Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/536

Rh 480 BUILDING CARPENTRY. because the abutment of the inner partis better supported. This is certainly the case, but it supposes the whole rafter to go to the bottom of the socket, and the beam to be thicker than the rafter. Some may think that this will weaken the beam too much, when it is no broader than the rafter is thick ; in which case they think that it requires a deeper socket than Nicholson has given it. Perhaps the advantages of Nicholson s construction may be had by a joint like fig. 9, No. 2. Whatever is the form of these butting joints, great care should be taken that all parts bear alike ; and the artist will attend to the magnitude of the different surfaces. In the general compression the greater surfaces will be less compressed, and the smaller will therefore change most. When all has settled, every part should be equally close. Because great logs are moved with difficulty, it is very troublesome to try the joint frequently to see how the parts fit ; therefore we must expect less accuracy in the interior parts. This should make us prefer those joints whose efficacy depends chiefly on the visible joint. It appears from all that has been said on : this subject, that a very small part of the cohesion of the tie-beam is sufficient for withstanding the horizontal thrust of a roof, even though very low pitched. If, therefore, no other use is made of the tie-beam, one much slenderer may be used, and blocks may be firmly fixed to the ends, on which the rafters might abut, as they do on the joggles on the head and foot of a king-post. A tie-beam may have to carry a floor or ceiling (sometimes the workshops and store-rooms of a theatre), and therefore requires a great scantling, but frequently beams have little to do, and contain an unneces sary quantity of timber. It is therefore of importance to ascertain the most perfect manner of executing such a joint, and wo have directed the attention to the principles that are really concerned in the effect. In all hazardous cases the carpenter calls in the assistance of iron straps; and they are frequently necessary, even in roofs, not withstanding this superabundant strength of the tie-beam. But this is generally owing to bad construction of the wooden joint, or to the failure of it by time. There needs but little to be said of the joints at a joggle worked out of solid timber; they are not near so diffi cult as the last. When the size of a log will allow the joggle to receive the whole breadth of the abutting brace, it ought certainly to be made with a square shoulder ; or, which is still better, an arch of a circle having the other end of the brace for its centre. Indeed, this in general will not sensibly differ from a straight Hue perpendicular to the brace. By this circular form the settling of the roof makes no change in the abutment ; but when there is not sufficient stuff for this, we must avoid bevel joints at the shoulders, because these always tend to make the brace slide off. The brace in Plate XXIII., fig. 10, No. 1, must not be joined as at b, but as at a, or in some equivalent manner. When the very oblique action of one side of a frame of carpentry does not extend, but compresses, the piece on which it abuts, there is no difficulty in the joint. Indeed a joining is unnecessary, and it is enough that the pieces abut on each other ; and we have only to take care that the mutual pressure be equally borne by all the parts, and that it do not produce lateral pressures, which may cause one of the pieces to slide on the butting joint. A very slight mortise and tenon is sufficient at the joggle of a king-post with a rafter or straining beam. It is best, in general, to make the butting plain, bisecting the angle formed by the sides, or else perpendicular to one of the pieces. In fig. 10, No. 2, where the straining beam a l&amp;gt; cannot slip away from the pressure, the joint a is preferable to b, or indeed to any uneven joint, which never fails to produce very unequal pressures on the different parts, by which some are crippled, others are splintered off, &c. When it is necessary to employ iron straps for strength ening a joint, considerable attention is necessary that we stra F may place them properly. The first thing to be determined is the direction of the strain. This must be resolved into a strain parallel to each piece, and another perpendicular to it; and then the strap which is to be made fast to any of the pieces must be so fixed that it shall resist in the direc tion parallel to the piece. Frequently this cannot be done ; but we must come as near to it as we can. We can hardly give any general rules. Fig. 30 shows the nature of the strap or stirrup by which the king-post carries the tie-beam. The strap that we i, observe most generally ill placed is that 3 which connects the foot of the rafter with the beam. It only binds down the rafter, and does not act against its horizontal thrust. It should be placed farther back on the beam, g with a bolt through it, which will allow it to ^ | turn round. It should embrace the rafter almost horizontally near the foot, and should be notched square with the back of the rafter. It is represented in fig. 11, Plate XXIII. By moving round the eye-bolt, it follows the rafter, and cannot pinch and cripple it, which it always does in its ordinary form. We are of opinion that straps which have eye-bolts in the very angles, and allow all motion round them, are of all the most perfect. A branched strap, such as may at once bind the king-post and the two bracc.s which butt on its foot, will be more serviceable if it have a joint. When a roof warps, those branched straps fre quently break the tenons, by affording a fulcrum in one of their bolts. An attentive and judicious artist will consider how the beams will, act on such occasions, and will avoid giving rise to these great strains by levers. In the fore going reasoning upon the direction of straps, regard has been had especially to the economizing of their immediate strength ; but it may happen that the principal purpose of the strap will be answered by its pressing the rafter firmly upon the beam, and this effect may be produced by a certain deviation from the horizontal position, with but little diminution of the strength of the strap, a deviation which has also the advantage of allowing the strap to embrace the whole of the beam, without weakening it by driving a bolt through it. We must not, howevor, run the risk of crippling the end of the beam. A skilful carpenter never employs many straps, considering them as auxiliaries foreign to his art, and subject to imperfections in workmanship which he cannot discern or amend. We must refer the reader to Nicholson s Carpenter and Joiners A ssistant f or a more particular account of the various forms of stirrups, screwed reds, and other iron-work, for carrying tie-beams, &c. The diagrams of Plate XXIV. will illustrate the use of Furt the before-described joints on a smaller scale in the further ij operations of the carpenter s work. Ordinary scarfing is the cutting away equally from the J Q^, ends, but on the opposite sides, of two pieces of timber, for the purpose of tying or connecting them lengthwise, and is done to wall-plates and bond-timber. The usual mode is by cutting about three-fifths through each piece on the upper face of the one and the under face of the other, about 6 or 8 inches from the end, as in fig. 1, Plate XXIV., transversely, making what is technically termed a calf or kerf, and longitu dinally from the end, from two-fifths down on the same side, so that the pieces lap together with a sort of half dovetail. The heavy supervening weight of the wall and joists renders it impossible that they should be drawn apart without tear ing the fibres asunder or lifting the weight. Nevertheless