, and the first
train passed through the Britannia bridge in 1850. Though each girder has
been made continuous over the four spans it has not quite the proportions
over the piers which a continuous girder should have, and must be regarded
as an imperfectly continuous girder. The spans were in fact designed as
independent girders, the advantage of continuity being at that time
imperfectly known. The vertical sides of the girders are stiffened so that
they amount to 40% of the whole weight. This was partly necessary to meet
the uncertain conditions in floating when the distribution of supporting
forces was unknown and there were chances of distortion.

[Illustration: FIG. 16.--Britannia Bridge.]

[Illustration: FIG. 17.--Britannia Bridge (Cross Section of Tubular
Girder).]

Wrought iron and, later, steel plate web girders were largely used for
railway bridges in England after the construction of the Conway and Menai
bridges, and it was in the discussions arising during their design that the
proper function of the vertical web between the top and bottom flanges of a
girder first came to be understood. The proportion of depth to span in the
Britannia bridge was 1/16. But so far as the flanges are concerned the
stress [v.04 p.0540] to be resisted varies inversely as the depth of the
girder. It would be economical, therefore, to make the girder very deep.
This, however, involves a much heavier web, and therefore for any type of
girder there must be a ratio of depth to span which is most economical. In
the case of the plate web there must be a considerable excess of material,
partly to stiffen it against buckling and partly because an excess of
thickness must be provided to reduce the effect of corrosion. It was soon
found that with plate webs the ratio of depth to span could not be
economically increased beyond 1/15 to 1/12. On the other hand a framed or
braced web afforded opportunity for much better arrangement of material,
and it very soon became apparent that open web or lattice or braced girders
were more economical of material than solid web girders, except for small
spans. In America such girders were used from the first and naturally
followed the general design of the earlier timber bridges. Now plate web
girders are only used for spans of less than 100 ft.

Three types of bracing for the web very early developed--the Warren type in
which the bracing bars form equilateral triangles, the Whipple Murphy in
which the