Elementary Principles of Carpentry

Abstract

MR. TARN has for a good many years enjoyed a high reputation amongst the profession of architects as a writer upon the practical principles of building regarded mathematically. Tredgold's treatise on Carpentry has for a very long time indeed possessed the highest reputation as a much more than elementary book of reference upon that important department of building construction which deals with timber work; it has been republished time after time in the form of the old-fashioned substantial quarto which used to be in vogue before we were encouraged to expect to read as we run. It is quite in accordance with the fitness of things that Tredgold and Mr. Tarn should come together, and the English building world will scarcely require to be told that the result is satisfactory. The new edition before us is in fact a readaptation once more of the excellent material of the old standard treatise to the changing condition of our mechanical knowledge and skill. The author's mode of treating his subject has been retained intact; and we still have the well-known sections upon pressures, resistances, floors, roofs, domes, partitions, centers, bridges, joints, and timber. Whether this particular arrangement is the best, is a question scarcely worth asking, at least on behalf of the less fastidious criticism of those practical designers of carpentry who must here constitute the overwhelming majority of readers; but the editor has certainly not found it to be any bar to the importation of new matter in his own way. In one section he has introduced Prof. Clerk Maxwell's now universally appreciated system of diagrams of pressures, whereby the mere application of a common drawing scale to the component lines of easily constructed geometrical figures saves all further trouble and uncertainty in ascertaining the precise strains which the several members of a truss have to bear. In other sections the accepted formulæ of calculation, given only empirically by Tredgold, are mathematically demonstrated. The familiar tables of strength which supply the values of constants are “corrected” to accord with recent experiments more delicately and adroitly conducted, and several new tables of the kind are added. The consequent revision of Tredgold's “rules” and tables of scantlings has been thoroughly and carefully done; and various modes of more advanced construction are duly developed. That difficult subject, the theoretical thrust of domes—for in practice there ought to be none—is taken in hand mathematically, and a short chapter is added on stone vaulting. The important items of scaffolding, shoring, coffer dams, and so on, have been also introduced. The remarkable timber bridges of America—rough and ready science of the best—have been taken account of as they ought; and certain amendments which are made in respect of the plates serve in a reasonable measure to substitute new trussing for old. Lastly, the description of the nature and properties of timber is largely modified to meet the advanced knowledge of the day. With all this, the Tredgoldian character of the treatise is dutifully preserved; and so we may say it ought to be, for to modernise Tredgold too much would certainly not improve him. One of the prominent merits of the work consists in the unusually large number of illustrative plates, all to a useful scale. If these do not represent many of the more modern designs in timber work, they frequently offer examples to the student which are all the better in one respect—they exemplify that substantiality of construction which it is too much the tendency of scientific precision almost to discourage. It is a good maxim in carpentry as in most other departments of building, to make the structure not only strong but stronger than strong; and Tredgold always leans in that direction. The word economy is much employed amongst us; but, whereas its original and proper signification pointed only to skilful administration, its meaning with us is very much like mere parsimony. Waste of material is the bugbear of our builders, and almost still more of our architects. It need not be denied that mathematical science is in a certain way provocative of such parsimony; indeed, lightness of construction is regarded as an academical virtue in both architecture and engineering. But a moment's reflection ought to satisfy alike the most scientific and the least that true science is as much averse to parsimony of substance as common sense is. The strength of building materials can only be determined by extremely delicate experiments upon “breaking” strains, from the results of which the “safe” strains have to be deduced by estimate; and this, no doubt, becomes matter of opinion. The question is, what proportion of the breaking strain shall be recognised—almost arbitrarily—as the safe strain? With the single exception of iron, timber is the material with reference to which this matter of opinion is the most definitively settled. The reason is this:—The breaking strain must be instantaneously applied; this is essential to precision of tabulation. The safe strain is that proportion of this instantaneous breaking strain which the material will bear permanently without any risk of its elasticity being eventually overcome and a commencement made of that disturbance of the structure of the material which, once begun, increases in a geometrical ratio until the end is ruin. It is accepted, therefore, that the proportion of an ascertained instantaneous breaking strain which has to be recognised as the limit of a permanent safe strain is. one-third, one-fourth, and so on, according to the character of the material. What does this mean? It means that a greater strain than this proportion would in time, with one accidental circumstance and another, produce a commencement of instability. Perhaps it is to be regretted that this question is still disposed of so empirically as it is; we might at least in these days have express observations made and reduced to what system might appear. Tredgold's rules turn very much upon the manifestations of flexure; and this, of course, is not only another way of dealing with the matter, but one which affords at any rate a basis upon which mathematical formulas may be arrived at. On the whole, Tredgold is an old-fashioned writer, empirical and practical; but he is none the worse for that, perhaps all the setter. Mr. Tarn has accepted the duties and responsibilities of a scientifically, and we have pleasure in testifying that he does his work well, and that he does not overdo it.

Elementary Principles of Carpentry.

By Thomas Tredgold Sixth Edition, by E. Wyndham Tarn, M.A., Architect. (London: Crosby Lockwood and Co., 1885.)