ductility
and ductility demand are not fully understood.

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