es. On the other hand, suspension bridges
require lofty towers and massive anchorages. The defect of the suspension
bridge is its flexibility. It can be stiffened by girders and bracing and
is then of mixed type, when it loses much of its advantage in economy.
Nevertheless, the stiffened suspension bridge will probably be the type
adopted in future for very great spans. A bridge on this system has been
projected at New York of 3200 ft. span.

The immense extension of railways since 1830 has involved the construction
of an enormous number of bridges, and most of these are girder bridges, in
which about half the superstructure is in tension and half in compression.
The use of wrought iron and later of mild steel has made the construction
of such bridges very convenient and economical. So far as superstructure is
concerned, more material must be used than for an arch or chain, for the
girder is in a sense a combination of arch and chain. On the other hand, a
girder imposes only a vertical load on its piers and abutments, and not a
horizontal thrust, as in the case of an arch or suspension chain. It is
also easier to erect.

A fundamental difference in girder bridges arises from the mode of support.
In the simplest case the main girders are supported at the ends only, and
if there are several spans they are _discontinuous_ or _independent_. But a
main girder may be supported at two or more points so as to be _continuous_
over two [v.04 p.0534] or more spans. The continuity permits economy of
weight. In a three-span bridge the theoretical advantage of continuity is
about 49% for a dead load and 16% for a live load. The objection to
continuity is that very small alterations of level of the supports due to
settlement of the piers may very greatly alter the distribution of stress,
and render the bridge unsafe. Hence many multiple-span bridges such as the
Hawkesbury, Benares and Chittravatti bridges have been built with
independent spans.

Lastly, some bridges are composed of cantilevers and suspended girders. The
main girder is then virtually a continuous girder hinged at the points of
contrary flexure, so that no ambiguity can arise as to the stresses.

[Illustration: FIG. 1.--Trajan's Bridge.]

Whatever type of bridge is adopted, the engineer has to ascertain the loads
to be carried, and to proportion the parts so that the stresses due to the
loads do not exceed limits found by experience to be safe. In many
countries the