Civil Engineering Reference
In-Depth Information
f
bf
bt
f
f
bf
f
bw
t
f
Neutral axis
d
h
d
-
t
f
M
w
M
f
t
w
bt
f
f
bf
b
FIGURE 7.7
Preliminary proportioning of girder flanges.
An estimate of flange size can be made based on a preliminary web height and
web thickness,
t
w
≥
V
/0.35
F
y
h
, where
V
is the maximum applied shear force, as
M
f
b
h
−
V
0.35
F
y
(
6
)
=
M
f
b
h
−
V
2.10
F
y
A
f
=
.
(7.36b)
The preliminary web height is typically estimated from typical
L/d
ratios for eco-
nomical design (see Chapter 3), available plate and section sizes, plate slenderness,
underclearance requirements, and/or aesthetic considerations.
7.2.6 P
LATE
G
IRDER
D
ESIGN
Modern plate girders typically consist of welded flange and stiffened web plates
(Figure 7.8). Railway bridge girders are generally of large size, and ensuring the sta-
bilityofplateelementsincompressionzonesandconsiderationofstressconcentration
effects in tension zones are critical components of the design. Simple span plate gird-
ers of about 150 ft or less are generally economical for railway bridge construction.
Longer spans are feasible for continuous construction. However, continuous spans are
Top flange
Longitudinal
stiffener
Bearing
stiffener
Web plate
Transverse
stiffener
Bottom flange
FIGURE 7.8
Cross section of the modern plate girder.
