This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
Fishing. A stud or post may be lengthened, or fished, as it is called, by the addition of an extra piece of the required length, the ends being cut square and the pieces secured together by nailing on the opposite sides of the stud two pieces of 1-inch board about 2 feet long, as shown at Fig. 20.
This is done only in balloon-frame construction, as in braced-frame construction, the distance between the interties and plate, or sill, should never be greater than the available length of studs. But in balloon frames, where the studs are butt jointed against both the sill and the plate without any intermediate joint, the studs frequently require to be fished, especially under the centers of gable roofs, where the height of the house is greatest. Studs can be obtained in all lengths from 13 to 24 feet, and any length in excess of 24 feet can be obtained only by splicing two pieces together.
88. In making a splice joint, attention must be given to the character of the strain to which it will be subjected, whether tension, compression, transverse strain, or torsion, and provision must be made for it to resist that strain, and at the same time to fulfil any other conditions of utility or beauty that may be required of it.
89. One of the simplest form of splice for a tie-beam or other tension member is shown in Fig. 21, where the timber a is simply laid against the timber b and bolted up with the three bolts shown at c. Between the bolt heads, or nuts, and the face of the timber there should be interposed, in each case, a washer d to prevent crushing or tearing the surface of the timber when the nut is tightened.
This makes a clumsy and awkward looking joint, and should seldom be used, except for temporary structures, but aside from this defect it is excellent for its purpose, and, by reason of the bolts, makes a stronger tension member than would the joint previously described for the splicing of studs, when spikes alone are used to secure the fish-plates.
Scarfing. Where the joint is to be subjected to compression or tension, or both, and at the same time must preserve an appearance of neatness and good workmanship, we must resort to what is called scarfing, or to fishing with iron plates, or both.
Scarfing consists in the cutting and fitting of the ends of two pieces of timber in such a manner that they will enter into and fit each other so as to form a comparatively secure joint, simply from the conditions of their joining surfaces. They are usually made additionally secure by means of hardwood keys and iron bolts, or screws, but the primary consideration should be, to make the unsecured joint as strong as possible, by means of the proper proportioning of its details and parts.
91. This proportion would vary according to the strains for which the beam was intended, or to which it was likely to be subjected. In Fig. 22 is shown a form of joint which is applicable to a timber which would be submitted to compression, as the two abutting surfaces at cd and ef are equal to the entire cross-section of the timber, and are at right angles to the line of compression, but under tension the pieces would immediately pull apart unless secured, as shown, by means of bolts and straps, in which case the entire tensile strain would have to be borne by the straps and the sections of beam immediately under them and between the bolt holes and the end of the beam; and the bolts themselves would be subjected to a shearing strain.
Again, in Fig. 23, we have a joint which, under compressive strains, would be all that is ordinarily required, but if submitted to tension, the entire strain would be borne by the keys r, s and the iron bolts m. It is evident in Fig. 23 that the failing of this joint under a tensile strain would be caused by one or more of the following five conditions: The keys r, s might shear through and permit the two pieces to slide apart; or, if secured by bolts, the bolts might shear and have the same result. The wood of the beam might split or shear off on some line from o to dc, allowing the pieces to pull apart without disturbing the keys; or, the beam might part under the tension at the thinnest point, as at b; or, the bolts might remain intact, but the material between them and the end of the beam might be sheared off and permit the beams to separate.
92. It is evident, therefore, that in order to secure the greatest tensile strength combined with the most economical distribution of material, the following conditions must exist in any scarf joint, no matter what its form may be:
The sections of the timber subjected to a shearing strain must be so proportioned that their shearing value will be equal to the portions of the timber subjected to a tensile strain; and if the joint is additionally secured by means of iron bolts or plates, or both, the combined tensile strength of all the members submitted to tension must be equal to the combined strength of the members submitted to shear.
For example, if we use a joint of the form shown in Fig. 23 for a beam subjected to tensile strain, and omit the four bolts m, we must proportion the parts so that the shearing strength of the keys r, s, and the shearing strength of the material between o and de or between p and ij will equal the tensile strength of the section at b. For, no matter how much stronger any one of these details may be than is required of it, the beam will not acquire additional strength thereby, as it will only be as strong as its weakest part.
The importance of analyzing the conditions under which a beam will be strained is, therefore, very evident, and it is the comprehension of these conditions, and the knowledge of how to provide for them, that enables us to select a proper form of scarf joint in each particular case.