Of the Cohesive Powers of Wood resisting a force acting Perpendicular to the Direction of the Fibre.
On this subject very little new matter can be col lected to add to that given under the article carpen try ; for although numerous experiments have been made since that article was written, on most of the other strains to which materials are exposed, this has been almost wholly neglected. We know of nothing that has been done, except two or three experiments by Mr. Tredgold, and one by Mr. Barlow, which lat ter, however, was on the lateral adhesion of the fibres, but those of the former were made on their trans verse strength ; these are as below.
Kind of Wood. Resistance on a square inch.
Oak 2316 lbs.
Poplar 1782 lbs. Tredgold.
Larch from 979 to 1700.
Lateral ad- 2 • 600 Barlow.
Lesion of If we allowed ourselves to form any general esti mate from these few experiments, we might say that the resistance to the strain across the fibres is about double that of the direct cohesion, but that the lateral adhesion is only about one half of the direct strength. We cannot, however, state this as correct in all cases, nor even as approximative; a few experiments on this subject would be highly interesting.
On the Resistance of Timber and other Materials to a Transverse Strain.
We have entered at some length upon this subject, under the article CARPENTRY, having there examined the result of all the most common strains of this kind to which materials may be exposed ; but as in the preceding cases we had not then the means of furnish ing satisfactory experimental data for computing the amount of the resistance, the dimensions of the tim ber being given, or for computing the dimensions re quisite to resist any proposed strain or load, we shall therefore principally confine ourselves in this place to laying before our readers such experimental results, as will enable them to submit the principles laid down in the article quoted to practical cases, with such few additional rules as may appear necessary.
It is shown, article CARPENTRY, p. 502, that the
strength of a rectangular beam to resist a transverse strain varies directly as the breadth and square of the depth, and inversely as the length ; consequently, if from a series of well-conducted experiments we have been enabled to determine the breaking weight on a piece of timber, of any given species and of given di mensions, we may thence compute the weight neces sary to break a piece of timber of the same kind, load ed in a similar way, and of any dimensions whatever.
It has also been demonstrated in the same article, that the strains upon a beam fixed at one end in a wall, and loaded at the other, is four times greater than when the same weight is hung upon the middle of the same beam, and the latter supported at its two ex tremities.
It has also been shown experimentally that when a beam is fixed at both its extremities in a wall, and loaded in the middle, its strength is to that, when only supported at its two ends, as 3 to 2.
And lastly, that when a weight is uniformly distri buted over a beam, its mechanical action fracture is only one half of what it is when collected in the middle.
It follows, therefore, that from a series of experi ments made on the resisting power of timber or other material, in any one of these cases, the resistance in any other may be found, or rather perhaps we ought to say, that the resistance in all of them is the same, it being merely the strain that is altered by the differ ent modes of fixing and loading. Let 1 = the length, b the breadth, and d the depth of any rectangular piece of timber,.all in inches, and 'W the weight in pounds, requisite to break it. Let also S be the weight requisite to break a piece of similar timber, whose length, breadth, and depth, are each one inch, then form our first bd. 11V As : 1 : W: 1 bd.
Which S will be a constant number of reference for computing the strength of any piece of timber of the same kind, under all variety of dimensions and modes of fixing and loading.