ACTUAL STRENGTH OF MATERIALS.

150. Any material will bear a much larger load for a short time than for a long one. The weight that does not so injure materials as to render them unsafe, is from one third to one fourth only of the ultimate strength. Throughout the present work one fourth will be the most that will in any case be used.

WROUGHT IRON.

151. Extension.

	lbs. per square inch.
Mean of 17 experiments by Barlow (p. 270)	62,720
Weisbach’s Mechanics (Vol. ii., p. 71)	60,500
Overman’s Mechanics, (p. 408, 409)	61,333
Brown, Rennie, and Telford, (mean)	65,251

The mean	62,451

Reducing by 4 for safety	15,613

Or in round numbers 15,000 lbs. per square inch, is the resistance of wrought iron to extension, to be used in practice.

152. Compression.—Great discrepancies appear among writers on the strength of materials, as to the compressive strength of wrought iron. Though all estimate the resistance to compression, as great as to extension, yet no one in summing up the general result of experiment, places the former at more than from 50 to 75 per cent. of the latter. William Fairbairn gives, as the relative resistances to extension and compression in bars applied as girders, 2 to 1.

We have by Weisbach	56,000
We have by Rondelet	70,000
We have by Hodgkinson	65,000

The mean	63,667

Reducing by 4	15,917

In round numbers	16,000	lbs. per square inch.

As far as practice is any guide, from 8,000 to 12,000 pounds per inch is the most to be used. The ratio of 90 to 66, seems to express very nearly the action as in the most reliable structures; which will, therefore, be adopted, or 11,000 pounds per square inch nearly. The resistance to compression is very much greater after wrought iron has been somewhat compressed.

CAST-IRON.

153. Extension.—This material is seldom used to resist a tensile force. That the tables may be complete, however, the following is given:—

By Weisbach	20,000	pounds.
By Barlow	18,233	pounds.
By Overman	20,000	pounds.
By Rennie	18,000	pounds.
By Hodgkinson	16,577	pounds.
By the British Iron Commission	15,711	pounds.

The mean	18,087	pounds.

Reducing by 4	4,522	pounds.

In round numbers	4,500	pounds.

154. Compression.

By Weisbach	109,800	pounds.
By Hodgkinson	107,520	pounds.
By Iron Commission	100,000	pounds.
Stirling’s toughened	130,000	pounds.

Mean of Common	105,773	pounds.

Mean of Stirling’s	130,000	pounds.

Reducing by 4 for safety (Common)	26,443	pounds.

Reducing by 4 for safety (Stirling’s)	32,500	pounds.

In round numbers (Common)	25,000	pounds.

In round numbers (Stirling’s)	30,000	pounds.

155. Following are given the condensed results of the preceding figures, which may be relied upon as giving perfectly safe dimensions in practice.

Wrought Iron.	Cast-Iron.
15,000	4,500	Tensile strength,
11,000	25,000	Compressive strength.

For additional remarks on iron, see chap. IX.

156. Nature and Strength of American Woods.

Name of the wood.	Weight per cubic foot.	Resistance to Extension.	Resistance to Compression.	Value of S.	Elasticity.

White Pine	26	12,000	6,000	1,229
Yellow Pine	31	12,000	6,000	1,185
Pitch Pine	46	12,000	6,000	1,727	4,900
Red Pine	35	12,000	6,000	1,527	7,359
Virginia Pine	37	12,000	6,000	1,456
Spruce	48	12,000	6,000	1,036
Larch	33	12,000	6,000	907	2,465
Tamarack	26	12,000	6,000	907
White Cedar	22	8,000	4,000	766
Canada Balsam	34	12,000	6,000	1,123
White Oak	48	15,000	7,500	1,743	8,595
Red Oak	41	15,000	7,600	1,687
Live Oak	72	15,000	7,200	1,862
White Beech	44	18,000	9,100	1,380	5,417
Red Beech	48	18,000	9,000	1,739
Birch	44	15,000	7,000	1,928
Black Birch	41	15,000	7,200	2,061
Yellow Birch	36	15,000	7,200	1,335
Ash	38	16,000	8,100	1,795	6,581
Black Ash	35	16,000	8,000	861
Swamp Ash	57	16,000	8,000	1,165
Hickory	51	15,000	7,200	2,129
Butternut	54	15,000	7,600	1,465
Ironwood	54	16,000	8,100	1,800
Rock Elm	45	16,000	8,011	1,970	2,799


The mean tensile strength of wood is	14,080	lbs.
Reducing by 4 for safety	3,520	lbs.
Reducing for want of seasoning	2,000	lbs.
The reduced mean compressive strength	1,000	lbs.
Reduced resistance to detrusion	150	lbs.
Ratio of tensile to compressive strength	2 to 1.
Mean value of S in formula (WL = 4Sbd²) for the woods most used in practice	1,250.

157. The lateral adhesion of fir was found, by Barlow, to be six hundred pounds per square inch. (Lateral adhesion is the resistance which the fibres offer to sliding past each other in the direction of the grain; as, in pulling off the top of a post where it is halved on to the chord.)

158. As regards the nature of timber, seasoning, time of cutting, etc., although these are important items, still, generally, commercial considerations outbalance all else. The most complete treatise on the nature of woods, is “Du Hamel, L′exploitation des bois;” from which it appears that the best oaks, elms, and other large trees, are the product of good lands, rather dry than moist. They have a fine, clear bark, the sap is thinner in proportion to the diameter of the trunk, the layers are less thick, but more adherent the one to another; and more uniform than those of trees growing on moist places. The grain of the latter may look very fine and compact, but microscopic examination shows the pores to be full of gluten.

The density of the same species of timber, in the same climate, but on different soils, will vary as 7 to 5; and the strength, both before and after seasoning, as 5 to 4.

In trees not beyond their prime, the density of the butt is to that of the top, as 4 to 3; and of centre to circumference, as 7 to 5. After maturity, the reverse occurs in both cases.

Oak, in seasoning, loses from ¼ to ⅓ of its weight; but its strength is increased from 30 to 40 per cent.

GENERAL TABLE OF THE NATURE OF MATERIALS.

159. The tensile strength of wrought iron assumed as 1,000.

Material.	Tension.	Compression.	Cross Strain.	Sum.	Weight per cubic ft.	Sum divided by weight per cub. ft.
Cast-Iron	300	1,666	31.68	1,997.68	450	4.4
Wrought Iron	1,000	733	55.40	1,788.40	480	3.7
Wood	133	66	5.60	204.60	30	6.8

The advantage possessed by iron over wood, is in durability only. The above figures show how much more of the strength of the material is consumed by its own weight in iron than in wood. In actual practice, however, the method of making joints and other details often render iron the lightest material.