A Treatise On The Principles And Practice Of Dock Engineering

15° DOCK ENGINEERING. imported in logs, 12 to 14 inches square, and the latter in scantlings of 9 inches by 3 inches and in lengths up to 16 feet. Oregon pine is noted for the great length and girth of its logs, but it is not a very strong or durable wood. Signs of decay have been observed in a very short time. It is, however, very useful for temporary shoring, and can be obtained from 30 to 130 feet in length with 12 to 30 inches quarter-girth. Hemlock, from the Southern States of South America, is in demand for deals and sleepers. TABLE XIII.—Weight and Strength of Timber. Timber. Weight in Lbs. per Cub. Et. Transverse Strength in Lbs. Timber. Weight in Lbs. per Cub. Ft. Transverse Strength in Lbs. Greenheart, . 62 to 75 900 to 1,500 Elm, 34 to 37 350 to 450 Mora, 57,, 68 1,100 ,, 1,250 Beecli, 43,, 53 550 „ 700 Purpleheart, . 62 Oak, 49 ,, 61 500 ,, 650 Bullet tree, 67 Pitchpine, 41,, 58 500 ,, 700 Kakaralli, 63 Birch, 45 ,, 49 550 „ 650 Jarrah, . 63 to 64 500 to 650 Fir,.. 34 ,, 36 400 „ 700 Karri, 63 „ 64 650 „ 850 Pine, 32,, 34 350 ,, 500 Red gum, 53,, 63 650 „ 720 Spruce, . 29 „32 400 „ 500 Ironbark, . 72 950 ,, 1,050 Chestnut, 35 „41 550 „ 650 Blue gum, 63 to 71 550 „ 850 Cedar, . 35 „47 400 ,, 500 Stringy bark,. 58 450 „ 650 Ash, 43,, 53 600 ,, 700 Teak, 41 to 52 600 ,, 700 Note. —The transverae strength given above is the concentrated central breaking weight of a beani 1 inch wide, 1 inoh deep, and 1 foot clear span. Sélection of Timber.—A thorough insight into the merits and defects of different logs can only be obtained by much experience and close personal investigation. The selection of timber for important marine works should, accordingly, only be entrusted to a competent and reliable man. It would be a diflicult matter to enumerate all the indications of weakness in logs, and many defects are quite latent to the inexperienced eye. Shakes or splits should be looked for and their extent gauged by tapping. Discolora- tion is a bad sign, as also are sponginess and the appearance of wormholes on the surface. Timber with large or dead knots is unsuitable. The lieart should be central. Rankine* states the following general indications of strong and durable timber : — “ In the same species, that specimen will in general be the strongest and the most durable which has grown the slowest, as shown by the narrowness of the annual rings. “ The cellular tissue, as seen in the medullary rays (when visible), should be hard and compact. “ The vascular or fibrous tissue should adhere firmly together, and should show no woolliness at a freshly cut surface, nor should it clog the teeth of the saw with loose fibres. * A Manual of Civil Engineering, p. 441.