Tbe Britannia Tabular Bridge (figs. S, 9), built over the Menai Strait at Bangor to carry the Chester and Holyhead Railway, was constructed by Stephenson and Fairbairn 0846–IS5o), and opened to railway traffic in 1S5o. It is composed of two continuous wrought-iron beams 15r1 feet long and weighing- four thousand six hundred and eighty tons each. The tubes are supported on three masonry piers and two abutments, there being thus four spans. Of these, the two channel spans are 46o feet each, and the two stone spans 23o feet each. The tubes are firmly fixed upon the centre tower, but are supported on roller-beds on the lateral piers and abutments, leaving the structure free to expand and contract.
The Conway Tubular Bridge, built over the Menai Strait by Stephenson and opened in 184S, has a span of 400 feet. The Victoria Bridge, built also by Stephenson (1859), over the St. Lawrence River at Montreal, has one span of 33o feet and twenty-four of 239 feet each.
Disadz,antaisres of Tubular bridges have several dis advantages. They are exceeding-lv heavy and expensive, and on account of their peculiar structure are specially sensitive to changes of temperature and extremely liable to deterioration by rusting. For these reasons, as well as because of the better knowledge among eng-ineers of the theory of strains on the members of bridge structures, and greater experience and skill in producing structnral forms of iron adapted for the purpose, the tubular 07 solid-wall girder for wide spans has given place to the lattice- or open work framed structures, which with equal strength are relatively much lighter, and therefore more economical. These girders (as in the case of the wooden-lattice g,irder) are compound beams formed of two parallel or curved upper and lower members, called " chords," connected by certain members vertical and oblique, called " web-members," which stiffen the chords and transform the various strains due to the weight of the girder itself and of its load into longitudinal strains, which by the character of the structure are conveyed up and down along the members of the girder to the piers and abutments.' Framed Girder or Truss.--There are many varieties in the form and construction of bridge trusses, for each of which certain advantages arc claimed. The framed girder or trus§ (fi/. 42, figs. 1-5) consists generally of two parallel chords (an upper and a lower one) diminishing in cross-section toward the ends of the bridg-e. In one modification of this, the so-called " suspension-truss," devised by Bolin= in the United States, there is employed a system of inclined suspension-rods uniting the feet of the posts directly with the ends of the upper chord, and therefore rendering the lower chord unnecessary. The bridge roadway with this form of truss is carried on top of the single chord, the posts and oblique ties being in this case beneath the roadway. Numerous bridges of this type exist in the United States, especially along the line of the Baltimore and Ohio Rail road. The Fink truss (fi/. 45, fig. also of American design, is a sus
pension truss of somewhat different construction, and in its original form or with certain modifications is extensively in use in the United States for rail way bridges. Figures to 5 (p. 42) exhibit various forms of bridge trusses.
Lenticular and Bowstring Girders.—The framed girder above described may be modified by giving both chords or one of them a curved fonn. In the first case there results the so-called " lenticular girder" (fiL 44, .fig. 4); in the last, the bowstring girder. The first specimen of the lenticular girder bridge of which there is record was that bnilt at Saltash, in Eng land, by Brunel, in 1859; the two larger spans of this bridge are each 455 feet. Figure 17 (i5/. 43) exhibits a form of the lenticular girder devised by Pauli and employed by Gerber in the construction of the iron bridge over the Rhine at Mayence (345 feet span). In the Schwedler system (figs. '3-15), a bowstring girder, the designer aims to economize material in the frame. In this construction the vertical web-members are in compression and the diagonals in tension.
Members subjected to tension are counnonly made of flat bars; for those subjected to compression the stiffer ang-le-irons (1) or the channel form (3), are preferred (figs. 26, 27), or they are made of several pieces joined together by riveting to form a column of great stiffness (fig. 25).
Laitice the earlier specimens of iron bridges the close lat tice g,,,irder made of flat bars was much affected, but, as it lacked sufficient stiffness, numerous verticals were specially introduced (/5/. 43, fig. 12). Present practice departs widely from this: each separate piece entering into the construction of an iron bridge serves a specific purpose and is especially adapted to resist the strains that will come upon it from the load on the structure. Some important differences exist in the practice of European and American engineers. The chief of these lies in the manner of uniting the chords to the web-members. European engineers prefer to make this attach ment bv riveting.; in America, on the other hand, it is the universal practice to use cylindrical pins of iron or steel for this purpose. In American practice the lower chord is usually fonned of flat bars placed edgewise and united by pins (p. 42, jig. 13). The web-members subject to tension are made of iron bars (fiat, square, or round) with eyes at the ends for the insertion of the pin-connections (fis-. 8). The compression-members, which at first were made of cast iron, are now made exclusively of wrought iron or steel. They are hollow columns built up of flat pieces or curved segments of vari ous forms riveted together. Many of the leading bridge-constructors have special forms of columns which they prefer to employ. Some of these are shown on Plate 42 (figs. 6, 7).