h a p t e r 6 / Arches
The Romans were the first to use arches as major structural elements, employing
them, mainly in semicircular form, in bridge and aqueduct construction and for roof
supports, particularly the barrel vault. Their choice of the semicircular shape was due
to the ease with which such an arch could be set out. Generally these arches, as we
shall see, carried mainly compressive loads and were therefore constructed from stone
blocks, or voussoirs, where the joints were either dry or used weak mortar.
During the Middle Ages, Gothic arches, distinguished by their pointed apex, were used
to a large extent in the construction of the great European cathedrals. The horizontal
thrust developed at the supports, or springings, and caused by the tendency of an arch
to ‘flatten’ under load was frequently resisted by flying buttresses. This type of arch was
also used extensively in the 19th century.
In the 18th century masonry arches were used to support bridges over the large number
of canals that were built in that period. Many of these bridges survive to the present
day and carry loads unimagined by their designers.
Today arches are usually made of steel or of reinforced or prestressed concrete and
can support both tensile as well as compressive loads. They are used to support bridge
decks and roofs and vary in span from a few metres in a roof support system to sev-
eral hundred metres in bridges. A fine example of a steel arch bridge is the Sydney
harbour bridge in which the deck is supported by hangers suspended from the arch
(see Fig. 1.6(a) and (b) for examples of bridge decks supported by arches).
Arches are constructed in a variety of forms. Their components may be straight or
curved, but generally fall into two categories. The first, which we shall consider in this
chapter, is the three-pinned arch which is statically determinate, whereas the second,
the two-pinned arch, is statically indeterminate and will be considered in Chapter 16.
Initially we shall examine the manner in which arches carry loads