The following table expresses the forces in tons which must be applied for each square inch in the section of the metals, in order that they may be torn apart or crushed:
┌───────────────────────┬──────────────────────┬──────────────────────┐ │ │ Tenacity per square │Crushing pressure per │ │ │ inch, in tons. │square inch, in tons. │ ├───────────────────────┼──────────────────────┼──────────────────────┤ │Cast iron │ 8 │ 50 │ │Wrought iron │ 30 │ 17 │ │Iron wire │ 40 │ ... │ └───────────────────────┴──────────────────────┴──────────────────────┘
Besides the direct strains which tend to simply elongate or compress the materials of a structure or of a machine, there are modes of applying forces which give rise to transverse strains, tending to twist or wrench the pieces or to bend them, or rupture them by causing one part of a solid to slide away from the rest. Strains of this kind no doubt come into play in certain subordinate parts of bridges of any kind; but if we divide bridges according to the nature of the strains to which the essential parts of the structure are subject, we may place in a class where the materials are exposed to crushing forces only, all bridges formed with stone and brick arches; and in a second class, where the material is subjected to extension only, we can range all suspension bridges; while the third class is made up of bridges in which the material has to resist both compression and extension. This last includes all the various forms of girder bridges, whether trussed, lattice, or tubular. The only remark that need be here made on arched bridges is, that when cast iron was applied to the construction of bridges, the chief strength of the material lying in its resistance to pressure, the principle of construction adopted was mainly the same as that which governs the formation of the arch; but as cast iron has also some tenacity, this permitted certain modifications in the adjustment of the equilibrium, which are quite out of the question in structures of brick and stone.