Page 305 - Handbook of Structural Steel Connection Design and Details

Partially Restrained Connections

290 Chapter Four

forces are inertial rather than gravity-type. For seismic design of
connections, AISC 358-05 require that the expected (or mean)
strength of the beam be used rather than its nominal (or 5% frac-
tile) strength. To accomplish this, nominal yield and ultimate
strength values are multiplied by either a R or R factor, which
y t
varies with the material type. In addition, to account for peak con-
nection strength, strain hardening, local restraint, additional rein-
forcement, and other connection conditions and additional factor
(C ) is used, where C is taken as the average of the yield plus
pr pr
ultimate strength divided by the yield strength. C need not be
pr
taken as greater than 1.2. This capacity-design approach is differ-
ent from the static (that is, nonseismic) case where the connection
can be designed for forces derived from the structural analysis, and
without regard to what the actual ultimate capacity and failure
mode of each of the connection components is.
Before looking at examples of CW-BB connections for both static
and seismic loading cases, a number of important design issues need
to be understood. These issues, discussed in detail below, are of par-
ticularly significance for CW-BB connections, but the principles
involved are applicable to most strong PR connections:
1. Proportioning of flange connection: Whenever possible the yield
strength of the connection elements (top and bottom plates) should be
matched to that of the beam flange. This will ensure that distributed
yielding takes place and that severe local buckling will not ensue.
Severe local buckling can result in an early fracture of the beam
flanges if cyclic loads are present. Astaneh-Asl (1995) recommends
that for yielding on the gross section:

b t R F ≅ b t R F (4.1a)
p p y yp f f y yf
where b and t are the width and thickness and the subscripts p and
f refer to the plate and beam flange, respectively. Usually, the expect-
ed yield strength of the materials is not known when the design is
done. For designs not involving seismic forces, the nominal material
properties, as opposed to the nominal ones, can be used throughout.
For the case of seismic forces the same assumptions can be made with
regards to sizing the plate, but both the R and C factors must be
y pr
applied to avoid undesirable modes of failure. To avoid a tensile rup-
ture of the flange, by AISC 358 (2005), Section F13:

1 Y F y 1
t
# b a1 2 b 2 (4.1b)
d b f
2 F u 8

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