Some of the arched bridges built by the Romans remain in use to this day to attest the skill of their architects. The Ponte Molo at Rome, for example, was erected 100 B.C.; and at various places in Italy and Spain many of the ancient arches still exist, as at Narni, where an arch of 150 ft. span yet remains entire. Until the close of the last century the stone or brick arch was the only mode of constructing substantial and permanent bridges. And in the present century many fine bridges have been built with stone arches. The London and Waterloo Bridges across the Thames are well-known instances, each having several arches of wide span, attaining in the respective cases 152 ft. and 120 ft. The widest arch in England, and one probably unsurpassed anywhere in its magnificent stride of 200 ft., is the bridge across the Dee at Chester, built by Harrisson in 1820. At the end of last century cast iron began to be used for the construction of bridges, a notable example being the bridge over the Wear at Sunderland, of which the span is 240 ft. But with the subsequent introduction of wrought iron into bridge building a new era commenced, and some of the great results obtained by the use of this material will be described in the present article. In order that the reader may understand how the properties of wrought iron have been taken advantage of in the construction of bridges, a few words of explanation will be necessary regarding the strains to which the materials of such structures are exposed.
Such strains may be first mentioned as act most directly on the materials of any structure or machine, and these are two in number, namely, extension and compression. When a rope is used to suspend a weight, the force exerted by the latter tends to stretch the rope, and if the weight be made sufficiently great, the rope will break by being pulled asunder. The weight which just suffices to do this is the measure of the tenacity of the rope. Again, when a brick supports a weight laid upon it, the force tends to compress the parts of the brick or to push them closer together, and if the force were great enough, the brick would yield to it by being crushed. Now, a brick offers so great a resistance to a crushing pressure, that a single ordinary red brick may be capable of supporting a weight of 18 tons, or 40,320 lbs.—that is, about 1,000 lbs. on each square inch of its surface. Thus the bricks at the base of a tall factory chimney are in no danger of being crushed by the superincumbent weight, although that is often very great. The tenacity of the brick, however, presents the greatest possible contrast to its strength in resisting pressure, for it would give way to a pull of only a few pounds. Cast iron resembles a brick to a certain extent in opposing great resistance to being crushed compared to that which it offers to being pulled asunder, while wrought iron far excels the cast metal in tenacity, but is inferior to it in resistance to compression.
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 inch, in tons. | Crushing pressure per 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.
Fig. 139.
Fig. 140.
