The joint shown in Fig. 112 is simple and strong; but the strength consists wholly in the bolts, and in the friction of the parts produced by screwing the pieces firmly together. Should the timber shrink to even a small degree, the strength would depend altogether on the bolts. It would be made much stronger by indenting the pieces together, as at the upper edge of the tie-beam in Fig. 113, or by placing keys in the joints, as at the lower edge in the same figure. This process, however, weakens the beam in proportion to the depth of the indents.
Fig. 114 shows a method of scarfing, or splicing, a tie-beam without bolts. The keys are to be of well-seasoned, hard wood, and, if possible, very cross-grained. The addition of bolts would make this a very strong splice, or even white-oak pins would add materially to its strength.
Fig. 115 shows about as strong a splice, perhaps, as can well be made. It is to be recommended for its simplicity; as, on account of there being no oblique joints in it, it can be readily and accurately executed. A complicated joint is the worst that can be adopted; still, some have proposed joints that seem to have little else besides complication to recommend them.
In proportioning the parts of these scarfs, the depths of all the indents taken together should be equal to one third of the depth of the beam. In oak, ash or elm, the whole length of the scarf should be six times the depth, or thickness, of the beam, when there are no bolts; but, if bolts instead of indents are used, then three times the breadth; and when both methods are combined, twice the depth of the beam. The length of the scarf in pine and similar soft woods, depending wholly on indents, should be about 12 times the thickness, or depth, of the beam; when depending wholly on bolts, 6 times the breadth; and when both methods are combined, 4 times the depth.
Sometimes beams have to be pieced that are required to resist cross-strains - such as a girder, or the tie-beam of a roof when supporting the ceiling. In such beams, the fibres of the wood in the upper part are compressed; and therefore a simple butt joint at that place (as in Fig. 116) is far preferable to any other. In such case, an oblique joint is the very worst. The under side of the beam being in a state of tension, it must be indented or bolted, or both; and an iron plate under the heads of the bolts gives a great addition of strength.
Scarfing requires accuracy and care, as all the indents should bear equally; otherwise, one being strained more than another, there would be a tendency to splinter off the parts. Hence the simplest form that will attain the object is by far the best. In all beams that are compressed endwise, abutting joints, formed at right angles to the direction of their length, are at once the simplest and the best. For a temporary purpose, Fig. 112 would do very well; it would be improved, however, by having a piece bolted on all four sides. Fig. 113, and indeed each of the others, since they have no oblique joints, would resist compression well.
In framing one beam into another for bearing purposes, such as a floor-beam into a trimmer, the best place to make the mortise in the trimmer is in the neutral line (Arts. 120, 121), which is in the middle of its depth. Some have thought that, as the fibres of the upper edge are compressed, a mortise might be made there, and the tenon driven in tight enough to make the parts as capable of resisting the compression as they would be without it; and they have therefore concluded that plan to be the best. This could not be the case, even if the tenon would not shrink; for a joint between two pieces cannot possibly be made to resist compression so well as a solid piece without joints. The proper place, therefore, for the mortise is at the middle of the depth of the beam; but the best place for the tenon, in the floor-beam, is at its bottom edge. For the nearer this is placed to the upper edge, the greater is the liability for it to splinter off; if the joint is formed, therefore, as at Fig. 117, it will combine all the advantages that can be obtained. Double tenons are objectionable, because the piece framed into is needlessly weakened, and the tenons are seldom so accurately made as to bear equally. For this reason, unless the tusk at a in the figure fits exactly, so as to bear equally with the tenon, it had better be omitted. And in sawing the shoulders care should be taken not to saw into the tenon in the least, as it would wound the beam in the place least able to bear it.
Thus it will be seen that framing weakens both pieces, more or less. It should, therefore, be avoided as much as possible , and where it is practicable one piece should rest upon the other, rather than be framed into it. This remark applies to the bearing of floor-beams on a girder, to the purlins and jack-rafters of a roof, etc.
In a framed truss for a roof, bridge, partition, etc., the joints should be so constructed as to direct the pressures through the axes of the several pieces, and also to avoid every tendency of the parts to slide. To attain this object, the abutting surface on the end of a strut should be at right angles to the direction of the pressure; as at the joint shown in Fig. 118 for the foot of a rafter (see Art. 86), in Fig. 119 for the head of a rafter, and in Fig. 120 for the foot of a strut or brace. The joint at Fig. 118 is not cut completely across the tie-beam, but a narrow lip is left standing in the middle, and a corresponding indent is made in the rafter, to prevent the parts from separating sideways. The abutting surface should be made as large as the attainment of other necessary objects will admit. The iron strap is added to prevent the rafter sliding out, should the end of the tie-beam, by decay or otherwise, splinter off. In making the joint shown at Fig. 119, it should be left a little open at a, so as to bring the parts to a fair bearing at the settling of the truss, which must necessarily take place from the shrinking of the king-post and other parts. If the joint is made fair at first, when the truss settles it will cause it to open at the under side of the rafter, thus throwing the whole pressure upon the sharp edge at a. This will cause an indentation in the king-post, by which the truss will be made to settle further; and this pressure not being in the axis of the rafter, it will be greatly increased, thereby rendering the rafter liable to split and break.
If the rafters and struts were made to abut end to end, as in Figs. 121, 122 and 123, and the king or queen post notched on in halves and bolted, the ill effects of shrinking would be avoided. This method has been practised with success in some of the most celebrated bridges and roofs in Europe; and, were its use adopted in this country, the unseemly sight of a hogged ridge would seldom be met with.
A plate of cast-iron between the abutting surfaces will equalize the pressure.
Fig. 124 is a proper joint for a collar-beam in a small roof: the principle shown here should characterize all tie-joints. The dovetail joint, although extensively practised in the above and similar cases, is the very worst that can be employed. The shrinking of the timber, if only to a small degree, permits the tie to withdraw - as is shown at Fig. 125. The dotted line shows the position of the tie after it has shrunk.
Locust and white-oak pins are great additions to the strength of a joint. In many cases they would supply the place of iron bolts; and, on account of their small cost, they should be used in preference wherever the strength of iron is not requisite. In small framing, good cut nails are of great service at the joints; but they should not be trusted to bear any considerable pressure, as they are apt to be brittle. Iron straps are seldom necessary, as all the joinings in carpentry may be made without them. They can be used to advantage, however, at the foot of suspending-pieces, and for the rafter at the end of the tie-beam. In roofs for ordinary purposes, the iron straps for suspending-pieces may be as follows: When the longest unsupported part of the tie-beam is -
10 feet, the strap may be 1 inch wide by 3/16 thick.
In fastening a strap, its hold on the suspending-piece will be much increased by turning its ends into the wood. Iron straps should be protected from rust; for thin plates of iron decay very soon, especially when exposed to dampness. For this purpose, as soon as the strap is made let it be heated to about a blue heat, and, while it is hot, pour over its entire surface raw linseed oil, or rub it with beeswax. Either of these will give it a coating which dampness will not penetrate.