The Design of Modern Steel Bridges - TEDI

94 The Design of Modern Steel Bridges
dead load in the case of continuous construction. For composite beams the
above limit applies to the overall depth, i.e. concrete slab plus steel girder, and
there is an additional limit of 1/30 for the depth of the steel cross-section.
There are also limits on the deflection under live load and impact, which often
govern the depth of cross-section; these are 1/800 of the span generally, but
1/1000 of the span for bridges in urban areas used by pedestrians. Where
several longitudinal girders are interconnected by crossbracings or diaphragms
for efficient lateral distribution of load, the deflection for this purpose may be
calculated by assuming that all the girders will deflect equally.
There are no specific limitations on girder depths or deflections in the British
Standard[2], except that attention is drawn to the need for camber for the sake
of appearance, drainage and headroom clearance. In the case of a nominally
straight bridge, a sagging deflection exceeding 1/800 of the span is also
discouraged.
5.2 Analysis for forces and moments
To design a plate girder, it is necessary to first obtain the bending moment,
shear force and axial force acting on its various sections. The open crosssection
of a plate girder is torsionally very flexible and hence it is generally
assumed that a plate girder section cannot resist any torsion. Axial force occurs
in a plate girder when the bridge deck is subjected to longitudinal forces due to,
say, braking. Owing to vertical loads, axial force occurs when the plate girder
is part of a portal frame or is supported by inclined cables.
If any load is applied over one side of the bridge deck, the beams directly
under the load obviously deflect more than the others; the consequent transverse
bending of the deck slab distributes some of the load on to beams away
from the load. This transverse sharing of the load may be further improved by
the provision of transverse diaphragms across the width of the bridge deck and
connected to the longitudinal beams. Transverse diaphragms over the supports
of the longitudinal beams prevent the latter from twisting and are virtually
essential. The usefulness of intermediate diaphragms should be judged by
balancing the improved lateral distribution of load against the cost of providing,
connecting and maintaining them.
In a bridge deck constituted by a set of plate girders supporting a concrete
deck, the most convenient way to obtain the bending moments and shear forces
is by the assumption that the deck consists of a grillage of longitudinal and
transverse beams. The continuous concrete slab is replaced by a series of
discrete parallel beams spanning between the steel beams. If there are transverse
members connected to the main longitudinal girders, then the grillage
consists of these longitudinal and transverse girders. Generally the concrete
deck is made to act compositely with the steel girders by the provision of shear
connectors; in such cases the concrete slab is taken as a flange of the steel