Page 294 - Handbook of Structural Steel Connection Design and Details

Partially Restrained Connections

Partially Restrained Connections 279

achieving certain percentage of the fixed-end moment or reaching a
deflection limit seem more reasonable (Leon, 1994).

4.2.2 Connection strength
A connection can also be classified in terms of strength as either a
full-strength (FS) connection or a partial-strength (PS) connection. An
FS connection develops the full plastic moment capacity M of the
P
beam framing into it, while a PS connection can only develop a por-
tion of it. For classifying connections according to strength, it is com-
mon to nondimensionalize the vertical axis of the M- curve by the
beam plastic moment capacity (M ) as is shown in Fig. 4.3.
p,beam
Connections not capable of transmitting at least 0.2 M at a rotation
p
of 0.02 rad are considered to have no flexural strength. Because many
PR connections do not exhibit a plateau in their strength even at
large rotations, an arbitrary rotation value must be established to
compare connection strength (M ) to the capacity of the beam. For
p,conn
this purpose a rotation of 0.02 rad is recommended by the author.

4.2.3 Connection ductility
Connection ductility is a key parameter either when the deformations
are concentrated in the connection elements, as is the typical case in
PR connections, or when large rotations are expected in the areas
adjacent to the connections, as in the case of ductile moment frames
with welded connections. The ductility required will depend on the
flexibility of the connections and the particular application (that is,
braced frame in a nonseismic area versus an unbraced frame in a
high-seismic area).
A connection can be classified as ductile based on both its absolute
and its relative rotation capacity (Fig. 4.4). The horizontal axes in Fig.
4.4 show both total connection rotations and connection ductilities.
Three connection curves are shown: (a) two of the curvs are for con-
nections in special moment frames (SMFs), one with hardening or
non-degrading behavior (ND) and one with moderate degradation (D),
and (b) one of the curves is for a degrading connections in an interme-
diate moment frame (IMF). The total rotation (in terms of milliradi-
ans or radians 10 ) is how typical moment-rotation curves for con-
3
nection tests are reported. In general, only the envelopes of the cyclic
results are shown, and a very coarse relative limit between ductile
and nonductile connections can be set a total rotation of 0.04 rad.
The relative ductility index ( / ) can be used for comparing
u y
the rotation capacity of connections with similar moment-rotation
characteristics. In order to compute a relative ductility ( ), a yield
rotation ( ) must be defined. For PR connections, such as the one
y

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