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.

Wood, 2000 1000 150

Wro't Iron, 15000 11000

Cast Iron, 4500 25000


=Tension.= If a weight of 2000 lbs. were hung to the lowest end of a
vertical beam, so that the line of action of the weight and axis of
the beam formed one and the same straight line--the tension on the
beam would be 2000 lbs. But, if the beam were inclined, and the force
acted in a vertical direction, then the strain would be increased in
the ratio of the increase of the diagonal of inclination over the
vertical;--suppose the beam is 20 ft. long and inclined at an angle of
45°--and let 2000 lbs., as before, be suspended from its lower end.
Now the diagonal being 20°,--the vertical will be 14.014 ft.--and the
strain will be found as follows,--

14.014 : 20 :: 2000 : 2854--lbs.

The greater the angle of inclination from the horizontal, the less the