Page 317 - Handbook of Structural Steel Connection Design and Details

Partially Restrained Connections

302 Chapter Four

The relative stiffness, assuming the beam is 24 ft long is

6
K conn L beam s6.25 3 10 ds28 3 12d
5 5 5 22.1
EI beam s29000ds3270d

Note that this will put this connection in the FR range. This calculation can
only be considered as an estimate as from the computations above it is clear
that slip is the main contributor to the deformation. The assumed slip
(1/32 in) is a reasonable value for the first cycle. As the connection is cycled,
this slip will probably increase to approximately 0.25 in and the secant stiff-
ness will decrease correspondingly, probably putting the connection into the
mid-PR range.
The rest of the checks should proceed as for Example 4.1, with
additional checks for the column for (a) continuity plates, (b) doubler
plates (unlikely), and (c) beam-to-column moment ratio. For the
beam, additional checks for block shear and shear connection demand
should be performed.

4.3.2 Column-bolted–beam-bolted
connections (T-stubs)

Bolted T-stub connections were a popular connection in moment-
resisting frames before field-welded connections became economical,
and along with end-plate connections still represents the most effi-
cient kind of column-bolted–beam-bolted (CB-BB) connection. The
mechanistic model for this type of connection is shown in Fig. 4.5,
while the possible yield and failure modes are shown in Fig. 4.8. The
important conceptual difference between a CW-BB and a CB-BB is
that for T stubs the springs that represent the connection to the col-
umn flange have lower strength and stiffness. This is because they
represent the flexural deformations that can take place in the flanges
of the tee as well as any axial deformation of the bolts to the column
flange. Both of these are flexible when compared to the axial stiffness
of a weld, which can be considered to be an almost rigid element. In
addition for the CB-BB connections, the spring representing the bolts
needs to include the prying action, which can significantly increase
the force in the bolts at ultimate. Figure 4.15 shows prying action in a
very flexible T stub. In this case the flexibility of the flange of the
stub results in an addition prying force (Q) at the tip of the stub
flange. This force increases the nominal force in the bolts above its
nominal pretension value (T).
For the case of the T stub, the springs shown in Fig. 4.5 can have a
wide range of strength and stiffnesses, depending primarily on the

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