In which w= breaking weight in lbs., b = breadth, d = depth, and I = length, in inches. Actual tests will probably show wide variations in both directions from the mean calculated strength. STRENGTH OF COPPER AT HIGH TEMPERATURES. The British Admiralty conducted some experiments at Portsmouth Dockyard in 1877, on the effect of increase of temperature on the tensile strength of copper and various bronzes. The copper experimented upon was in rods .72-in. diameter. The following table shows some of the results: Up to a temperature of 400° F. the loss of strength was only about 10 per cent, and at 500° F. the loss was 16 per cent. The temperature of steam at 200 lbs. pressure is 382° F., so that according to these experiments the loss of strength at this point would not be a serious matter. Above a temperature of 500° the strength is seriously affected. STRENGTH OF TIMBER. Strength of Long-leaf Pine (Yellow Pine, Pinus Palustris) from Alabama (Bulletin No. 8, Forestry Div., Dept. of Agriculture, 1893. Tests by Prof. J. B. Johnson.) The following is a condensed table of the range of results of mechanical tests of over 2000 specimens, from 26 trees from four different sites in Alabama; reduced to 15 per cent moisture: Specific gravity. 3 WL Transverse strength, bha Butt Logs. Middle Logs. Top Logs. 0.23 to 4.69 Av'g of all Butt Logs. 0.767 12,614 9,460 842 to 2,697 1,926 1.34 to 4.21 09 to 4.65 9,300 4,587 to 9,100 2.98 7,452 1,598 17,359 0.449 to 1.039 0.575 to 0.859 0.484 to 0.907 4,762 to 16,200 7,640 to 17,128 4,268 to 15,554 do do. at elast. limit. 4,930 to 13,110 5,540 to 11,790 2,553 to 11,950 Mod, of elast., thous. lbs. 1,119 to 3,117 1,136 to 2,982 Relative elast. resilience, inch-pounds per cub. in. Crushing endwise, str. per sq. in.-lbs. Crushing across grain,| strength per sq. in.,lbs. 675 to 2,094 656 to Tensile strength per sq. in. 8,600 to 31,890 6,330 to Shearing strength (with grain), mean per sq. in. 4,781 to 9,850 5,030 to 1,445 584 to 1,766 29,500 4,170 to 23,280 464 to 1,299 539 to 1,230 484 to 1156 866 Some of the deductions from the tests were as follows: 1. With the exception of tensile strength a reduction of moisture is accompanied by an increase in strength, stiffness, and toughness. 2. Variation in strength goes generally hand-in-hand with specific gravity. 3. In the first 20 or 30 feet in height the values remain. constant; then occurs a decrease of strength which amounts at 70 feet to 20 to 40 per cent of that of the butt-log. 4. In shearing parallel with the grain and crushing across and parallel with the grain, practically no difference was found. 5. Large beams appear 10 to 20 per cent weaker than small pieces. 6. Compression tests endwise seem to furnish the best average statement of the value of wood, and if one test only can be made, this is the safest, as was also recognized by Bauschinger. 7. Bled timber is in no respect inferior to unbled timber. The figures for crushing across the grain represent the load required to cause a compression of 15 per cent. The relative elastic resilience, in inchpounds per cubic inch of the material, is obtained by measuring the area of the plotted-strain diagram of the transverse test from the origin to the point in the curve at which the rate of deflection is 50 per cent greater than the rate in the earlier part of the test where the diagram is a straight line. This point is arbitrarily chosen since there is no definite "elastic limit" in timber as there is in iron. The “ 'strength at the elastic limit" is the strength taken at this same point. Timber is not perfectly elastic for any load if left on any great length of time. The long-leaf pine is found in all the Southern coast states from North Carolina to Texas. Prof. Johnson says it is probably the strongest timber in large sizes to be had in the United States. In small selected specimens, other species, as oak and hickory, may exceed it in strength and toughness. The other Southern yellow pines, viz., the Cuban, short-leaf and the loblolly pines are inferior to the long-leaf about in the ratios of their specific gravities; the long-leaf being the heaviest of all the pines. It averages (kiln-dried) 48 pounds per cubic foot, the Cuban 47, the short-leaf 40, and the loblolly 34 pounds. Strength of Spruce Timber.-The modulus of rupture of spruce is given as follows by different authors: Hatfield, 9900 lbs. per square inch; Rankine, 11,100; Laslett, 9045; Trautwine, 8100; Rodman, 6168. Trautwine advises for use to deduct one-third in the case of knotty and poor timber. Prof. Lanza, in 25 tests of large spruce beams, found a modulus of rupture from 2995 to 5666 lbs.; the average being 4613 lbs. These were average beams, ordered from dealers of good repute. Two beams of selected stock, seasoned four years, gave 7562 and 8748 lbs. The modulus of elasticity ranged from 897,000 to 1,588,000, averaging 1,294,000. Time tests show much smaller values for both modulus of rupture and modulus of elasticity. A beam tested to 5800 lbs. in a screw machine was left over night, and the resistance was found next morning to have dropped to about 3000, and it broke at 3500. Prof. Lanza remarks that while it was necessary to use larger factors of safety, when the moduli of rupture were determined from tests with smaller pieces, it will be sufficient for most timber constructions, except in factories, to use a factor of four. For breaking strains of beams, he states that it is better engineering to determine as the safe load of a timber beam the load that will not deflect it more than a certain fraction of its span, say about 1/300 to 1/400 of its length. The above table should be taken with caution. The range of variation in the species is apt to be much greater than the figures indicate. See Johnson's tests on long-leaf pine, and Lanza's on spruce, above. The weight of yellow pine in the table is much less than that given by Johnson. (W. K.) Compressive Strengths of American Woods, when slowly and carefully seasoned.-Approximate averages, deduced from many experiments made with the U. S. Government testing machine at Watertown, Mass., by Mr. S. P. Sharpless, for the Census of 1880. Seasoned woods resist crushing much better than green ones; in many cases, twice as well. Different specimens of the same wood vary greatly. The strengths may readily vary as much as one-third part more or less from the average. End- lbs. per * Specimens 1.57 ins. square × 12.6 ins. long. + Specimens 1.57 ins. square X 6.3 ins. long. Pressure applied at mid-length by a punch covering one-fourth of the length. The first column gives the loads producing an indentation of .01 inch, the second those producing an indentation of .1 inch. (See also page 306). Expansion of Timber Due to the Absorption of Water, (De Volson Wood, A. S. M. E., vol. x.) Pieces 36 x 5 in., of pine, oak, and chestnut, were dried thoroughly, and then immersed in water for 37 days. The mean per cent of elongation and lateral expansion were: Expansion of Wood by Heat.-Trautwine gives for the expansion of white pine for 1 degree Fahr. 1 part in 440,530, or for 180 degrees 1 part in 2447, or about one-third of the expansion of iron. Ash Shearing Strength of American Woods, adapted for Pins or Treenails. J. C. Trautwine (Jour. Franklin Inst.). (Shearing across the grain.) Beech 5223 66 ........... ........................ 7285 THE STRENGTH OF BRICK, STONE, ETC. A great advance has recently been made in the manufacture of brick, in the direction of increasing their strength. Chas. P. Chase, in Engineering News, says: "Taking the tests as given in standard engineering books eight or ten years ago, we find in Trautwine the strength of brick given as 500 to 4200 lbs. per sq. in. Now, taking recent tests in experiments made at Watertown Arsenal, the strength ran from 5000 to 22,000 lbs. per sq. in. In the tests on Illinois paving brick, by Prof. I. O. Baker, we find an average strength in hard paving brick of over 5000 lbs. per square inch. The average crushing strength of ten varieties of paving-brick much used in the West, I find to be 7150 lbs. to the square inch. A recent test of brick made by the dry-clay process at Watertown Arsenal, according to Paving, showed an average compressive strength of 3972 lbs. per sq. in. In one instance it reached 4973 lbs. per sq. in. A test was made at the same place on a "fancy pressed brick.' The first crack developed at a pressure of 305,000 lbs., and the brick crushed at 364,300 lbs., or 11,130 lbs. per sq. in. This indicates almost as great compressive strength as granite paving-blocks, which is from 12,000 to 20,000 lbs. per sq. in. The following notes on bricks are from Trautwine's Engineer's Pocket book: Strength of Brick.-40 to 300 tons per sq. ft., 622 to 4668 lbs. per sq. in. A soft brick will crush under 450 to 600 lbs. per sq. in., or 30 to 40 tons per square foot, but a first-rate machine-pressed brick will stand 200 to 400 tons per sq. ft. (3112 to 6224 lbs. per sq. in.). Weight of Bricks.-Per cubic foot, hest pressed brick, 150 lbs.; good pressed brick, 131 lbs.; common hard brick, 125 lbs.; good common brick, 118 lbs.; soft inferior brick, 100 lbs. Absorption of Water.-A brick will in a few minutes absorb 1⁄2 to 34 lb. of water, the last being 1/7 of the weight of a hand-moulded one, or of its bulk. Tests of Bricks, full size, on flat side. (Tests made at Watertown Arsenal in 1883.)-The bricks were tested between flat steel buttresses. Compressed surfaces (the largest surface) ground approximately flat. The bricks were all about 2 to 2.1 inches thick, 7.5 to 8.1 inches long, and 3.5 to 3.76 inches wide. Crushing strength per square inch: One lot ranged from 11,056 to 16,734 lbs.; a second, 12,995 to 22,351; a third, 10,390 to 12,709. Other tests gave results from 5960 to 10.250 lbs. per sq. in. Crushing Strength of Masonry Materials. 66 Retaining Walls.") tons per sq. ft. Brick, best pressed.. 40 to 300 Chalk. Granite................................ 20 to 30 300 to 1200 (From Howe's tons per sq. ft. Limestones and marbles. 250 to 1000 Strength of Granite.-The crushing strength of granite is commonly rated at 12,000 to 15,000 lbs. per sq. in. when tested in two-inch cubes, and only the hardest and toughest of the commonly used varieties reach a strength above 20,000 lbs. Samples of granite from a quarry on the Con⚫ necticut River, tested at the Watertown Arsenal, have shown a strength of 35,965 lbs. per sq. in. (Engineering News, Jan. 12, 1893). Strength of Avondale, Pa., Limestone-(Engineering News, Feb. 9, 1893).-Crushing strength of 2-in. cubes: light stone 12,112, gray stone 18,040, lbs. per sq. in. 66 Transverse test of lintels, tool-dressed, 42 in. between knife-edge bear ings, load with knife-edge brought upon the middle between bearings: Gray stone, section 6 in. wide x 10 in. high, broke under a load of 20,950 lbs. Modulus of rupture. 2,200 Light stone, section 844 in. wide x 10 in. high, broke under........ 14,720 ** Modulus of rupture.. Absorption.-Gray stone...........................................051 of 1% Light stone.. Transverse Strength of Flagging. (N. J. Steel & Iron Co.'s Book.) EXPERIMENTS MADE BY R. G. HATFIELD AND OTHERS. 1,170 .052 of 1% b = width of the stone in inches; d = its thickness in inches; 1 = distance between bearings in inches. The breaking loads in tons of 2000 lbs., for a weight placed at the centre of the space, will be as follows: Thus a block of Quincy granite 80 inches wide and 6 inches thick, resting on beams 36 inches in the clear, would be broken by a load resting midway 80 X 36 between the beams = 36 X.62449.92 tons. STRENGTH OF LIME AND CEMENT MORTAR. (Engineering, October 2, 1891.) Tests made at the University of Illinois on the effects of adding cement to lime mortar. In all the tests a good quality of ordinary fat lime was used, slaked for two days in an earthenware jar, adding two parts by weight of water to one of lime, the loss by evaporation being made up by fresh additions of water. The cements used were a German Portland, Black Diamond (Louisville), and Rosendale. As regards fineness of grinding, 85 per cent of the Portland passed through a No. 100 sieve, as did 72 per cent of the Rosendale. A fairly sharp sand, thoroughly washed and dried, passing through a No. 18 sieve and caught on a No. 30, was used. The mortar in all cases consisted of two volumes of sand to one of lime paste. The following results were obtained on adding various percentages of cement to the mortar: Tensile Strength, pounds per square inch. Days. Days. Days. Days. Days. Days. Days. Age........ Lime mortar 328 30 66 66 Rosendale.. 11 18 13 1816 21 2212 23 Portland.. |