CHAPTER XVII. CONSTRUCTIONAL CARPENTRY. CARPENTRY is the art of constructing works in timber. The art, of course, more or less belongs to the carpenter and builder, but the engineer's services are required when works of larger magnitude than dwelling-houses are concerned, and it is in connection with such works that the general principles of carpentry will now be discussed. The engineer should understand the principles of design, the most common joints and fastenings, and the most economical and safe use of the various scantlings employed. The heavy carpentry work to be described specially concerns a civil engineer in municipal practice. It includes 1. Large timber roofs. 2. Arch centerings. 3. Shoring and Strutting. 4. Timber Bridges. 5. Piles and Pile driving (before described fully). 6. Timbering of Trenches, The first thing to do is to investigate the properties of good timber. These are set forth in Chapter XIII. on Materials of Construction, and will not be reverted to here. The first thing to consider are theoretical points of design in general, and particularly the joints and fastenings employed on large timber structures. First of all, we have the common fish joint (figs. 318 and 319). The two ends to be joined abut against each other, and on each side are plates of wood (or preferably wrought iron), with bolts passing through them. This joint is economical both of labour and material, and is much used in temporary structures. In fact, it may be said that in Britain any works of carpentry which an engineer will be called upon to design will probably be of a temporary nature, such as a bridge to take traffic during the rebuilding of an existing one, temporary bridges to take light railways used on large. works such as reservoirs, and gantries for cranes on big masonry works like reservoir dans. ¡ya Square FIG. 319. + Timber is more or less liable to shrink, and such shrinkage will cause much stress on the bolts. To avoid this, hard-wood keys are often inserted as well as the bolts, as shown in fig. 320. Folding wedges are also When designing, always make the sectional area of the fish pieces equal to that of the tie, and provide bolts having a combined total sectional area of the sectional area of the timber after the boltholes have been cut. used for the same purpose. The second variety of neat and effective joint, are shown in fig. 321. joint is what is called a scarfed joint. It is both a and the usual proportions in terms of the depth bE is what we call the detrusion area = 7 ab, while bc = 4 <--7 ab. --→ 5/14 14 D FIG. 321. D. 14 is the compressile area D, while ab and cd are the tensile areas = The failure of these joints shows under what particular stress the joint has failed. Fig. 322 shows the method of each other; they are fastened by 22x Steel Straps Bolts and fish plates may also be used. They may also be assisted by oak wedges. The duty of such wedges is to provide for shear stress. In a long span we have to build up the beams. procedure. Two beams are placed over iron straps, and have hard-wood keys as shown. A more elaborate method is shown by fig. 323. It is very costly, however. Another joint not so common in engineering works is that called the tusk tenon. It would be employed for such purposes as fitting the cross joists to a timber bridge. It is shown dimensioned in terms of the depth in fig. 324. 2×2 Oak Keys --12' Now it is essential that the joints in all beams should be parallel to the load upon them (viz. usually vertical). The tenon which we have just described fits into the other member by means of a slot called a mortise. These mortises are best made by hand, but are now largely cut by machinery, and are usually known by the ends being as in fig. 325 instead of exactly square. Another joint sometimes used on large beams is the splayed scarf. It is tightened up by means of folding wedges, as shown by fig. 326. As regards strengthening wood beams, what is termed a flitch beam was described fully, with calculations, under Beams. There were also discussed in the same chapter some calculations regarding trussed wood beams, and we treated them graphically in "Graphic Statics." The object of trussing a beam is to increase the bearing power of any particular section by providing iron or steel stay rods to take up part of the tension while the timber itself takes up the compression, the rods in turn being assisted by cast-iron struts. out of 9"x9 30' 0' FIG. 323. Their use is frequently resorted to in structures such as light road bridges and temporary work, but the practice has fallen into disfavour lately, partly owing to steel sections being so cheap, the unsatisfactory manner in which blacksmiths carry out the forg ings, and finally to the unequal expansion set up between the timber and steel. A good example of a trussed wood beam occurs in figs. 327, 328, and 329. It would be suitable for a span of 30 ft., and is calculated as described under Beams. A fuller example will appear later on. We now have to consider struts and pillars of timber when supporting beams. Regarding the joints between beam and pillar, called the bearing joints, two good forms occur in figs. 330, 331. These are designed in accordance with the theory of pillars and struts, fully described in that chapter, and are such that they more or less fix the pillar at the top end, and FIG. 325.-Machine-cut Mortise. "Boll plate FIG. 326. Plate. It is consequently make it much stronger, especially that shown in fig. 331. for such a reason that these bearing joints must never be made as in fig. 332, although the author has seen ignorant carpenters do so. For temporary works the halved joint shown by fig. 333 is very satisfactory, but the double-halved joint shown in fig. 334 is more so, only it requires very careful fitting. When a post has suitable bracing, a common fished joint may be used, but when no such bracing is provided, fish plates must be provided on all four sides. The splayed scarf, fig. 326, can also be used for posts. In most heavy work, however, some form of bracing at the top of posts is provided for by means of struts. Two good forms are shown in figs. 335 and 336. To find the required scantlings would only require a very simple stress diagram. We refer the reader to a previous chapter. In figs. 335 and 336, however, care must be taken that the joints shown in A are sufficiently set back from the ends of the beam, so as to prevent failure by detrusion, viz. "Bars C.I. Struts about 30' FIG. 327. shearing off of the shaded area. The same conditions apply to joints at the feet of principal rafters and tie rods in roof trusses, and between the struts on king and queen posts. A more satisfactory way of making such joints, where practicable, is as shown in figs. 337 and 338. For fastening timbers, instead 1/2" dia Y/" dia. FIG. 328. FIG. 329. of nails, dogs, spikes, or bolts, we often use what are termed treenails. They should be made of best oak, having an ultimate resistance across the grain of 4000 lbs. per square inch. Their diameter should be of the thickness of the pieces joined. The use of iron or steel straps in carpentry is much in vogue; TTT FIG. 330. FIG. 331. FIG. 332. they are sometimes essential, but are only more or less satisfactory, because the timber, always liable to shrink, becomes loose in the straps. They are also liable to corrosion (especially in oak, due to the presence of gallic acid), and in all cases should undergo a process of preservation. A common and effective way is to heat the iron to the melting-point of lead (630° F.) and dip them in cold linseed oil. They may also be similarly dipped in coal-tar, or they may be galvanised. The latter process fails, however, in sea air, or near sulphur fumes. For some purposes it is necessary to bend timber slightly to an arched form. It is then called a knee. Its execution requires a special cramp. This cramp is shown in fig. 339. The strip of boiler plate goes right round. FIG. 333. "Bolt. Elevation. So→→→ Plan FIG. 334. When beams are built up of timber placed side by side, they are called builtup ribs. They may be considered as strong as a solid rib of the same depth, and of a thickness less one piece. When, however, the layers are laid flat, it becomes a laminated beam. The several pieces are called the lamina. They are weaker than solid ribs in the following way. A laminated beam of 5 ·Bolts Bolt Bolts FIG. 335. FIG. 336. laminæ is only as strong as a solid one of the same dimensions. In most timber structures the tenacity or resistance to direct crushing should not be more than 1000 lbs. per sq. in. This in practice gives a factor of safety of 10. Assume it to be required to construct a timber bridge to carry a roadway over a river, canal, or other natural obstacle. First of all we decide upon the span=S. Then say we place the beams at a distance B, centre to centre, and make the depth of beam = d. All measurement in feet. Then to find the thickness of the beams we have the equation T=. BS2 (225) |