The simplest bridge that can be built, is a single beam, or stick of
timber, spanning the opening between the abutments--but this is only
of very limited application--(only for spans of 20 feet and less)
owing to the rapid increase in sectional dimensions which is required
as the span becomes greater.
Next comes the single beam supported by an inclined piece from each
abutment meeting each other at the middle point of the under side of
the beam--or, another arrangement, of two braces footing securely on
the beam and meeting at a point above the middle point of the beam,
which is suspended from the apex of the triangle formed by them, by
means of an iron rod--These arrangements may be used up to 50 feet.
For any span beyond 50 feet, modifications of this arrangement are
used which will be described hereafter. Now let us investigate shortly
the different strains that the various parts of a bridge have to
bear--and the strength of the materials used. The theory of strains in
bridge trusses is merely that of the Composition and Resolution of
Forces. The various strains, to which the materials of a bridge are
subjected--are compression, extension and detrusion.
Wood and Iron are the materials more generally employed in bridge
construction--and in this pamphlet we shall take the following as the
working strength of the materials--per square inch of section.
Tension. Compression. Detrusion.