Page 367 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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LATERAL-FORCE DESIGN

LATERAL-FORCE DESIGN 8.21

the plastic resistance of the ductile element on the mean or expected resistance rather than the nom-
inal resistance, because expected resistance is normally larger than the nominal capacity. If the
expected resistance is larger than anticipated, the ductile element may not achieve its full ductility
before a peripheral, less ductile element fails. The AISC provisions address this issue by multiply-
ing the nominal plastic resistance of a ductile element by an expected strength factor R y . Modern
structural steels often vary widely from the nominal yield stress, and thus R y is defined by

F
R = ye (8.18a)
y
F y
where F ye and F y are the expected and the nominal minimum specified yield stress, respectively. This
R y value can be established through testing or, in the absence of test data, specification-defined val-
ues of between 1.1 and 1.6 are provided in the provisions, depending on the grade of steel. R y is used
to evaluate the uncertainty in material properties and how this affects the seismic performance of the
building. Similar balance checks are sometimes required using the ultimate tensile stress of the steel.
The ratio, R t , of the expected ultimate tensile stress to the nominal tensile stress is defined as
R = F te (8.18b)
t
F t
where F te and F t are the expected and the nominal tensile stresses, respectively. The AISC provides
values between 1.1 and 1.3 for the steels commonly used in seismic design.
Second, many steel structural systems achieve their ductility by plastic deformation in the steel
near welded joints. Welds and the heat-affected zone immediately adjacent to welds may have dif-
ferent properties than steel members, and there is a greater probability of local or internal flaws in
welded joints than in steel sections. The FEMA recommendations for special moment-resisting
frames resulting require that the welds be a matching metal and require a minimum toughness of
welds in regions where large inelastic strain demands occur. This requirement assures adequate
inelastic strain capacity of the welded joint to achieve ductile performance. AISC provisions add
these requirements to demand-critical welds for other structural systems. These demand-critical
welds must use a filler metal capable of providing a minimum Charpy V-notch (CVN) toughness of
20 ft⋅lb at −20°F and 40 ft⋅lb at 70°F.
Third, columns are extremely critical elements in all structural systems, since the columns must
support gravity load regardless of the earthquake excitation. As a result, the forces and moments in
columns are very uncertain when complex inelastic deformations of the frames occur. When the fac-
tored axial load on the column exceeds 40% of the nominal capacity, the columns must have ade-
quate resistance to satisfy additional load-factor combinations provided in the IBC. These additional
load combinations assure that columns are designed with adequate resistance to support all combi-
nations of earthquake loads and dead loads on the structure. The reader is referred to the IBC provi-
sions for the specific load combinations.

8.7.1 Limitations on Moment-Resisting Frames
Structural tests have shown that steel moment-resisting frames may provide excellent ductility and
inelastic behavior under severe seismic loading. Because these frames are frequently quite flexible,
drift limits often control the design. The NEHRP seismic provisions recognize this ductility and
assigns R = 8.0 to special moment-resisting frames (Art. 8.4).
Slenderness Requirements. Special steel moment-resisting frames must satisfy a range of slen-
derness requirements to control buckling during the plastic deformation in a severe earthquake. The
unsupported length, L b , of bending members must satisfy
.
0 086 rE
L ≤ y (8.19)
b
F y

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