DETRUSION.
148. Detrusion, or crushing across a fixed point, is such as occurs wherever a brace abuts against a chord, or where a bridge bears upon a bolster or wall plate; also the shearing of bolts, pins, and rivets.
GENERAL RESISTANCE OF MATERIALS.
149. The resistance to extension, to compression, (as regards simple crushing,) and to detrusion, is as the area of cross section; i. e., if we double the area, we double the strength. The resistance to a cross strain is as the breadth, as the length inversely, and as the square of the depth; i. e. if we double the breadth we double the strength; if we double the length, we divide the strength by two; and if we double the depth, we multiply the strength by four.
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.