STRENGTHENING OF STONE MASONRY ARCH BRIDGES THREE ...

Structural Scheme And Principles
The stone blocks of ashlar work are 3 dimensional, and can be readily cut to form 3
dimensional planar bridge geometries. If ashlar stone is to be used this should be
expressed and exploited. Random and coursed rubble stonework lends itself to simpler
geometries in section, but can curve in plan with ease, which can be useful in wing
walls.
Parapets are not usually part of the principal structure, and traditionally have varied
from simple boulders to classical statuary, they also can be in a different material such
as timber or metal. Not only are they an important part of the elevation, they are also
usually the only part seen when passing over the bridge, and can therefore be treated
sometimes in a separate way from the main structure.
Arch bridges are always under compression. The force of compression is pushed
outward along the curve of the arch toward the abutments. The average line of the
forces should be as near the centre line as possible, and certainly within the kern.
Fig. 3.3 – Basic structural scheme
In any structure, except a simple pier or column, it is impossible to have compression
without tension. In the case of an arch, the tension is in the ground, which is therefore a
member that costs nothing. If we take this argument further, it can prove that arch spans
can be made longer than beam spans. Although the ground under an arch is in tension,
the ground just outside the abutments is compressed by the thrust of the arch. Between
Leonardo da Vinci Program 24