CHAPTER XXVII. IRON ROOFS AND ROOF-TRUSSES, WITH DETAILS OF CONSTRUCTION. OWING to the increasing cost of lumber, and the necessity of erecting buildings as nearly fire-proof, and with as little inflammable material in the roof, as possible, it is becoming quite a common practice to roof large and expensive buildings with iron roofs, which, of course, involves the use of iron roof-trusses: hence it is important that the architect and progressive builder should have a general idea of the construction and principles involved in iron roof-trusses, and be familiar with the best forms of trusses for different spans, conditions of loading, etc. II] T I-Beam. Deck-Beam. Channel-Bar. T-Bars. Angle-Irons. Besides being non-combustible, iron roof-trusses are superior to wooden trusses in that they may be built much stronger and lighter, and are much more durable. Various forms of trusses have been constructed to suit different conditions of span, load, height, etc., and of these the following examples have been found to be the best and most economical. Before proceeding to describe these various forms of trusses, we would call the reader's attention to the sections of beams, angleirons, T and channel bars, shown in Fig. 1. It will frequently be necessary to refer to these sections; as they are the principal shapes of rolled iron entering into the construction of iron roofs, and it is of great importance that an architect or builder be familiar with their forms and names. For convenience in describing the different forms of iron roofs, we shall divide them into the following classes: 1st, Truss-roofs with straight rafters, which are simply braced frames or girders. 2d, Bowstring-roofs with curved rafters of small rigidity, and with a tie-rod and bracing. 3d, Arched roofs, in which the rigidity of the curved rafter is sufficient to resist the distorting influence of the load without additional bracing. Trussed Roofs. For small spans, the most economical and simplest form of truss is that represented in Fig. 2. (Owing to the LEBANON FURNACE. Fig. 2. small scale to which it is necessary to draw these figures, we have represented the pieces by a single line, which has been drawn heavy for strut-pieces, and light for ties and rods.) This truss was built by the Phoenix Iron Company for the roof of a furnace-building. It consists of two straight rafters of channel or T bars, two struts supporting the rafters at the centre, a main tie-rod, and two inclined ties assisting the tie-rod to support the end of the struts. The lines on the top of the truss represent the section of a monitor on the roof, which is not a part of the truss, but only supported by it. One of the great merits of this truss is that it has but few pieces in compression, viz., the rafters and two struts; which is a condi tion very desirable in iron trusses, owing to the fact that wroughtiron resists a tensile strain much better than a compressive one, and hence it is more economical to use wrought-iron in the form of ties than in the form of struts. It should be borne in mind that for ties, rods or flat bars of iron are the most suitable; while for struts, it is necessary to use some form of section that offers considerable resistance to bending, such as a T-iron, or four angle-irons riveted together in the form of a cross; for wrought-iron struts always fail by bending or buckling, and not by direct crushing. In Figs. 2-10 the pieces which are struts, or resist a compressive strain, are drawn with heavy lines, and those pieces which act as ties are drawn with a light line. 60 ft. Fig. 3. Fig. 3 represents a truss similar to that in Fig. 2, but having two struts instead of one, which is more economical where the span is over fifty-six feet, for the reason that it allows the rafters to be made of lighter iron. For spans of from seventy to a hundred feet, the form of truss shown in Fig. 4 has been found to be about the most economical and satisfactory in every respect. NEW MILL, PHOENIX IRON-WORKS, ROCK-ISLAND ARSENAL. Fig. 4. The rafters in this truss, for moderate spans, may be T-irons; and for larger spans, channel-bars and the ties and struts may be bolted to the vertical rib. For very large spans, channel-bars may be used, placed back to back, with the ends of the bracing bars between them. I-beams are also used for the rafters, but they have the objection of not being in a shape to connect readily with the other forms of iron. The flanges of an I-beam do not offer so good an opportunity for riveting as do those of angle and T irons and channel-bars. The ties are rods of round iron or flat bars; and the struts, commonly T-irons or angle-irons bolted together. MASONIC TEMPLE, PHILADELHIA. Fig. 5. Another form of truss, shown in Fig. 5, derived from the wooden queen post truss, is very commonly used for spans of from sixty to a hundred and forty feet. A modification of this truss is shown in Fig. 6, in which both struts and ties are inclined, instead of only the -134 ft. Fig. 6. struts, as in Fig. 5. The truss in Fig. 6 has the advantage that the struts are shorter, more nearly perpendicular to the rafters, and less strained. Bowstring-Roofs. — In designing iron roofs, it is sometimes desired to vary the ordinary straight pitch roof by using a curved rafter. Two examples of such roofs are shown in Figs. 7 and 8, ALTOONA STATION, PENNSYLVANIA RAILROAD. Fig. 7. which were constructed by the Phoenix Iron Company of Philadelphia. These may be considered as the simplest forms of bowstring-roofs. The principal use of the bowstring-roof proper is for roofing |