As regards the first example (fig. 8) of suspension-truss construction, we mav mention that the span in the clear is 39X feet, that the upper part has a lantern for ventilation supported by cast-iron columns, and filially that the purlins carrying the rafters are of wood and are fastened upon the iron struts, as seen in Figure 14. The ridge purlin alone is made of iron, and this rather serves to strengthen the suspension trusses in the length, and therefore does not lie over the braces.
Truss: _Birmingham second exam ple, Figures 15 to 21, exhibits the details of the iron-truss roof of the Cen tral Railway-station in Birmingham, England, and is an instance of the " bowstring truss "—a system quite different from the foregoing. A girder made of one vertical plate 15 inches wide, with four angle-irons and of cir cular form, takes the place of the braces; Figure 16 gives the cross-sectiou of such a girder at its ends and middle. The purlins lie over the iron girders, while underneath them a system of vertical supports and diagonal ties connected at their heel by the suspension rods, 4 inches thick, is arranged so as to stiffen the whole. All the single ties and plates have stiff joints, the connection being made partly by riveting, as seen in the detail of the Figure, and partly by bolts; thus collectively they form an immovable framework that spans a hall 208 feet in width.
This framework (fig. 15) rests at one side upon the wall of the depot building, where it is bolted down; on the other side it rests upon hollow cast-iron pillars, whose upper ends are seen in section in Figure 21. To prevent unfavorable effects upon the columns by its contraction and expan sion due to changes of temperature, it has a bearing of steel rollers held together in a frame at the tops of the columns (fig. 24 The construction of the vertical supports and their connection with the diagonal braces and tension rods are particularly interesting. Details are shown in horizontal section in Figure 18, in vertical section in Figure 20 and in Figure 17a. Of the above-mentioned construction of the support, it may be added that Figure 17, a and b, gives front and side views and Figure 19 the hori zontal section, and that it consists of four angle-irons separated in their length by intermediate pieces bolted to cast-iron shoes above and below, which are common to both. The diagonal ties in cross-section are right angled, are 44_. inches wide by 34 of an inch thick, and are fastened by three screws each, above and below, between corresponding flaps on the shoes.
Roof.. Diana 22 gives a very interesting- exam ple of the second class of iron roofing—namely, that of the arch-rib; it represents the roof of the Diana Bath, at Vienna, built by Etzel. The
arch-ribs which form the chief support of the whole roof are constructed like stone or briek arches, except that here the individual parts—made of cast iron—are immovably connected by flanges and screws, thus forming a rigid whole without material horizontal thrust. As the roof is symmet rical on the two sides, with an angle of thirty degrees, and is, moreover, arranged tangentially to the arch-ribs, there arise in each main couple be tween the arch-rib and the surfaces of the roof and the side walls three similar triangles with equal sides, which are filled up with open cast-iron work-. On it are rafter-like pieces, which serve for the reception and secur ing of the wooden purlins. The span of the whole framework in the clear, between the foot-points of the arches, is ‘5,3 feet.
The Drexel Building,in Philadelphia, a typical illustration of recent American practice, worthy of note as a fine example of a great building devoted to business purposes, exhibits certain novelties of construction. The windows are made very wide, the piers are reduced to a mini mum, and in the centre of each pier there is placed an iron column on which the floor girders bear, thuS leaving the masonry to carry only its own weight. The interior consists entirely of iron construction, the col umns being cased with hollow brick as a protection against fire. The necessity of building six stories over the existing banking building of Drexel S: Co., and also over a large apartment for the Stock Exchange, the portion of the building- above in both cases haying no internal sup ports, presented serious difficulties, but the problem was solved by using heavy iron trusses shaped somewhat like the letter A, rising through four stories of Lite building (nearly 5o feet), with their bases supported on heavy iron pilasters built up against the inner faces of the walls. These pilas ters, which were made in sections of convenient length, bolted together end to end and anchored to the walls, were afterward encased in marble, mak ing a very handsome appearance. Those in the banking building were set up after business-hours, the entire work being carried on without any in terference with the daily business routine. The cut (pi. 5, fig-. 2) shows the construction of the trusses which support the floors and roof. The ground-plan of the building is in the form of a huge letter H, each wing being- 220 by 56 feet and rising 135 feet above the sidew-alk.