Civil Engineering Reference
In-Depth Information
regarded as class 4. Where the web is considered to resist shear forces only
and is assumed not to contribute to the bending and normal force resistance
of the cross section, the cross section may be designed as class 2, 3, or 4,
depending only on the flange class. EC3 also specifies cross-sectional
requirements for plastic global analysis.
3.8.2 Bending Moment Resistance of Steel Plate Girders
Steel plate girders are used to carry larger loads over longer spans that are
possible with traditional universal rolled I-sections. Plate girders are used
in bridges mainly as main girders (see Figure 1.20) and as cross girders in
bridges having wide cross sections and bridges carrying multiple traffic
tracks. Steel plate girders are constructed by welding steel plates together
to form I-sections as shown in
Figure 3.17
.
The web of a plate girder is rel-
atively thin, and stiffeners are required mainly to prevent buckling due to
compression from bending and shear. Stiffeners are also required at load
points, supports, and within panels. The depth of a plate girder (
h
) may
be taken as one-tenth to one-twelfth of the span (
L
/10 to
L
/12) for main
girders and as one-seventh to one-ninth of the span (
L
/7 to
L
/9) for cross
girders. The breadth of flange plate is made about one-fifth of the depth (
h
/
5). The deeper the girder is made, the smaller the flange plates required.
However, the web plate must then be made thicker or additional stiffeners
are provided to meet particular design requirements. Flange cover plates can
be kept constant throughout, curtailed as shown in
Figure 3.20
,
or single
flange plates can be reduced in thickness when reduction in bending
moment permits as shown in
Figure 3.21
.
Long plate girders are commonly
divided into parts to facilitate transportation process, connected at the field
of construction using splices, as shown in
Figure 3.22
.
The figure shows typ-
ical welded and bolted splices of plate girders.
Cover plate
Flange plate
B
B
Elevation
Section B-B
Figure 3.20 Curtailed cover plates.
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