Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/535

Rh JOINTS.] BUILDING 479 a centre, which was yielding to its load, and had pushed aside one of the piers above 4 inches uare Where the beams stand square with each other, and the nts. strains are square with the beams and in the piano of the frame, the mortise and tenon is the most perfect junction. A pin is generally put through both in order to keep the pieces united, in opposition to any force which tends to part them. Every carpenter knows how to bore the hole for this pin, so that it shall draw the tenon tight into the mortise, and cause the shoulder to butt close, and make neat work ; and he knows the risk of tearing out the bit of the tenon beyond the pin, if he draw it too much. We may jusb observe, that square holes and pins are much preferable to round ones for this purpose, bringing more of the wood into action, with less tendency to split it. x-tail Ship-carpenters have an ingenious method of making long Jging wooden bolts, not passing completely through, which take a very fast hold, though not nicely fitted to their holes, which they must not be, lest they should be crippled in driving. They call it fox-tail wedging. They stick into the point of the bolt T, Plate XXIII., fig 2, thin wedges cf hard wood, so as to project a proper distance ; when these reach the bottom of the hole by driving the bolt, they split the end of it, and squeeze it hard to the side. This may be practised with advantage in carpentry. If the ends of the mortise are widened inwards, and a thin wedge be put into the end of the tenon, it will have the same effect, and make the joint equal to a dove-tail ; but this risks the splitting the piece beyond the shoulder of the tenon, which would be unsightly, and may be avoided by two very thin wedges a and c being struck in near its angles, projecting equally; at a very small distance within these are to be placed two shorter ones l&amp;gt;, d, and more within these if necessary. In driving this tenon, the wedges a and c will take first, and split off a thin slice, which will easily bend without breaking. The wedges b, d will act next, and have a similar effect, and the others in succession. The thickness of all the wedges taken together must be equal to the enlargement of the mortise towards the bottom. i rtise When the strain is transverse to the plane of the two
 * . tenon, beams, great care must be taken by the artist in placing

his mortise A mortise in a girder for receiving the tenon of a binding-joist of a floor should be as near the upper side as possible, because the girder becomes concave on that side by the strain. But as this exposes the tenon of the bind ing-joist to the risk of being torn off, we are obliged to mortise further down. The form in Plate XXIII., fig. 3, generally given to this joint is extremely judicious. The sloping part a b gives a very firm support to the additional bearing e d, without much weakening of the girder. This form should be copied in every case where the strain has a similar direction The joint that most of all demands the careful attention of the artist is that which connects the ends of beams, one of which pushes the other very obliquely, putting it into a state of extension. The most familiar instance of this is the foot of a rafter pressing on the tic-beam, and thereby drawing it away from the other wall. When the direction is very oblique (in which case the extending strain is the greatest), it is difficult to give the foot of the rafter such a hold of the tie-beam as to bring many, of its fibres into the proper action. There would be little difficulty if we could allow the end of the tie-beam to project to a small distance beyond the foot of the rafter; but, indeed, the dimensions which are given to tie beams for other reasons are always sufficient to give enough of abutment when judiciously employed. Unfortunately this joint is very liable to fail from the effects of the weather. It is much exposed, and frequently perishes by rot, or becomes so soft and friable that a very small force is suOicient either for pulling the filaments out of the tie-beam, or for crushing them together. We are therefore obliged to secure it with particular attention, and to avail ourselves of every circum stance of construction. One is naturally disposed to give the rafter a deep hold by a long tenon, but it has been frequently observed in old roofs that such tenons break off. Frequently they are observed to tear up the wood that is above them, and push their way through the end of the tie-beam. This in all probability arises from the first sagging of the roof, by the compression of the rafters and of the head of the king post. The head of the rafter descends, and the angle with the tie-beam is diminished by the rafter revolving round its step in the tie-beam. By this motion the heel or inner angle of the rafter becomes a fulcrum to a very long and powerful lever much loaded. The tenon is the other arm, very short ; and being still fresh, it is therefore very powerful. It therefore forces up the wood that is above it, tearing it out from between the cheeks of the mortise, and then pushes it along. Carpenters have therefore given up long tenons, and give to the toe of the tenon a shape which abuts firmly, in the direction of the thrust, on the solid bottom of the mortise, which is well supported on the under side by the wall-plate. This form, represented in Plate XXIII,, fig. 4, has no tendency to tear up the end of the mortise. The tenon has a small portion a b cut perpendicular to the surface of the tie-beam, and the rest b c is perpendicular to the rafter. But if the tenon is not sufficiently strong (and it is not so strong as the rafter, which is thought not to be stronger than is necessary), it will be crushed, and then the rafter will slide out along the surface of the beam. It is there fore necessary to call in the assistance of the whole rafter. It is in this distribution of the strain among the various abutting parts that the varieties of joints and their merits chiefly consist. We can only mention a few here that have met with most general approval. The aim in fig. 5, Plate XXIII., is to make the abutments Various exactly perpendicular to the thrusts. The action is the forms of same as against the joggle on the head or foot of a-&amp;gt; oints - king-post. This is a very effectual joint ; it is not, how ever, much practised. It is said that the sloping seam at the shoulder lodges water ; but the great reason seems to be a secret notion that it weakens the tie-beam. Fig. G exhibits a form that is more general, but certainly worse. The shoulder-joint is sometimes formed like the dotted lineabedcfg of fig. G. This is much more agreeable to the true principle, and would be a very perfect method, were it not that the intervals b d and df are so short that the little wooden triangles b e d, d cf will be easily pushed off their bases b d, df. Fig. 7, No. 1, seems to have the most general approbation, but we fail to perceive its peculiar merits. It is the joint recommended by Price, and copied into books of carpentry as the true joint for a rafter foot. The visible shoulder-joint is flush with the upper surface of the tie-beam. The angle of the tenon at the tie nearly bisects the obtuse angle formed by the rafter and the beam, and is therefore somewhat oblique to the thrust. The inner shoulder ac is nearly perpendicular to b d. The lower angle of the tenon is cut off horizontally as at c d. Fig. 8 is a section of the beam and rafter foot, showing the different shoulders. Fig. 7, No. 2, is a simple,)-, and in our opinion a preferable joint. We observe it practised by the most eminent carpenters for all oblique thrusts ; but it surely employs less of the cohesion of the tie-beam than might be used without weakening it, at least when it is supported on the other side by the wall-plate. Fig. 7, No. 3, is also much practised by the best carpenters. Fig. 9, No. 1, is pro posed by Mr Nicholson as preferable to fig. 7, No. 3,