The Design of Modern Steel Bridges - TEDI

Chapter 5
Rolled Beam and Plate
Girder Design
5.1 General features
Rolled I-section steel joists and universal beams are very convenient for
bridges of up to 25 m span. Apart from a pair of vertical stiffeners over their
end supports, these do not require any other fabrication. For longer spans,
I-section girders made up of plates are used. Before welding became popular,
flange plates were connected to a web plate by riveting through angles, as
shown in Fig. 5.1(a); where a single flange plate was not adequate, several
plates were used as shown in Fig. 5.1(b).
As the bending moment fell along the span, the outer plates were stopped or
‘curtailed’. Welding removed the need for the flange angles and also removed
the gaps between adjacent elements where water could collect and initiate
rusting (see Fig. 5.1(c)). Curtailment of the flange area is achieved in welded
construction by using thinner and/or narrower flange plates in regions of
reduced bending moments, butt-welded to each other at the ends. There is a limit
to the thickness of the flange plate that can be conveniently used, since material
properties like weldability, notch toughness, through-thickness ductility and
even yield stress deteriorate with increase in thickness, and risks of lamination
and other inclusions increase. When a single plate is not adequate, the required
flange area is provided by using several flange plates as shown in Fig. 5.1(d); the
outer plates are made successively narrower than the inner ones, to which they
are connected by fillet welds along the longitudinal edges. The outer plates are
discontinued as the bending moments fell along the span; the discontinuity at the
end of each curtailed flange plate is, however, a potential fatigue problem and
needs careful detailing.
There may be other variations and combinations; for example, the flange
may be made up of several plates riveted together and then welded to the web
plate; the web may be made up of two thicker plates near the flanges and one
thinner plate at the middle of the cross-section, butt-welded to each other along
the longitudinal edges; the web depth may be varied along the span. Instead of
a plate, a channel section may be used as the flange.
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