Fig. 153 shows in elevation a very simple form of scarf, evidently well adapted to resist compression. The bearing surfaces are large, and perpendicular to the compressing force. Its form does not help it to resist tension. Under a tensile strain it would depend entirely upon the shearing strength of the bolts to hold it together. Nor is it adapted for a cross strain, which would bend the iron plates and tear out the bolts.
A modification of this scarf is sometimes formed like that in Fig. 157, but when intended to resist compression only, the keys kh are not required.
Any scarf containing oblique bearing surfaces is not adapted to resist compression, for reasons already given.
The scarf shown in Fig. 154 is often used for beams to resist a tensile strain. It will hold without the aid of bolts or straps, but the triangle abc offers a weaker resistance to the pressure of the wedges than when the joint is left square, as in Fig. 156.
Fig. 153. Scarf to resist Compression.
Fig. 154. Scarf to resist Tension.
The oblique surfaces of this scarf make it ill adapted to resist compression, and the angles which receive the splayed ends are liable to be split by their pressure.
Fig. 155. Joint fished with Iron Plates, and Scarfed to resist Tension.
Fig. 155 is a modification of the last, often used in preference.
The form of scarf shown in Fig. 156 is well adapted to resist both tension and compression, even independently of bolts and plates.
It is evidently weak in cross section, on account of the timber being so much cut away, and therefore it is not fit to withstand a transverse strain.
Fig. 156. Scarf to resist Tension and Compression.
The wedges shown in the centre are most useful when bolts are to be added, in which case they bring the parts of the joint up to their eventual position before the bolts are inserted, so that there may be no cross strain upon the latter.
Fig. 157 shows a modification of the last, in which the tabling is avoided, and the necessary resistance to tension is given by means of keys of hard wood, k k, as shown; or pairs of wedges such as that shown in Fig. 156 may be used with advantage. Scarf to resist Cross Strain. - When a beam is subjected to a transverse strain, the fibres of its upper part are compressed, and those of the lower portion distended, as shown in an exaggerated form by the dotted lines in Fig. 158.
Fig. 157. Scarf with Keys kk to resist Tension and Compression.
Fig. 158. Scarf to resist Cross Strain.
In scarfing such a beam, therefore, the indents in the upper or compressed portion should be kept square and perpendicular to the pressure, while those in the lower, or distended part, may be oblique, as they have to resist tension only.
The strength of the scarf is increased by inclining a b so as to have as great a thickness as possible at c b. The angle at b tends to hold the pieces together.
It has been found by experiment1 that a joint to resist cross strain is stronger when scarfed vertically through its depth, as in Fig. 159, than when the scarf is formed flatwise across its width, as is usually the case.
Fig. 159. Joint Scarfed vertically and Fished to resist Cross Strain.
If, in addition to transverse pressure, the beam is exposed to a strain in the direction of its length, its resistance to tension is afforded by placing a wrought-iron plate over the joint on the lower side as shown in Fig. 160.
Fig. 160. Scarf to resist Cross Strain and Tension.
In this scarf the angle at a is rather weak, but the line a b is necessarily oblique, in order to get a sufficient thickness at b c to resist the transverse strain.
Fig. 161 shows the usual way of scarfing wall plates. The wedge-shaped portion is technically known as the "calf," or "kerf."
Fig. 161. Scarfed Wall Plate.