Popular Science Monthly/Volume 2/March 1873/The Strength of Timber
|THE STRENGTH OF TIMBER.|||
ALTHOUGH it is of less importance to investigate the strength of timber at the present time than it was formerly, in consequence of the diminished use of that material in permanent structures, and the more general employment of iron, still it will always be a very valuable material for certain purposes, and ought not to be neglected. Timber is variously used, even now, in permanent works, and is applied much more extensively in temporary structures—such as centerings and scaffolding. Hence its properties are well worthy of careful attention; and the student should be familiar, not only with the external appearance of the principal kinds of wood, but also with their relative strength, stiffness, toughness, and durability.
One of the most obvious inferences to be drawn from the experiments previously recorded is, that very wide variations exist in the strength and other elastic properties of different metals, and even of different specimens of the same metal. If we could investigate the properties of timber with the same care which has been bestowed on the metals, we should find that there is an even greater variation in the properties of different kinds of wood. This arises, in part, from the fact that timber is much affected by a number of external and internal conditions, during its growth and seasoning, and in its subsequent treatment, which gradually modify and change its properties.
It will only be necessary here to speak of the powers of resistance of a few among the many kinds of wood now employed in the mechanical arts. The greater number of the varieties of wood owe their commercial value to special characteristics, such as beauty of grain and capability of being polished—the description of which does not fall within the scope of the present article.
As a general rule, we may judge of the hardness of a wood by its specific gravity, if it is in its natural state. But the density may be increased by artificial compression, and this increase of density is generally accompanied by increase of strength. Some varieties of wood, as, for instance, lignumvitæ, are so dense that they sink in water, while some of the softer woods have not half the density of that fluid. The presence of gum or resin in any wood adds both to its strength and durability. Many woods will last a long time if kept constantly under water, but scarcely any wood is very durable when allowed to become wet and dry alternately.
The strength of a piece of timber depends upon the part of the tree from which it is taken. Up to a certain age, the heart of the tree is the best; after that period, it begins to fail gradually. The worst part of a tree is the sap-wood, which is next the bark. It is softer than the other parts of the wood, and is liable to premature decay. The deleterious component of the sap-wood is absorbed, if the tree is allowed to grow for a longer period, and in time the old sap-wood becomes proper timber-fibre similar to heart-wood. Hence, the goodness of a tree, for timber purposes, depends on the age at which the tree was cut down. When young, the heart-wood is the best; at maturity, with the exception of the sap-wood, the trunk is equally good throughout; and, when the tree is allowed to grow too long, the heart-wood is the first to show symptoms of weakness, and deteriorates gradually.
The best timber is secured by felling the tree at the age of maturity, which depends on its nature as well as on the soil and climate. The ash, beech, elm, and fir, are generally considered at their best when of 70 or 80 years' growth, and the oak is seldom at its best in less time than 100 years, but much depends on surrounding circumstances. As a rule, trees should not be cut before arriving at maturity, because there is then too much sap-wood, and the durability of the timber is much inferior to that of trees felled after they have arrived at their full development.
The strength of many woods is nearly doubled by the process of seasoning, hence it is very thriftless to use timber in a green state, as it is not only weak, but is exposed to continual change of bulk, form, and stability. After timber is cut, and before it is properly seasoned, the outside is found to crack and to split more than the inside of the mass, because it is more exposed to the desiccating effect of the surrounding atmosphere, but, as the outside dries, the air gradually finds its way to the interior. If timber is cut up by the saw when green, and allowed to season or dry in a gradual manner, it is found to be the most durable. In the arts, however, artificial drying is often resorted to, as in the case of gun-stocks. These are put into a desiccating chamber, where a current of air at 90° or 100° is passed over them, at such a rate as to change the whole volume of air in the chamber every three minutes, and it is found that a year of seasoning may thus be saved. The walnut-wood is as good, after this process, as if the seasoning had been accomplished by time and exposure, and works more smoothly under the cutting instruments of the stock-machinery.
Wood will always warp after a fresh surface has been exposed, and will likewise change its form by the presence of any moisture, either from that contained in the atmosphere or from wetting the surface. The effect of moisture on dry wood is to cause the tubular fibres to swell; hence it is that, if a plank or board is wetted upon one side, the fibres there will be distended, and the plank, in consequence, must bend.
The natural law that governs the shrinking or contraction of timber is most important to practical men, but it is too often overlooked.
The amount of the shrinkage of timber in length, when seasoning, is so inconsiderable that it may in practice be disregarded. But the shrinkage in transverse directions is much greater, and presents some peculiarities which can only be explained by examining the structure of the wood, as resulting from its mode of growth. An examination of the end section of any exogenous tree, such as the beech or oak, will show the general arrangement of its structure. It consists of a mass of longitudinal fibrous tubes, arranged in irregular circles, which are bound together by means of radial plates or rays, which have been variously named: they are the "silver grain" of the carpenter, or the "medullary rays" of the botanist, and are in reality the same in their nature as the pith. The radial direction of these plates or rays, and the longitudinal disposition of the woody fibre, must be considered in order to understand the action of seasoning. For the lateral contraction or collapsing of the longitudinal fibrous or tubular part of the structure cannot take place without first tearing the medullary rays, hence the shrinking of the woody bundles finds relief by splitting the timber in radial lines from the centre parallel with the medullary rays, thereby enabling the tree to maintain its full diameter. If the entire mass of tubular fibre composing the tree were to contract bodily, then the medullary rays would, of necessity, have to be crushed in the radial direction to enable it to take place, and the timber would thus be as much injured in proportion as would be the case in crushing the wood in a longitudinal direction.
If an oak or beech tree is cut into four quarters, by passing the saw twice through the centre at right angles, before the splitting and contracting have commenced, the lines a c and b c in Fig. 1 would be of the same length, and at right angles to each other, or, in the technical language of the workshop, they would be square; but, after being stored in a dry place, say for a year, a great change will be found to have taken place, both in the form and in some of the dimensions. The lines a c and b c will still be of the same length as before, but from a to b the wood will have contracted very considerably, and the two lines a c and b c will not be at right angles to each other, the angle being diminished by the portion shown in black in Fig. 1. The medullary rays are thus brought closer by the collapsing of the vertical fibres.
But, supposing that six parallel saw-cuts are passed through the tree, so as to form it into seven planks, what will be the behavior of the several planks? Consider the centre plank first. After due seasoning and contracting, it will be found that the middle of the board still retains the original thickness, from the resistance of the medullary rays, while the thickness will be gradually reduced toward the edges for want of support, and the entire breadth of the plank will be the
same as it was at first for the foregoing reasons, and as shown in Fig. 2. Then, taking the planks at each edge of the centre, by the same law their change and behavior will be quite different: they will still retain their original thickness at the centre, but will be a little reduced on each edge throughout, but the side next to the heart of the tree will be pulled round or bent convex, while the outside will be the reverse, or hollow, and the plank will be considerably narrower throughout its entire length, more especially on the surface of the hollow side. Selecting the next two planks, they will be found to have lost none of their thickness at the centre, and very little of their thickness at the edges, but very much of their breadth as planks, and will be curved round on the heart-side and made hollow on the outside. Supposing some of these planks to be cut up into square prisms when in the green state, the shape that these prisms will assume after a period of seasoning will entirely depend on the part of the tree to which they belong, the greatest alteration would be perpendicular to the medullary
|Fig. 3.||Fig. 4.|
rays. Thus, if the square was originally near the outside, as seen in Fig. 3, then the effect will be as shown in Fig. 4, namely, contraction in the direction from a to b. After a year or two the square end of the prism will become rhomboidal, the distance between c and d being nearly the same as at first, but the other two edges brought closer together by the amount of their contraction. By understanding this natural law, it is comparatively easy to predict the future behavior
of a board or plank by carefully examining the end-wood, in order to ascertain the part of the log from which it has been cut, as the angle of the ring-growths and the medullary rays will show this, as in Figs. 5 and 6. If a plank has the appearance of the former, it must have been cut from the outside, and for many years it will gradually shrink in the breadth; while the next plank, shown in Fig. 6, must have been derived from near the centre or heart of the tree, and it will not shrink in the breadth but in thickness, with the full dimension in the middle, but tapering to the edges.
The foregoing remarks apply more especially to the stronger exogenous woods, such as beech, oak, and the stronger firs. The softer woods, such as yellow Canadian pine, are governed by the same law; but, in virtue of their softness, another law comes into force, which to some degree affects their behavior, as the contracting power of the tubular wood has sufficient strength to crush the softer medullary rays to some extent, and hence the primary law is so far modified. But even with the softer woods, such as are commonly used in the construction of houses, if the law is carefully observed, the greater part of the evils of shrinking would be obviated. Hence, also, it is that when a round block, as a mast, is formed out of a tree, it retains its roundness because it contracts uniformly or nearly so, whereas, if a round spar is formed out of a quartering of the same tree it will become an oval, or otherwise contorted toward that shape.
It would, not be in accordance with the object to enumerate all the woods that are employed in the arts, therefore a few only are selected, or such as are employed for purposes where strength is the primary object, viz., ash, beech, elm, fir, hornbeam, mahogany, oak, and teak.
Ash is a coarse wood, but possessed of considerable strength, and is distinguished for its great toughness and elasticity, and is usually employed where severe shocks and wrenches have to be encountered, such as for agricultural implements, the felloes and spokes of wheels, and the shafts of carriages, for hammer-shafts, and for spring purposes generally wherever wood is employed for that purpose.
From its great flexibility it is seldom employed where rigidity is a desideratum. The combination of strength with flexibility is the characteristic of ash, and when the wood is from a young tree, or a tree not too old, it is an invaluable wood in many respects; but as the tree becomes older, the change to brittleness sets in and soon renders it less valuable. It is also remarkable for its endurance when kept dry, but when exposed to damp or to wet it rapidly decays. The numerical value of its properties varies considerably, but in general terms it may be stated that, as compared with oak, good ash has frequently a still greater tenacity and likewise a greater degree of toughness, but, from its flexibility, especially when young, it has considerably less stiffness, which unfits it for many purposes.
Beech has frequently considerable strength, and is chiefly distinguished for its uniformity, its smoothness of surface, and closeness of grain. It likewise possesses no little beauty, and takes a good polish, more especially when its silver grain is skilfully exposed. When well seasoned and not too old, it is frequently used for the cogs of mill-gearing gearing, and is usually considered by millwrights as next to hornbeam, both in strength, toughness, and general suitability for that purpose. It requires, however, to be kept very dry, for in damp situations it quickly wears out, but, when beech is immersed in water constantly, its endurance is considerable. The strength of beech is nearly the same as that of oak; it is also tougher, but its stiffness is inferior to that of oak, even to the extent of 25 per cent.
Elm, although a cross-grained, rough wood, and mostly used for rough purposes, is yet held in great estimation for its toughness and non-liability to split by the driving of bolts. It is much used in the construction of blocks for pulley-tackle, for heavy naval gun-carriages, and for the naves of carriage-wheels. It is a wood which is little affected by constant immersion in water, but decays rapidly when alternately wet and dry, and consequently is not very durable for purposes involving exposure to a wet climate. Its chief defect in ordinary use is its great liability to warp, and twist, and get out of form; and, as regards strength, toughness, and rigidity, it is inferior to oak, as well as in almost every other respect.
The fir and pine woods are members of a large family, and are of great variety, and differ much in most of their properties. These classes of timber, in addition to being employed for building purposes, are likewise the chief materials that are used in great works, where the question of strength combined with cost becomes the most prominent consideration. The most durable varieties are the larch, the pitch-pine, and the firs, from Memel and Norway, and are valued mostly on account of the large quantity of resin, pitch, and turpentine, which they contain. The Canadian pine, variously termed white or yellow, is not a strong wood, but is much used by engineers for making patterns or models, on account of its smoothness of surface, its non-liability to warp, its comparative freedom from knots, and the facility with which it can be cut. The white or yellow pine is not nearly so strong or so stiff as oak, yet sometimes it is almost equal to it in its tenacity and toughness. In such a large family as that of the resinous firs and pines, there is almost an equal variation in their strength, toughness, and rigidity.
Hornbeam is a wood which is comparatively little used, except by engineers, for the teeth or cogs of wheels, and for mallets, for which purposes it is perhaps superior to all other woods, and this is mostly due to its great toughness and remarkably stringy coherence of fibre. Its cohesive strength and other properties depend much upon its age, as a plank, and still more on the age of the tree from which the plank was taken. When in the most favorable condition, it is fully equal to the average of oak (even when considered merely as a wood), but when cut from older trees, and when over-seasoned, it is frequently found worthless, and has soon to be renewed. When of proper age and quality, it has no equal for its own special purposes.
Mahogany is a beautiful, close-grained wood, but is used not so much on account of its strength, but more frequently because of its non-liability to shrink, warp, or twist, and from the peculiar property of taking a firm hold of glue. In the last respect it is superior to any other wood. Mahogany differs greatly in regard to its closeness, hardness, strength, and beauty. That from Honduras, called "bay-wood," is much inferior to that called "Spanish" mahogany, which comes from the West Indies; the former is much used in the construction of light textile machinery, but chiefly on account of its cheapness; and the latter is used for furniture or for other ornamental purposes. As regards strength, this wood is inferior to oak in all respects, and its great characteristic defect is unsuitability for exposure to the weather, or, indeed, for any purpose where it is made alternately wet and dry. When so subjected, it rapidly decays, and loses all its good qualities.
Oak, taken as a whole, is one of the strongest and most durable of woods, and is especially adapted for exposure to the weather of a damp climate, and is indeed suitable for almost every purpose where the properties of strength, stiffness, and toughness, combined with endurance, are required. Its value for ship-building is proverbial, and in its employment for the staves of casks, for tree nails, for carriage-wheels, and for all such purposes requiring lightness and strength in combination, it is equally useful. From time immemorial it was esteemed the best timber for heavy roofs, and the condition in which some of these grand old roofs have reached our era fully attests the wisdom of the selection.
Oak is found of many degrees of quality, but probably none, taking every property into account, is superior to that which grows in England, and which is perhaps more durable than any other. Some of the foreign oaks are as good in some respects, but, as a whole, English is the best.
- From "Strength of Materials and Structures."D. Appleton & Co.