Fla i Curz-eri Lattice regards roof-trusses with straight or curved lattice rafters, reference may be inade to Plate 4 (figs. 23-28), where two examples of the kind will be found. The first (figs. represents the roof construction of the Otto Circus, in Berlin—a pavilion roof with straight lattice rafters. The second (fig. 28) exhibits the roof of the main hall of the St. Pancras Station, in London—a saddle roof with curved lattice girders.
Otto Circus.—The roof of the Otto Circus is polygonal, with a span of 12.2.1. feet in the clear; it consists essentially of twenty straight wrought iron lattice girders tapered toward their upper ends, connected above by a cast-iron ring (fig. 27), and below by I-shaped wrought-iron intermediate pieces. The ring at the top is required only on account of the lantern set upon it, for without the latter the rafters would come directly together in one point; the lower polygonal pieces, on the other hand, form an essential feature, a sort of anchor by which the horizontal thrust of the construction is entirely eliminated. The lattice girders (shown in detail, fi/. 4, jig. 24) are 3o-", inches deep at the lower and 22 inches at the upper end, and con sist, each, of b.vo pair of rolled angle-irons and the fiat bars set diagonally between. Two curved tie pieces are set between each two lattice rafters, as shown in Figure 23. Figures 25 to 27 represent the details of the lan tern, Figure 25 giving the method of connecting at the heel of the rafters belonging to it, and Figure 26 the connection at the upper end; Fig-,tire 27, the union of the tension rods proceeding from the heel of each rafter in a ring common to all. The whole was constructed at the celebrated Borsig Works, in Berlin.
SI. Pancras roof of the St. Pancras Railway-station, in London (Jig. 28), which was built under the direction of W. H. Barlow, has a span of 24o feet in the clear, and rises from the walls of the building and directly from the platforms to a heig-ht of r2434. feet in the clear. Each lattice girder lias the form of a basket-handle arch composed of font segments of 56.6 feet and 159.5 feet radius respectively, and is 6 feet broad. The angles between the arch, the roof, and the wall are filled in with orna mental open cast-iron work. The principal rafters are placed at intervals of 29 feet, and from one to another are stretched fifteen iron-lattice purlins, each of which carries three intermediate rafters of I-shaped cross-section. In the middle portion the roof is covered with glass to a breadth of 8o feet; the remaining portion is covered with slate.
Frankfort Central imposing example of a mod ern iron-roof structure of the lattice-girder type—perhaps the largest of the kind in the world—is the train-hall of the new Central Railway-station at Frankfort-on-the-Alain (,6/. 5, fig. f). The framework, which has a total span of 55r feet and a length of 6ro feet, consists of three semicircular arched naves, each of which rises in the centre to a height of 93/, feet. The graceful outlines of the interior are especially impressive, and the upper parts of the heavy iron structures, in consequence of their enormous height, look from below like a spider-web. This interesting building is referred to more fully under Railway-stations (p. 2r2).
Iron has not heretofore been used extensively for the construction of entire building-s, for the reason that stone possesses decided advantages as a material for walls, particularly in producing—imposing architectural effects, unless in cases where the iron is made directly to imi tate stone. It has also been found by experience that iron in bulk does not furnish a proper substance to incorporate in walls, on account of its great expansibility iinder the influence of heat; but when -used in the shape of thin lamince, as hoop-iron laid within walls in the bed-joints of the brick or stone, it has no injurious effect, while it is very advantageous in that form as a tie to the structure.
The metal is, however, employed at the present day extensively in edi fices not requiring solid walls, such as pavilions, observatories, exhibition buildings, markets, public-halls, conservatories, etc., which, eyen when the dimensions are large, should be expressly desig-ned to appear as light as possible, and, while possessing all necessary solidity, should be espe cially fitted to admit light and air into the interior. To attain this result, glass is extensively introduced, and becomes a very important material in the structure. In shop-fronts iron is also used to a great extent, to dis pense with the ponderous masses of stone formerly employed, and for the opportunity of display necessary to such edifices. For the same reason, it is applied to warehouses and public buildings, and is much used in the United States for ornamental facades. In other styles of buildings the use of this metal is exceptional. The principal experiments in this direction have been made in England, where the cheapness of the material has led to its being largely' employed in the erection of dwelling-houses for work ingmen.