way bridges it
commonly consists of cross girders, attached to or resting on the main
girders, and longitudinal rail girders or stringers carried by the cross
girders and directly supporting the sleepers and rails. For spans over 75
ft., expansion due to change of temperature is provided for by carrying one
end of each chain girder on rollers placed between the bearing-plate on the
girder and the bed-plate on the pier or abutment.

Fig. 14 shows the roller bed of a girder of the Kuilenburg bridge of 490
ft. span. It will be seen that the girder directly rests on a cylindrical
pin or rocker so placed as to distribute the load uniformly to all the
rollers. The pressure on the rollers is limited to about p = 600 d in lb
per in. length of roller, where d is the diameter of the roller in inches.

[Illustration: FIG. 14.--Roller Bed of a Girder.]

In the girders of bridges the horizontal girder is almost exclusively
subjected to vertical loading forces. Investigation of the internal
stresses, which balance the external forces, shows that most of the
material should be arranged in a top flange, boom or chord, subjected to
compression, and a bottom flange or chord, subjected to tension. (See
STRENGTH OF MATERIALS.) Connecting the flanges is a vertical web which may
be a solid plate or a system of bracing bars. In any case, though the exact
form of cross section of girders varies very much, it is virtually an I
section (fig. 15). The function of the flanges is to resist a horizontal
tension and compression distributed practically uniformly on their cross
sections. The web resists forces equivalent [v.04 p.0539] to a shear on
vertical and horizontal planes. The inclined tensions and compressions in
the bars of a braced web are equivalent to this shear. The horizontal
stresses in the flanges are greatest at the centre of a span. The stresses
in the web are greatest at the ends of the span. In the most numerous cases
the flanges or chords are parallel. But girders may have curved chords and
then the stresses in the web are diminished.

[Illustration: FIG. 15.--Flanged Girder.]

At first girders had solid or plate webs, but for spans over 100 ft. the
web always now consists of bracing bars. In some girder bridges the members
are connected entirely by riveting, in others the principal members are
connected by pin joints. The pin system of connexion used in the Chepstow,
Saltash, Newark Dyke and other early English bridges is now r