In railway infrastructure, the gr
ade is usually predetermined a
nd so the design of bridges is
constrained to meet the railway grade. Because
of this, rail overpass bridges are very often in
the form of half-through girders,
in which the headroom limita
tion requires that the I-section
beams that form the half-through girders are loaded
at the level of the tension flange, rather than
at the compression flange level in more conventi
onal bridges. Half-through girder bridges are in
general comprised of two parallel I-secti
on beams joined by a concrete deck at the
bottom/tension flange level as ill
ustrated in Figure 1. Loading
the beam at the tension flange
level through the deck results in the top flange
being subjected to compression, and the top
flange is not braced against overall buckling.
The compression flange of the I-section is
restrained only by the stif
fness of the usually flexible web
and the tension flange is provided
with continuous torsional restrain
t through its connection to the
concrete deck. An arrangement
analogous to this one is that of a heavily
loaded beam supported on
seats (Bradford, 1989).