al which are most economical,
especially in the superstructure. In the case of bridges of large span the
cost and difficulty of erection are serious, and in such cases facility of
erection becomes a governing consideration in the choice of the type to be
adopted. In many cases the span is fixed by local conditions, such as the
convenient sites for piers, or the requirements of waterway or navigation.
But here also the question of economy must be taken into the reckoning. The
cost of the superstructure increases very much as the span increases, but
the greater the cost of the substructure, the larger the span which is
economical. Broadly, the least costly arrangement is that in which the cost
of the superstructure of a span is equal to that of a pier and foundation.

For masonry, brick or concrete the arch subjected throughout to compression
is the most natural form. The arch ring can be treated as a blockwork
structure composed of rigid voussoirs. The stability of such structures
depends on the position of the line of pressure in relation to the extrados
and intrados of the arch ring. Generally the line of pressure lies within
the middle half of the depth of the arch ring. In finding the line of
pressure some principle such as the principle of least action must be used
in determining the reactions at the crown and springings, and some
assumptions must be made of not certain validity. Hence to give a margin of
safety to cover contingencies not calculable, an excess of material must be
provided. By the introduction of hinges the position of the line of
resistance can be fixed and the stress in the arch ring determined with
less uncertainty. In some recent masonry arched bridges of spans up to 150
ft. built with hinges considerable economy has been obtained.

For an elastic arch of metal there is a more complete theory, but it is
difficult of application, and there remains some uncertainty unless (as is
now commonly done) hinges are introduced at the crown and springings.

In suspension bridges the principal members are in tension, and the
introduction of iron link chains about the end of the 18th century, and
later of wire ropes of still greater tenacity, permitted the construction
of road bridges of this type with spans at that time impossible with any
other system of construction. The suspension bridge dispenses with the
compression member required in girders and with a good deal of the
stiffening required in metal arch