Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/531

Rh BUILDING UP BEAMS.] should not be used for floors, as they are softer ; and Swedish deals are bad for framing, as they warp. For interior finishings generally, Baltic red and white woods, and the American red and yellow pine, are to be preferred. We must first proceed to consider the means by which form in the work of the carpenter is to be secured, and the co .mections by which the various strains are excited and communicated. The following practical remarks on various joints are abridged from the article by Prof. Robison in the former editions of this work. .jints. The joinings practised in carpentry are almost infinitely various, and each lias advantages which make it preferable in some circumstances. Many varieties are employed merely to please the eye. We do not concern ourselves with these ; nor shall we consider those which are only employed in connecting small works, and can never appear on a great .scale ; yet even in some of these, the skill of the carpenter may be discovered by his choice ; for in all cases it is wise to make every, even the smallest, part of his work as strong as the materials will admit. He will be particularly attentive to the changes which will necessarily happen by the shrinking of timber as it dries, will consider what dimensions of his framings will be affected by this and what will net, and will then dispose the pieces which are less essential to the strength of the whole in such a manner that their tendency to shrink shall be in the same direction with the shrinking of the whole framing. If he do other wise, the seams will widen, and parts will be split asunder. He will dispose his boardings in such a manner as to contribute to the stiffness of the whole, avoiding at the same time the giving them positions which will produce lateral strains on truss beams which bear great pressures ; recollecting, that although a single board has little force, yet many united have a great deal, and may frequently perform the office of very powerful struts. Our limits confine us to the joinings which are most essential for connecting the parts of a single piece of a frame (when it cannot be formed of one beam, either for want of the necessary thickness or length), and the joints for con necting the different sides of a trussed frame. lilding Much ingenuity has been bestowed on the manner of &amp;gt; beams, building up a great beam of many thicknesses, and many singular methods are practised ; but when we consider the manner in which the cohesion of the fibres performs its office, we see that the simplest are formed on the same principles as the most refined, and they are less apt to induce false notions of the strength of the assemblage. Thus, were it required to build up a beam for a great lever or a girder, so that it may act nearly as a beam of the same size of one log, it may either be done by plain joggling, as in Plate XXII. fig. 1, A, or by scarfing, as in B or C. If it is to act as a lever, having the gudgeon on the lower side at C, we believe that most artists will prefer the form B and C. We may fre quently gain a considerable accession of strength by this building up of a beam, especially if the part which is stretched by the strain be of oak, and the other part be fir. Fir being so much superior to oak as a pillar, and oak so much preferable as a tie, this construction seems to unite both advantages. But much better methods of making powerful levers, girders. &c., are obtained by trussing. Observe that the efficacy of both methods depends entirely on the difficulty of causing the piece between the cross joints to slide along the timber to which it adheres. Therefore, if this be moderate, it is wrong to make the notches deep ; for as soon as they are so deep that their ends have a force sufficient to push the slice along the line of junction, nothing is gained by making them deeper ; and this requires a greaterexpenditure of timber. Scarfings of beams are frequently made oblique, as in Plate XXII. fig. 2; but this seems a bad practice. It 477 begins to yield at a point where the wood is crippled and splintered off, or at least bruised out a little. As the pressure increases, this part, by squeezing broader, causes the solid parts to rise a little upwards, and gives them some tendency, not only to push their antagonists along the base, but even to tear them up a little. For similar reasons we disapprove of the favourite practice of many artists to make the angles of their scarfings acute, as in fig. 3. This often causes the two pieces to tear each other up. The abutments should always be perpendicular to the directions of the pressures. This law is also to be extended to the abutments of different pieces of a frame, and the artist must even attend to the shrinking of the timbers by drying. When two timbers abut obliquely, the joint should be most full at the obtxise angle of the end ; because, by drying, that angle grows more obtuse, and the beam would then be in danger of splintering off at the acute angle. It is evident that the nicest work is indispensably Wedging, necessary in building up a beam. The parts must abut on each other completely, and the smallest play or void takes away the whole efficacy. It is usual to give the abutting joints a small taper to one side of the beam, so that they may require moderate blows of a maul to force them in, and the joints may be perfectly close when the external surfaces are even on each side of the beam. T!ut we must not exceed in the least degree, for a very taper wedge has great force ; and if the pieces be driven together by very heavy blows, the whole is left in a state of violent strain, and the abutments are perhaps ready to splinter off by a small addition of pressure. It is not unusual to leave some abutments open enough to admit a thin wedge reach ing through the beam. Nor is this a bad practice, if the wedge is of material which is not compressed by the driving or the strain of service. Iron would be preferable for this purpose, and for the joggles, were it not that, by its too great hardness, it cripples the fibres of timber to some distance. In consequence of this it often happens, that in beams which are subjected to desultory and sudden strains (as in the levers of reciprocating engines), the joggles or wedges widen the holes, and work themselves loose ; therefore skilful engineers never admit them, and indeed admit as few bolts as possible, for the same reason ; but when resisting a steady or dead pull, they are not so improper, and are frequently used. Beams are built up, not only to increase their dimensions in the direction of the strain (which we have hitherto called their depth), but also to increase their breadth, or the dimensions perpendicular to the strain. Sometimes the breadth of girder is doubled, if it is thought too weak for its load, and when the thickness of the flooring must not be increased. The mast of a great ship of war must be made bigger Masts and athwartship, as well as fore and aft. This is one of the uprights, nicest problems of the art; and professional men are by no means agreed in their opinions in regard to it. We shall content ourselves here with exhibiting the different methods. The most obvious and natural method is that shown in Plate XXII. fig. 4. It is plain that (inde pendent of the connection of cross bolts, which are used in them all when the beams are square) the piece C cannot bend in the direction of the plane of the figure without bending the piece D along with it. This method is much used in the French navy ; but it is undoubtedly imperfect. Fig. 5 exhibits another method. The two halves of the beam are tabled into each other in the same manner as in fig. 1. It is plain that this will not be affected by any unequal swelling or shrinking, because this is insensible in the direction of the fibres ; but when bent in the direction a b, the beam fig. 4 is weaker than bent in the direction