Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/532

Rh 478 BUILDING [CARPENTRY. cf. Each half of fig. 4 has, in every part of its length, a thickness greater than half the thickness of the beam. It is the contrary in the alternate portions of the halves of fig. 5. When one of them is bent in the direction AB, it is plain that it drags the other with it by means of the cross butmeuts of its tables, and there can be no longi tudinal sliding. But unless the work is accurately executed, and each hollow completely filled up by the table of the other piece, there will be a lateral slide along the cross joints sufficient to compensate for the curvature ; and this will hinder the one from compressing or stretching the other in conformity to this curvature. The imperfection of this method is so obvious that it has seldom been practised ; but it has been combined with the other, as is represented in fig. 6, where the beams are divided along the middle, and. the tables in each half are alternate, and alternate also with the tables of the other half. Thus 1, 3, 4 are prominent, and 5, 2, 6 are depressed. This construction evidently puts a stop to both slides, and obliges every part of both pieces to move together, a b and c d show sections of the built-up beam corresponding to AB and CD. No more is intended in this practice by any intelligent artist, than the causing the two pieces to act together in all their parts, although the strains may be unequally distributed on them. Thus, in a built-up girder, the binding joists are frequently mortised into very different parts of the two sides. But many seem to aim at making the beam stronger than if it were of one piece ; and this inconsiderate project has given rise to many whimsical modes of tabling and scarfing. The practice in the British dockyards is somewhat different from any of these methods. The pieces are tabled as in fig. 6, but the tables are not thin parallelepipeds, but thin prisms. The two outward joints or visible seams are straight lines, and the table 1 rises gradually to its greatest thickness in the axis. In like manner, the hollow 5, for receiving the opposite table, sinks gradually from the edge to its greatest depth in the axis. Plate XXII., fig. 7, No. 1, represents a section of a round piece of timber built up in this way, where the full line EF, GH is the section corresponding to AB of fig. G, and the dotted line EG, FH is the section corresponding to CD. This construction, by making the external seam straight, leaves no lodgment for water, and looks much fairer to the eye ; but it appears to us that it does not give so firm a hold when the mast is bent in the direction EH. The exterior parts are most stretched and most compressed by this bending ; but there is hardly any abutment in the exterior parts of these tables. In the very axis, where the abutment is the firmest, there is little or no difference of extension and compression. But this construction has an advantage, which, we imagine, much more, than compensates for these imperfections, at least in the case of a round mast ; it will draw together by hooping incomparably better than any of the others. Joggles of elm are sometimes used in the middle of the large tables of masts ; and when sunk into the firm wood near the surface, they must contribute much to the strength. But it is very necessary to employ wood not much harder than the pine, otherwise it will soon enlarge its bed and become loose, for the timber of these large trunks is very soft. Scarfing. The most general reason for piecing a beam is to increase its length, This is frequently necessary, in order to procure tie-beams for very wide roofs. Two pieces must be scarfed together. Numberless are the modes of doing this, and almost every master carpenter has his favourite nostrum. Some of them are very ingenious ; but here, as in other cases, the most simple are commonly the strongest. We do not imagine that any, the most ingenious, is equally strong with a tic consisting of two pieces of the same scantling laid over each other for a certain length, and firmly bolted together. We acknowledge that this will appear an artless and clumsy tie-beam, but it will be stronger than any that is more artificially made up of the same thickness of timber. The next simplest and most obvious scarfing is that represented in Plate XXII., fig. 8, Nos. 1 and 2. If considered merely as two pieces of wood joined, it is plain that, as a tie, it has but half the strength of an entire piece, supposing that the bolts (which are the only connections) are fast in their holes. No. 2 requires a bolt in the middle of the scarf to give it that strength, and in every other part is weaker on one side or the other. If the bolts were sufficiently numerous and sufficiently firm, so as to produce a great degree of adhesion or of friction between the parts, this joint might be made almost as strong as the entire beam, since there is nothing to prevent the co-operation of each side with the other throughout its extent ; but much of the strength would be lost if the bolts became loose, even in an inconsiderable degree. But the bolts are very apt to bend by the violent strain, and require to be strengthened by uniting their ends by iron plates, in which case it is no longer a wooden tie. The form of No. 1 is better adapted to the office of a pillar than No. 2, especially if its ends be formed in the manner shown in the elevation No. 3. By the sally given to the ends, the scarf resists an effort to bend it in that direction. Besides, the form of No. 2 is unsuitable for a post, because the pieces by sliding on each other by the pressure are apt to splinter off the tongue which confines their extremity. Figs. 9 and 10 exhibit the most approved form of a scarf, whether for a tie or for a post. The key represented in the middle is not essentially necessary ; the two pieces might simply meet square there. This form, without a key, needs no bolts (although they strengthen it greatly), but, if worked very true and close, and with square abutments, will hold together, and will resist bending in any direction. But the key is a very great improvement, and will force the parts together with perfect tightness, but it must not be over driven. The form of fig. 9 is by far the best (it is sometimes said to be tabulated, that is, to render the joints as close as possible, and the juncture more independent of any bolts which might be placed similarly to those in fig. 8, No. 1), be cause the triangle of fig. 1 is much more readily splintered off by the strain or by the key than the square wood of fig. 9. It is far preferable for a poet, for the reason given in speaking of fig. 8, No. 1 and No. 2. Both may be formed with a sally at the ends equal to the breadth of the key. In this shape fig. 9 is well suited for joining the parts of the long corner posts of spires and other wooden towers. Fig. 9, No. 2 differs from No. 1 only by having three keys ; the principle and the longitudinal strength are the same. The long scarf of No. 2, tightened by the three keys, enables it to resist bending much better. None of these scarfed tie-beams can have more than one third of the strength of an entire piece, unless with the assistance of iron plates ; for if the key be made thinner than one-third, it has less than one-third of the fibres to pull by We are confident, therefore, that when the heads of the bolts are connected by plates, the simple form of fig. 8, No. 1, is stronger than those more ingenious scarrings. It may be strengthened against lateral bending by a little tongue, or by a sally, but cannot have both. The strongest of all methods of piecing a tie-beam would Fish be to set the parts end to end, and grasp them between t)ean other pieces on each side, as in Plate XXIII., fig. 1. This the ship-carpenter calls fishing a beam ; it is a frequent prac tice for occasional repairs. Perronet used it for the tie-beams or stretchers by which he connected the opposite feet of