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.
97. The joint shown in Fig. 24 is similar in construction to that in Fig. 23, except that the line dc is carried through the timber in a slanting direction instead of parallel to the top and bottom, and is not continuous in each timber; that is, the line of juncture on timber a extends from d to I and then makes a break lo at right angles to dl and then proceeds from o tor at the same angle it started. The extent of this slant from the surface of the timber depends upon the length of the joint, as the cuts dc and ef should never be less than one-sixth the depth of the timber when at right angles to de, and when the angle c dl is less than 90 degrees the distance c d should be increased.
98. This style of joint is not as strong as that in Fig. 23 when subjected to compression, but is somewhat stronger under tensile strain, as part of the stress is taken up by the iron bar jn, which is applied in the same manner as in Fig. 22. It is well adapted to a transverse strain, however, and when so intended, slight modification should be made, according to circumstances. When a beam is submitted to a transverse strain, its top is in compression and its under side is in tension; therefore, a combination of two forms of joint should be made to suit the two conditions. When the joint shown in Fig. 24 is used as a beam with a transverse strain, it is better to let the line c d run at right angles to the top of the beam and extend half way through, as the line cd in Fig. 22. This adapts the upper half to a compressive strain, while the under side may be scarfed, as o ef in Fig. 24, and have an iron bar or plate on the bottom.
The rectangular hole, or keyway, lop has its long side on the cuts oe and dl, and the keys r and s have each a full bearing surface on one timber instead of a half bearing on each timber, as the keys in Fig. 23. Under compression, however, there is a tendency for the joint to slide on the line de and split off the small bearing triangles cdx and fex'. The iron bar could hardly be expected to prevent this, as it would probably bend away from the timber between the bolts, and a square butt end, as above described, should, in such a case, be used. In this joint the keys r, s must be driven with caution, as the least overdriving would tend to split the timber on the line dx or ex'.
99. Shrinkage in the material in this joint would first cause it to open slightly on the line de; then, if the timber were under compression, this would be compensated by the sliding of the two pieces into close contact again, and the consequent loosening of the keys r, s; but if the timber were subjected to a tensile strain, the joint would remain open on the line de until the keys r and s were driven tighter to close the joint, and the bolts;m were tightened to hold it in place after the shrinkage had been taken up.
100. Fig. 25 is a modification of Fig. 24, in having the line de continuous and passing it through the center of the keyway rs, and also in having the meeting line of the two timbers cut at an obtuse angle c kg. This renders the beam much more reliable under a cross-strain tending to bend it in the direction of its width, and the pointed toe also tends to keep the timbers flush. Like Fig. 24, this joint is better suited to a tensile than compressive strain, but its behavior after shrinkage is more like that shown in Fig. 23. If used in a position where it will be subjected to a transverse strain, it should have the upper half prepared as described in Art. 08.
101. A method of lengthening timber and preserving a considerable degree of strength, both for longitudinal and for transverse strains, is shown in Fig. 26. Three thicknesses a, b, and c of the same size of timber are bolted together with four bolts m, the joints fg and hde being squarely sawed and tightly butted, in order to secure a good bearing. The bolts must fit the holes exactly, and the nuts on their ends must be screwed as tight as possible, as upon the bolts depends the entire strength of the joint.
102. In proportioning these scarf and lap joints, in oak, ash, elm, or hard pine, the whole length of the joint should be about six times the depth of the timber, when no bolts or plates are used, and the permanence of the union of the pieces depends upon the keys alone; but when bolts are used, the length of the joint should be about three times the depth of the beam, and when both means are adopted, or iron plates are interposed between the bolt heads and the timber, the joint can be as short as twice the depth of the beam. With pine and similar soft woods the length of the scarf should be twelve times the depth of the timber when there are no bolts to sustain the piece, and when reinforced by bolts, six times the depth; or with bolts and plates, a length of four times the depth of the timber is considered sufficient.
103. In Fig. 27 is shown a form of joint more suitable for a short post than either of the foregoing scarfed joints, but it is unfitted for tensile strain. The ends of the timbers a and b are carefully squared on the line c de, and in the center of the end of each piece is bored a hole to take the stud bolt, shown dotted at fg; at a distance from the end of each, and not less than two-thirds the thickness of the post, a square hole h is cut where the nut for the bolt is inserted, and through which it is screwed tight upon each end of the stud bolt. This brings both ends of the post into close contact, so that there will not be any perceptible compression at this point when the working load is imposed.
104. In Fig. 28 is a cheaper form of joint for a similar purpose, though this may also be used in positions where it will occasionally be subjected to slight tensile strain. It is an improved form of that shown in Fig. 20, but with indents cut at def, in order that any tensile strain will come upon the wooden fish-plates, or battens c, and the lugs of the timbers, and not entirely upon the nails, or spikes, as is the case in Fig. 20. The thickness of the plates c should each be equal to not less than one-quarter the depth of the timbers a and b, and the indents def should be not more than one-third the thickness of the fish-plates. The entire length of the joint from d to d' may be three times the timber depth, and the length of the indent from f to d about one-quarter the length of the joint. The fish-plates may then be bolted through the timber from side to side with four bolts, or maybe spiked to each side with from eight to twelve spikes, as shown.
105. In Fig. 29 the fish-plates are replaced by iron straps c not less in breadth than one-sixth the thickness of the timber, and sunk into the face of the wood a depth equal to their thickness, as shown at e. Holes are bored or punched in these straps to receive the lagscrews, or spikes, which serve to bind the timbers and straps together, and the iron, or hardwood dowel d, in the center of the posts, serves to keep the two pieces in alinement. This joint is suited only for compressive strains, and with Figs. 27 and 28, should be used only in a perpendicular position, unless the joint itself is independently supported.
This completes the description of some of the joints most commonly used in the framing of buildings, and we will now proceed to the details of general framing.