General Abstract. The resistance to the above mentioned strains varies as the the area of the cross section; so that by doubling the area we double the strength. Any material will bear a much greater strain for a short time than for a long one. The working strength of materials, or the weight which does not injure them enough to render them unsafe, is a mooted point, and varies, according to the authority, from 1-3 to 1-10 of the ultimate strength. The ratio of the ultimate strength to the working strength is called the factor of safety.
The following is a table of ultimate and working strengths of materials, and factors of safety:
| Weight | Materials. | Ult. Ext. | Ult. Comp. | Working Strengths. | Factor of Safety. | ||
|---|---|---|---|---|---|---|---|
| in lbs. | Exten. | Comp. | Tension | Comp. | |||
| 30 | Wood. | 14,000 | 7,000 | 2,000 | 1,000 | 7 | 7 |
| 480 | Wro't Iron. | 60,000 | 64,000 | 15,000 | 12,000 | 4 | 5.33 |
| 450 | Cast Iron. | 18,000 | 100,000 | 4,500 | 25,000 | 4 | 4 |
Lateral Adhesion. Lateral adhesion is the resistance offered by the fibres to sliding past each other in the direction of the grain, as when a brace is notched into a chord, or tie beam, at its foot, it is prevented by the lateral adhesion of the fibres from crowding off the piece, to the depth of the notch, against which it toes. Barlow's experiments give the lateral adhesion of fir as 600 lbs. per square inch, and the factor of safety employed varies in practice from 4 to 6, giving a working strength of from 150 to 100 lbs. per square inch.
TABLE OF COMPRESSIVE RESISTANCE OF TIMBER.
| Length given in Diameters. | Safety Weig't in Pounds. | Length given in Diameters. | Safety Weig't in Pounds. | Length given in Diameters. | Safety Weig't in Pounds. |
| 6 | 1000 | 24 | 440 | 42 | 203 |
| 8 | 960 | 26 | 394 | 44 | 185 |
| 10 | 910 | 28 | 358 | 46 | 169 |
| 12 | 860 | 30 | 328 | 48 | 155 |
| 14 | 810 | 32 | 299 | 50 | 143 |
| 16 | 760 | 34 | 276 | 52 | 132 |
| 18 | 710 | 36 | 258 | 54 | 122 |
| 20 | 660 | 38 | 239 | 56 | 114 |
| 22 | 570 | 40 | 224 | 58 | 106 |
| 60 | 99 |
In tensional strains, the length of the beam does not affect the strength; but in the beams submitted to compression, the length is a most important element, and in the table given above, the safety strains to which beams may be subjected, without crushing or bending, has been given for lengths, varying from 6 to 60 diameters.
PRACTICAL RULES.
Tensional Strain.
Let T = whole tensional strain.
" S = strength per square inch.
" a = sectional area in inches.
Then we have T = Sa.