Building Structures

STRUCTURAL SYSTEMS 31
Figure 1.39 Basic aspects of arches.
they were usually incidental compared to the gravity force. In
most contemporary construction, thinner and lighter arches
are produced and the pure arch action is seldom achieved.
The thrust of the arch—that is, the horizontal
component—is resolved in one of two ways. The most
direct way is to balance the force at the supports against each
other by using a tension tie across the base of the arch. This
very possibly, however, destroys the interior vaulted space
defined by the arch and is therefore not always acceptable.
The second way is to resolve the outward kick at each support
outside the arch. This means creating a heavy abutment or, if
the arch rests on top of a wall or column, creating a strut or a
buttress for the wall or column.
A major consideration in the structural behavior of an arch
is the nature of its configuration. The three most common
cases are those shown in Figure 1.40, consisting of the fixed
arch (Figure 1.40a), the two-hinged arch (Figure 1.40b), and
the three-hinged arch (Figure 1.40c).
The fixed arch occurs most commonly with reinforced
concrete bridge and tunnel construction. Maintaining the
fixed condition (no rotation) at the base is generally not
feasible for long-span arches, so this form is more often used
for short to medium spans. It may occur in the action of
a series of arches built continuously with their supporting
piers, as shown in Figure 1.40a. The fixed arch is highly
indeterminate in its actions and is subject to internal stress
and abutment forces as a result of thermal expansion.
The two-hinged arch is most common for long spans.
The pinned base is feasibly developed for a large arch and
is not subject to forces as a result of thermal change to the
degree of the fixed-arch base. This arch is also indeterminate,
although not to the degree as the fixed arch.
The three-hinged arch is a popular form for mediumspan
building roof structures. The principal reasons for this
popularity are the following:
The pinned bases are more easily developed than
fixed ones, making shallow bearing-type foundations
reasonable for the medium-span structure.
Thermal expansion and contraction of the arch will cause
vertical movements at the peak pin joint but will
have no appreciable effect on the bases. This further
simplifies the foundation design.