These are used in connection with any member of a roof which it may be advisable to have the power of adjusting, so as to tighten up the truss after it has been put together and into position.

The construction of these joints is similar to that of the joint used by carpenters for connecting a king post with a tie beam, as explained at p. 162.

Figs. 379, 380 show a simple example of a cottered joint applied to the end of a tie rod. This being flattened out, passes between two plates which are bolted to the shoe, and lie on each side of the web of the rafter.

Fig. 379.

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Fig. 380.

A rectangular slot is made through the plates and the end of the tie rod. In this slot are placed two iron wedges or " cotters " (c c), and the sides of the hole are protected and rendered smooth by means of wrought-iron gibs (jj), so that the wedges may slide easily when driven. As the wedges are driven inwards, they force the slot in the tie rod towards the shoe, so that it tends to coincide with the slot in the plates - thus the tie rod is shortened, and the roof tightened up. A somewhat similar example is given in Fig. 448, Plate X.

One cotter is frequently used instead of a pair (see Fig. 374), and has the same effect, for, as it is driven in, and the wider part enters the slot, it draws the two members in connection toward each other. Fig. 372 is an example of a cottered joint, the slot for which is formed in the shoe itself. Figs. 373, 374, 375 give details of a cottered joint connected with a wrought-iron shoe; and at H, Fig. 363, is an iron head of bad form (see p. 188), adapted for receiving two tension rods of a trussed-rafter roof, which are attached to it by cottered joints (see also Fig. 369). A taper of from 1/4" to 1/2" per foot of their length is generally given to the cotters.

Coupling Boxes

A coupling box or union joint (Figs. 381, 382) consists of a short hollow prism of iron with reverse screws tapped inside its ends, into which fit the screws on the ends of the portions of the rod to be connected; as the shackle is turned the ends are drawn inwards and the rod is shortened. The whole or part of the external surface is made polygonal so as to be capable of being turned by a spanner. The screws on the rod should have plus threads, that is threads standing above the surface of the rod, so as not to cut into or weaken it.

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