Civil Engineering Reference
In-Depth Information
7.2.6.2
Secondary Girder Elements
Stiffeners are secondary elements, but are of utmost importance to ensure the stability
of some main load carrying elements of plate girders. Specifically, the web plate
is usually stiffened by transverse, and sometimes, longitudinal stiffeners. The web
stiffeners generally consist of welded and/or bolted plates or angles, which may also
have an effect on the main member allowable fatigue stress range if the stiffener
attachments are within tensile regions of the plate girder
∗
(see Chapter 5).
7.2.6.2.1 Longitudinal Web Plate Stiffeners
Equation 7.57 indicates that if
h
/
t
w
4.18
√
E
/
f
c
, longitudinal stiffeners are required
to preclude web flexural buckling instability. The minimum recommended (AREMA,
2008) web plate thickness with longitudinal stiffeners is 50% of Equation 7.56 or
f
c
F
cr
≥
0.09
h
F
y
E
0.12
f
c
t
w
≥
E
.
(7.79)
The longitudinal stiffeners should be proportioned such that they have a flexural
rigidity,
EI
ls
, which creates straight nodes in the buckled plate. The plate buckling
coefficient,
k
, for critical buckling stress with a longitudinal stiffener at 25% of web
depth is
k
=
101. Using energy methods, it can be shown that (Bleich, 1952)
12.6
50
A
ls
ht
w
a
h
3
.
2
3.4
a
h
ht
w
12
(
1
I
ls
=
+
−
(7.80)
2
)
− υ
AREMA (2008)
†
uses a similar formula
ht
w
2.4
a
0.13
,
2
I
ls
=
−
(7.81)
h
where
I
ls
is the moment of inertia for a single longitudinal stiffener about the face of
the web plate (if longitudinal stiffeners are used both sides of the web, the moment
of inertia is taken about the centerline of the web
‡
),
A
ls
=
b
ls
t
ls
is the cross-sectional
area of the longitudinal stiffener, and
a
is the distance between intermediate transverse
stiffeners
(Figure 7.17).
Equation 7.81 also fits experimental data for pure bending
with longitudinal stiffeners at
h/
5 and small values of
A
ls
/ht
w
(0.05-0.25) (Salmon
and Johnson, 1980).
The thickness of the longitudinal stiffener,
t
ls
, to avoid local buckling is essentially
the buckling problem of uniform compression on a plate free at one side and partially
∗
For example, the bottom of transverse web stiffeners attached near the tension flange in simply supported
plate girders. Such attachments, particularly if welded, should be reviewed with respect to their effect
on allowable fatigue stress range.
†
Many other ASD design codes, recommendations, guidelines, and manuals use the same, or similar,
equation.
‡
It is usually not necessary and, therefore, unusual to use longitudinal stiffeners on both sides of the
web.
