A better form, however, is the strut of cruciform section, consisting of two T irons riveted back to back, as in Fig. 281.
Strong and light struts formed out of wrought-iron gas tubing fitted with cast-iron sockets at the ends are sometimes used. Another very good form was noticed in Part I. It consists of two flat bars, Figs. 290, 291, kept apart by cast-iron distance pieces, ccc, varying in length so as to form a strut tapering from the centre toward the ends.
1 Wray's Theory of Construction.
Such a strut is shown in Fig. 284, also in Plates VII. IX., and a similar one on an enlarged scale in Figs. 290, 291. L or T irons are sometimes substituted for the flat bars.
Fig.292. (ENLARGED SCALE) PLAN AT B.
Scale for Figs. 290,291, ½ inch = 1 foot. " " Figs. 292 1 inch=1 foot.
In very large roofs a strut on the same principle as the last mentioned may be constructed by using 4-angle irons kept apart by cross-shaped distance pieces. These crosses are made smaller and smaller as they approach the ends of the strut, which is therefore shaped somewhat like a weaver's shuttle, being wide in the middle and tapering towards its extremities.
Tie and Tension Rods in large roofs are of circular rod iron, or flat bars, as already described for smaller roofs - and are secured in a similar manner. T iron tie rods are very convenient for connections (see Plate XL)
Steel tie rods are now much used.
When a tie rod is long it is severed at the centre, sometimes at two or more points. The joints may in a round tie rod be formed as shown in Figs. 293, 294 (which are details of the joint at A, Fig. 311, Plate V.), or as in Figs. 338, 339, Plate VIII. When the tie rod is a T iron or flat bar a very simple joint may be made as in Fig. 295.
Plates IX. and X. give several illustrations of joints patented by Messrs. Handy side of Derby for roofs in which steel tie rods are used. The ends of the rods to be connected instead of being forked are bolted to steel straps of the form shown in Fig. 354. This has the great advantage of avoiding eyes, forks, or heads, which would be more difficult to form on steel rods than on those of iron. Moreover the joint is compact and neat in appearance, and each portion of the tie rod can be adjusted to the exact length required by merely turning it round.