Figs. 153 to 161 show sections of several forms of "scarfs" 3 - taken chiefly from Tredgold's work on Carpentry. It will be seen that they present a neater appearance than fished joints, inasmuch as the pieces are cut to fit one another, so that the resulting beam is of the same thickness throughout.
Much ingenuity has been expended in devising scarfs of very intricate form, but the simplest are the best, as they are the easiest to fit accurately together.
Many of the intricate forms given in books will be found to be useless upon being tested by the following principles laid down by Tredgold: -
"When two pieces of timber are tabled together, as shown in Fig. 152, if a tensile strain in the direction of the arrow comes upon the joint, it is evident that it would tend to shear off the pieces a h i c, cifd, by sliding them along the grain, also to crush the ends of the fibres at c i, and further to tear the beams asunder at b c, i k.
As "the weakest part is the strength of the whole," there would be no use in making b c wide enough to resist tearing if the piece a hic were so weak as to be dragged off, and vice versa.
In such a scarf, then, the strength of c i to resist compression, that of c ifd and c i h a to resist shearing, and of b c or i k to resist tearing, should all be equal.4
1 Seddon's Builders' Work. 2 Hid.
3 Sc. Scares. 4 See Tredgold's Rules, page 62 compressing force: there is a temptation to make them oblique (see Fig. 154), in order to hold the pieces together close side by side. This is not an objection when the beam is exposed only to tensile strains, but under compression, the angular point of one piece tends to tear or split the other.
In the succeeding figures it will be noticed that the scarfs are frequently aided in their resistance to strains by the use of fish plates, of hard wood keys, and of wedges. In applying these accessories to scarfs, their strength must be proportioned to that of the parts of the scarf itself - e. g. the strength of the fish plates (after being weakened by the holes for the bolts) must be equal to that of the beams to be united; and the resistance to shearing afforded by the keys must be equal to that of the portion of the scarf on either side.