Susfiension the ceiling joists cannot be directly supported in their length from below, the only other resource will be to use suspen sion from above; but this is seldom seen, because these joists are mostly used in the roofing of halls and hall-like apartments in which there is an attic space, and consequently no necessity for special arrangement of the ceiling joists. This system, forming the second class indicated above, is now very largely used, and may be subdivided into such as have (r) Trussed frames; (2) Arch-ribs set below; and (3) Flat or curved lattice rafters.
Trussed examples of roof constrnction of the first class, where the horizontal thrust is taken up by tension rods, will be found in Figures 8 to 14 and 15 to 21, together with the necessary details. The first example shows the roof construction with truss-frame according to the system of Polonceau, as used in the Market-Hall at Nancy; the second shows one of the same variety with truss in crescent form, as used in the great hall of the Central Depot at Birmingham.
The simplest case of roof construction with iron truss-work is that in which two iron struts having on their backs the purlins for the roofing material or for the rafters are connected directly together above, while a horizontal round tension rod is applied below. Trusses of this kind are generally placed at distances of about 13 feet from one another, and are further arranged according to the position and size of the window piers. If, because of a large span or to avoid too heavy sections, it seems neces .
sary to give the latter additional support in their length, it can be done in the manner first suggested by the French engineer Polonceau, as seen in Figure 8 (fi/. 4)—namely, by arranging the work so that the keel of each of the two short supports of the principal struts shall be connected with the solid apex of the latter by round iron suspension rods. The chief tension rod in this case must consist of several pieces.
The shapes of the individual parts of these constructions, their manner of connection, and the auxiliary pieces, will be seen among the details (jigs. 9-14) belonging to Figure 8. The main struts are made with the I-shaped cross-section, although their having to resist pressure in their length must be taken into consideration, notlyithstanding the fact that the cross-shaped section is better fitted for the purpose; this is done to make more convenient connections with other parts and for the reception of the purlins. The struts are easily made by rolling; the supports of the main
braces, on the other hand, which likewise have to sustain compression, are generally made with the cross-shaped cross-section. When the length is not great, cast iron may best be used, or for somewhat longer ones four rolled and riveted angle-irons. The tension and suspension rods, like all that are to be subjected to tension, are made of round wrought iron.
Where the length is somewhat great, these rods, particularly the hori zontal ones, must be provided with some arrangement for regulating their length. On account of the dividing of the rod which thus becomes neces sary, a light tension rod to prevent sagging is generally used. Figure 8 shows this arrangement with the tension rod in the middle. Figure 12 iepresents the detail, a being the elevation and b the plan. The two divis ions of the tension rod are inserted into a sleeve-nut at the two sides, with oppositely-directed screw-threads, while the rod goes through vertically in the middle and finishes with an ornamental screw. This rod is hung be tween two short iron beams above by screw bolts, as shown in Figure to, the rails, in their turn, being hung on a packing-plate connected with the main braces.
The most important connecting pieces in roof construction with trussed frames are at the points of meeting of the tension and suspension rods with the support of the chief braces. Figure it, a and b, shows one of these con necting, pieces in elevation and plan as generally used—that is, so that the individual rods and the support are attached between two wrought-iron plates by means of short bolts. By this method of connection, a slight movement of the rods becomes possible in the event of such changes in dimension as are caused by variations in temperature. For this reason, also, the connection between the braces and tension rods is not stiff, but loose; here a fork is used, which encloses the brace or cast-iron shoe, sepa ratelv shown in Figure io (pi. 4). This belongs to the same with its open part, its solid portion receiving the end of the tension rod, which is provided with a nut and thread. Sometimes a fork is placed on the rod, as on the upper ends of the diagonal suspension rods in Figure to.