Civil Engineering Reference
In-Depth Information
(to resist flexural buckling) and/or transversely (to resist shear buckling). The height
of web plates in long plate girders may be quite large and designers should carefully
review available plate sizes from steel making, fabrication, and shipping perspectives
inordertoavoidcostlylongitudinalsplicesandlimit,towithinpracticalrequirements,
transverse splices.
Weldedorboltedsplicesmaybeusedwhenrequiredduetoplatelengthlimitations.
These splices are generally designed for the shear and bending moment at the spliced
section and/or specific strength criteria recommended by AREMA (2008).
The connection of web and flange plates in modern plate girder fabrication is gen-
erally performed by high-quality automatic welding. These welds must be designed
to resist the total longitudinal shear at the connection as well as other loads directly
applied to the flanges and/or web (see Chapter 9).
7.2.6.1.1 Girder Tension Flanges and Splices
7.2.6.1.1.1 Tension Flanges
Overall girder bending capacity at service loads is
not reduced by the usual number and pattern of holes in a girder cross section (the
same capacity as for gross section but the stresses are distributed differently because
oflocalizedstressconcentrations).However,AREMA(2008)recommendsthatgirder
tension flanges be designed based on the moment of inertia of the entire net section,
I
x
n
, (using the neutral axis determined from the gross section
∗
) and the tensile yield
stress.Thisisparticularlyappropriatefortensionflangesplicesandasprotectionfrom
the effects of possible occasional tensile overload stresses. Many designers proportion
tension flanges based on net section properties being not greater than 85% of the gross
section properties.
The plate girder net section modulus,
S
x
n
, is determined as
I
x
n
c
t
≥
M
t max
F
all
M
t max
0.55
F
y
S
x
n
=
≥
(7.37)
or
S
x
n
≥
Δ
M
F
fat
,
(7.38)
where
c
t
is the distance from the neutral axis to the extreme fiber in tension,
M
t max
is the maximum tensile bending moment due to all load effects and combinations
(Table 4.5),
Δ
M
is the maximum bending moment range due to fatigue load (see
Chapter 5) and
F
fat
is the allowable fatigue stress range for the appropriate fatigue
detail category.
Residual stresses, which must be considered in the design of dynamically loaded
axial and flexural tensile members, and all axial compression members, are of neg-
ligible value in the design of statically loaded bending members. This is because the
presence of residual stresses may cause an initial inelastic behavior, but subsequent
∗
The neutral axis must consist of a smooth line that, because fiber stresses cannot suddenly change,
will vary only slightly at the typically few cross sections with holes. Therefore, the neutral axis will be
essentially at the location of the neutral axis of the gross section along the girder length.
