1304
THEORY AND PRACTICE OF ENGINEERING. Book II,
tlty of the other, which is to be joined to it, so as to suffer the diminished end of one pieceto overlap that of the other, and then uniting them by nails or wooden pins, which are calledtree-nails. This method is applied to plates, bond timbers and others where there is notmuch longitudinal compression or extension : where this kind of effect is to be providedfor, the upper as well as the lower timbers should be cut and let into each other ; theunder piece having a tenon formed at its extreme end, with a corresponding cuttingto receive it in the upper piece; that these tenons may be enabled to pass each other,it is necessary to cut away a part of the timbers in the middle of the length of the joint,equal to the length of the two tenons, so as to form ™ quare hole, through the middle ofthe timbers to be joined together, and this is afterwards closed up by driving an oak keyinto it; this also helps to drive the tenons to their respective mortises, and prevents thetimbers from being pulled asunder : the thickness of the key, in order that it may not becompressed, should be equal to a third of that of the piece into which it is inserted : anotherprinciple is here shown, which is more simple, the joint being cut obliquely ; to make thesetwo pieces stiff, the ends of both should be cut in an angular form. To strengthen thesescarfs, iron straps and screw-bolts are added, but no joining can be made so strong as the
timber itself.
In making joints, it must be remembered that all timber is liable to shrink when dry,and when wet to expand ; on this- account dovetail joints should be avoided as much aspossible, as they are liable to draw out, and ail joints should be made with reference totheir contraction and expansion, which sometimes tends to split off' portions of the framing :where iron bolts or straps are introduced care must be taken that their effect is not lost bythe changes that the timber undergoes ; the areas of the former should never be Jess thantwo-tenths of the area of the section of the beam ; it must also be recollected in making a
joint, that when force is applied to any portion, thefibres of the timber will slide upon each other.
Fishing a Beam is merely placing a piece ofthe same scantling to one side of the timber to beunited, and bolting them or hooping them toge-ther : separate pieces of timl>erare united either by scarfing,
notching, cogging, pinning,_ y
wedging, tenoning, &c. ^—
Scarfing consists in cutting '
away equally from the ends, but _ / /'/
on the opposite sides, of two A
pieces of timber for the purpose / /f/
of connecting them lengthwise: / / /
the usual method of scarfing ' / /
bond and wall plates is bycutting about ^ through eachpiece on the upper face of theone, and the under face of theother, about 6 or 8 inchesfrom the end transversely,making what is termed a kerf;and longitudinally from the
side, so that the pieces lap to-gether like a half dovetail.
Notching is either square ordovetailed, and is made use offor connecting the ends of wallplates and bond timbers at theangles, in letting joists down ongirders, binders, purlins or prin-cipal rafters.
Cogging or Cocking is a species of notch extending onone side, and having a narrow cog alone in the bearingpiece, flush with its upper face; it is principally made useof in tailing joists on wall plates.
Pinning consists in inserting cylindrical pieces of woodor iron through a tenon.
Wedging is the insertion of triangular prisms, whoseconverging sides are under an extremely acute angle, intoor by the end of a tenon, to make it fill the mortise socompletely as to prevent its being withdrawn.
Fig. 2062.
Fig. 2063.
Fig. 2064.
Fig. 2065.