Page 292 - Handbook of Structural Steel Connection Design and Details

Partially Restrained Connections

Partially Restrained Connections 277

extensive research (Ackroyd and Gerstle, 1982) has shown this proce-
dure to be generally safe, the final forces and deformations computed
from a simplified analysis can be different from those using and
advanced analysis program that incorporates the entire nonlinear M-
relationship shown in Fig. 4.2c.
The description of Type 3 construction used in previous versions of
the steel specification cannot properly account for the effect of connec-
tion flexibility at the serviceability, ultimate strength, or stability
limit states. The first LRFD specification (AISC, 1986) recognized
these limitations and changed the types of construction to fully
restrained (FR) and partially restrained (PR) to more realistically rec-
ognize the effects of the connection flexibility on frame performance.
The definition of PR connections in the first two LRFD versions of the
specification (AISC 1986, 1993), however, conformed to that used for
Type 3 in previous ASD versions. Research on PR connection behavior
has led to more comprehensive proposals for connection classification
(Gerstle, 1985, Nethercot, 1985, Bjorhovde et al., 1990, Eurocode 3
1992, to name but a few of the earlier ones) that clarify the combined
importance of stiffness, strength, and ductility in connection design.
The commentary of the more recent editions of the LRFD and unified
specification (AISC 360, 2005) contain much more detailed discussion
on connection classification schemes. The discussion here, which
remains consistent with that in the previous edition of this book, is in
substantial agreement with the main concepts that will appear in
those commentaries.

4.2.1 Connection stiffness
As noted earlier, the connection stiffness can be taken as the slope of
the M- curve. Since the curves are nonlinear from the start, it is pos-
sible to define this stiffness based on the tangent approach (such as
for K in Fig. 4.3) or on a secant approach (such as K or K ). A tangent
i serv ult
approach is viable only if the analysis programs available can handle
a continuous, nonlinear rotational spring. Even in this case, however,
the computational overhead can be large and this option is recom-
mended only for verification of the seismic performance of irregular
structures. In most designs for regular frames, a secant approach will
probably yield a reasonable solution at a fraction of the calculation
effort required by the tangent approach. In this case, the analysis can
be carried out in two steps by using linear springs. For service loads,
a K can be used for deflections and drift checks. The service secant
serv
stiffness can be taken at 0.0025 rad. A K , based on a secant stiffness
ult
to a rotation of 0.02 rad, can be used for checks related to ultimate
strength. Clearly, the deformations computed for the service load level
will be fairly accurate, since the deviation of K from the true curve
serv

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