Bridge Design Element 2: The Arch
Basic concepts
An arch is an architectural form that can be used in many
ways including the creation of doorways, ceilings, and
bridges. An arch is called a pure compression form
because its shape resolves all of the forces acting on it
into compression and eliminates all tension forces. This
is sometimes referred to as arch action. As the forces in
the arch are carried to the ground, the arch will push
outward at the base, called thrust. As the rise, or height
of the arch decreases, the outward thrust increases. In
order to maintain arch action and prevent the arch from
collapsing, the thrust needs to be restrained, either with
internal ties, or external bracing, called abutments.
Fixed arch vs hinged arch
There are three common configurations of arches:
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The fixed arch is most often used in reinforced concrete bridge and tunnel construction,
where the spans are short. This is the most basic form and is the one associated with
construction in early civilizations.
The two-hinged arch (below to the left) is the form most often used in modern bridges.
This type of arch has pinned connections at the base. Unlike the fixed arch, the pinned
base is able to rotate, allowing the structure to move freely and compensate for the
thermal expansion and contraction caused by changes in outdoor temperature.
The three-hinged arch (below to the right) is not only hinged at its base, like the twohinged arch, but at the mid-span as well. The additional connection at the mid-span
allows the three-hinged arch to move in two opposite directions, and compensate for any
expansion and contraction. It is most often used for medium-span structures, such as
large building roofs.
Types of arches
Arches have many forms, but all fall into three basic categories: Circular, pointed, and
parabolic. Arches can also be configured to produce vaults and arcades.
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Arches with a circular form, also referred to as rounded
arch, were commonly employed by the builders of
ancient, heavy masonry arches. Ancient Roman builders
relied heavily on the rounded arch to span large, open
areas. Several rounded arches placed in-line, end-to-end,
form an arcade, like in this Roman aqueduct.
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Pointed arches were most often used by builders of
Gothic-style architecture. The advantage to using a
pointed arch, rather than a circular arch, is that the
arch action in a pointed arch produces less thrust at
the base. This innovation allowed for taller and more
closely spaced openings, typical of Gothic architecture.
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The parabolic arch employs the principle that when
weight is uniformly applied to an arch, the internal
compression resulting from that weight will follow a
parabolic profile. Parabolic arches produce the most thrust at the base, but can span the
largest areas. They are commonly used in bridge design, where long spans are needed.
Construction
Since it is a pure compression form, the arch is useful because many building materials,
including stone and unreinforced concrete can resist compression but are weak when tension is
applied to them.
An arch is held in place by the weight of all of its members, making construction problematic.
One answer is to build a frame (historically, of wood) which exactly follows the form of the
underside of the arch. Blocks are then laid on it until the arch is complete and self-supporting.
In reinforced concrete construction, the principle of the arch is used so as to benefit from the
concrete's strength in resisting compressive stress. Where any other form of stress is raised, such
as tensile or torsional stress, it has to be resisted by carefully placed reinforcement rods or fibres.