Page 371 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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LATERAL-FORCE DESIGN

LATERAL-FORCE DESIGN 8.25

that are available to provide that performance. However, the AISC also requires that the performance
of the connection be clearly demonstrated before it is used in practice. To ensure this, a connection
test procedure for verifying connection performance is defined in Appendix S of the AISC seismic
provisions. With this test procedure, the connection must sustain a total rotation of 0.04 rad without
failure, or without deterioration of resistance below 80% of the nominal plastic-moment capacity of
the connection, for approval in special-moment-resisting frames. A number of connections are capable
of providing this performance, as shown in Fig. 8.13. In many cases, this performance is achieved
only within a given range of member sizes. Verification or documentation has not yet been provided
for the full range of applicability for most connection types.
The reduced-beam-section connection (see Figs. 8.12b and 8.13b) has the most widely docu-
mented performance of the connection types available to date. This connection achieved its ductility
by careful removal of a portion of the beam flange to assure that yielding occurs in the reduced
flange area before yielding occurs at the beam flange weld. The reduced section must be carefully
radiused and finished to avoid flaws and rough edges, and FEMA 350 provides guidance on these
requirements. The reduced beam section provides significant inelastic rotational capacity, but the
strain hardening of the connection is somewhat limited by the reduced flange width and the reduced
lateral stability of the yielded beam when yielding occurs several feet from the face of the column.
The welded-flange, welded-web connection (see Figs. 8.12c and 8.13c) requires considerable
care in cutting and finishing the weld access hole, and the web must be securely welded to the
column with both complete-joint-penetration welds and fillet welds to the shear tab as shown in
Fig. 8.12c. The weld access-hole preparation shown in this figure is now required by AISC provi-
sions for demand-critical flange welds in special-moment-frame connections. This connection has
developed large inelastic deformation capacity combined with significant strain hardening. Such
strain hardening is very beneficial, because it provides reserve strength and stiffness during large
seismic events, and the postyield stiffness may reduce the maximum inelastic demands on the struc-
ture. This may leave the structure more serviceable after a seismic event.
The bolted-flange plate connection (see Figs. 8.12d and 8.13d) permits complete field bolting of
the connection. It is clearly more difficult to design, because peak performance is achieved when the
connection design is balanced to achieve flexural yielding of the beam with subsequent shear yield-
ing of the connection panel zone and tensile yielding of the flange plate. This connection also has
large inelastic deformation capacity with large strain hardening, but the hysteretic behavior is some-
what pinched by the slip occurring at the bolts at larger seismic loads. The resistance at which bolt
slip occurs may represent a serviceability limit state that requires some attention for seismic design,
because frame deflections may be excessive if slip occurs at too small a seismic event. This connec-
tion is less well documented than those noted earlier, but it may provide the greatest inelastic duc-
tility of those described. It is clearly limited to connections with modest-sized members (W30 beams
or shallower), but it is a connection with considerable potential for seismic design. It is used infre-
quently but offers the potential of being an extremely ductile field-bolted connection. The reader is
referred to FEMA 355D for additional recommendations regarding the design and performance of
this connection.
Structural engineers today have a wide range of options for connection design, but the increased
options place greater demands on the designer with regard to documenting and verifying the seismic
performance of the connection.
Panel-Zone Yielding. Seismic bending moments in the beam cause large shear stresses in the col-
umn web in the panel zone of the connection (Fig. 8.14). Panel-zone yielding has been increasingly
important in recent years, because FEMA-sponsored research showed that shear yielding may reduce
connection peformance if it occurs before flexural yielding of the beam occurs. As a result, there has
been a slight reduction in the shear capacity of the panel zone in recent editions of the AISC seismic
provisions. Today the panel zone-shear force must be limited to
(8.22)
V n = 0.6F yc d c t wc
where F yc is the nominal yield stress of the column web, and d c and t wc are the depth and web thickness
of the column section, respectively. This shear capacity must be at least greater than the panel-zone

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