TIMBER-LINETIMBUKTU In America, Professor J. B. Johnson made a large number of tests for the Forest Department of the Board of Agriculture of the United States between 1891 and 1895. More than 300 trees were cut down and experimented with, the species under test embracing ten different kinds of pine and five different varieties of hard-wood trees. Records were made as to the nature of the soil and climate where the trees were grown; their conditions of growth, their age and size, and the season of felling. As in the tests made by Bauschinger, the percentage of moisture contained in the wood was very carefully observed, and it was found that this amount of moisture has a very great influence upon the resisting power of the wood, the strength increasing with the dryness of the material up to 3 or 4% of moisture, at which point the greatest strength of the wood is reached . Wood in such a dry condition , however, is never found in actual practice, timber in an ordinary well-warmed and well- ventilated situation probably containing at least 10%.
One general conclusion arrived at both by Bauschinger and Johnson was that the strength is much affected by the specific gravity of the timber. In all cases the strength increases proportionately with the density of the wood. A most complete series of tests upon the physical characteristics of the hard woods of Western Australia was completed for the government of Western Australia by G. A. Julius in 1907. This work was carried out in a most thorough manner, and as many as 16,000 tests were made, the conditions of test being based upon those laid down by Johnson. The results serve to show the great value of Australian timbers, and the comparisons made with the typical timbers of many other countries emphasize the fact that the Australian woods are equal to any in the world for hardness, strength and durability.
For use under special conditions a wood suited to the particular requirements must be selected. The following is a list of the best timbers for different situations: for general construction, spruce and pine of the different varieties; for heavy constructions, pitch pine, oak (preferably of English growth), teak, jarrah; for constructions immersed in water, Baltic pine, elm, oak, teak, jarrah; for very dry situations, spruce, pines, mahogany, teak, birch, sycamore.
There are no regulations in England limiting the working stresses that may safely be placed upon timber, although in some districts the least sizes that may be used for timbers in roofs and floors are specified. In some European and other countries, however, the safe working stresses of timber used for constructional purposes are defined. The building by-laws of the municipality of Johannesburg, in South Africa, contain the following table:
Safe Working Stresses for Timber. In tons per square inch.
Bending. Extreme Fibre Stress.
Timber .... Fir and Pine . Hardwood Note. -The compression stresses are for short struts and columns where the length does not exceed for timber 15 times the least transverse dimension, and where the ends are fixed. Where the ratio of the length to the least transverse dimension is higher, the load per square inch shall be proportionately reduced. No post of timber shall exceed in length 30 times its least transverse dimension.
REFERENCES. T. Tredgold, Principles of Carpentry, xii. ; R. E. Grandy, Timber Importer's Guide; G. A. Julius, Report of a Series of Tests upon the Physical Characteristics of the Hardwoods of Western Australia (1906-1907); J. B. Johnson, Report of Tests upon Timber made for the Forest Department of the Board of Agriculture of the United States (1891-1895); J. Bauschinger, " Report of Tests made upon Timber at Munich, Mittheilungen aus dem Mechanisch-Technischen Laboratorium der K. Technischen Hochschule in Munchen ; F. E. Kidder, Building Construction and Superintendence, vol. ii. ; Rivington, Notes on Building Construction, vol. iii. ; T. Laslett, Timber and Timber Trees; H. Stone, The Timbers of Commerce and their Identification; H. M. Ward, Timber and some of its Diseases; R. Hartig, Timbers and How to know them; J. Brown, The Forester; G. S. Boulger, Wood.
J. Broi CJ.BT.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)