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STRENGTH OF MATERIALS. 275
Cast-iron is variable ; it has usually five to six and a-half
times as much crushing as tensile strength, and when loaded
transversely it will deflect under the same load nearly twice as
much as wrought iron. It is especially useful for short pillars
or anything exposed to crushing stress, where there is little
danger of breakage by flexure; it is very much less reliable
when exposed to tensile or torsional stress.
Wood is not adapted to resist torsion, but is useful to resist
tensile, crushing and transverse stress, also to resist flexure.
It has nearly twice as much tensile as crushing strength.; there-
fore, it would seem specially well adapted, in all kinds of con-
struction, to be the member exposed to tensile stress, but where
wood and iron enter into construction together, iron is
always used as the member to take the tensile stress and wood
as the compressive member, because wood has suck low shear-
ing strength lengthwise with its fibers that, with any kind of
fastening at the ends, it will tear and split at the holes under
comparatively little stress; but this difficulty is easily overcome
when wood is used as the compressive member. Wood has
comparatively low tensile and crushing strength crosswise on the
fiber. _
This is well to remember with beams loaded transversely
and laid on posts. The beams may be sufficiently strong, but
under heavy load, if suitable precautions are not taken (see
page 250) the top of the post may press into the beam, especially
if the lumber is green.
Stone has high crushing strength but low tensile strength,
and, in consequence, very low transverse strength. It is very
well adapted for foundations when supported and laid in such a
way that its crushing strength comes into play, but when laid as
a beam to resist transverse stress it is very unreliable, as it will
break for a comparatively small load and it may break from a
blow or jar.
Brickwork is only suitable for crushing stress, and there is
great difference in the strength of different kinds of brick.
In calculating strength and stiffness in any kind of design-
ing, it should be remembered that it is only possible to deter-
mine the strength of any material by actual test, and that the
tabular and constant numbers here given are only an average
approximate.
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