Beams are joined in the direction of their length by " lapping," "fishing," and "scarfing."
This consists in simply laying one beam over the other for a certain length, and binding them together with straps, as shown in elevation in Fig. 149, or, if the joint is to stand a tensile strain, with bolts.
Fig. 149. Lapping.
1 A modification of the arrangement given in Rankine's Civil Engineering, p. 453.
Dr. Young1 says of this joint: "We acknowledge that this will appear an artless and clumsy tie-beam, but we only say that it will be stronger than any that is more artificially made up of the same thickness of timber."
The ends of the pieces are butted together, and an iron or wooden plate or "fish-piece " is fastened on each side of the joint by bolts passing through the beam. Fig. 150 is the plan of a joint fished with wooden plates, and Fig. 153 shows one fished with iron plates.
Fig. 150. Fishing with Timber Plates.
The bolts should be placed chequerwise (see Fig. 159), so that the fish plates and timbers are not cut through by more than one bolt hole at any cross section.
When subjected to tension, the chief strain comes upon the bolts (which are but slightly assisted by the friction between the "fish-pieces" and beam), these are loosened by the slightest shrinkage of the timber,, and they then press upon the fibres, crush them, and thus cause the joint to yield.
This dependence upon the bolts may be lessened by indenting or "tabling" the parts together, as at T T, Fig. 151, or by inserting keys, k k, but these arrangments decrease the section and strength of the beams.
When a beam is fished to resist compression, there should be plates on all the four sides.
A fished joint is manifestly unsuited to resist a cross strain.
The strength of fished joints in tension depends
(a) On the effective sectional area of the fish plates being together equal in tensile strength to the effective sectional area of the beam.
(6) On the sectional area of the bolts being sufficient on either side of the joint to resist shearing.
In practice it is usual to take the sectional area of the bolts as equal
Fig. 151. Fished Joint showing Keys kk, and Tabling T T.
1 Encyclopaedia Britannica on either side of the joint to at least 1/5 the effective sectional area of the tie.
(c) On placing the bolts in such a way, and at such distances, from the ends of the fish plates and butting ends of the timbers - that they will not draw through them - by shearing out the wood in front of them.
(d) On giving the bolts such bearing area as will prevent their cutting their way through the timbers on the fish plates. It is in this way that fished ties are most liable to yield.1
Scarfed joints are often fished with iron plates to assist the scarf (see Fig. 155, etc.) These plates also serve to protect the wood from being crushed by the bolts. They are sometimes turned down at the ends into the timber, so as to assist it in resisting tensional strains. It has been recommended that the indented ends should not be opposite to one another, as in Fig. 157, for in that position they cut into the timber at the same cross section, and weaken it more than if they are placed as in Fig. 159.2