The factor of safety for beams under a cross strain is one-fifth.
Table II.—Constants (D) for compression.
| Material | D[3] in lbs. for crushing only | Material | D[4] in lbs. for flexure for long pillars | ||
|---|---|---|---|---|---|
| Wet | Dry | ||||
| Spruce | 6,499 | to | 6,819 | Spruce | — |
Larch (fallen two months) | 3,201 | to | 5,568 | Larch | 1,645 |
Fir (white deal) | 6,781 | to | 7,293 | Fir, Riga | 2,035 |
| Fir, Memel | 2,361 | ||||
| Elm | 10,331 | Elm | 1,620 | ||
| Birch, English | 3,297 | to | 6,402 | Birch | — |
| Birch, American | 11,663 | ||||
| Ash | 8,683 | to | 9,363 | Ash | 1,840 |
| Oak, English | 6,484 | to | 10,058 | Oak, English | 2,068 |
Oak, Dantzic (very dry) | 7,731 | Oak, Dantzic | 2,410 | ||
Pillars of medium length—the constant for bending is taken at 2·9for all timbers. | |||||
The working load on pillars should not be greater than one-tenth of the breaking weight; but if the pillars are used for temporary purposes, and are over 15 and under 30 diameters one-eighth, and under 15 diameters one-fifth may be taken as the factor of safety.
Table III.—The Constants (E) for Breaking Weight under tensional stress.
| Material | E in lbs. per unit of 1 sq. inch | Authority | ||
|---|---|---|---|---|
| Spruce | 3,360 | |||
| Larch | 3,360 | |||
| Fir | 3,360 | Hurst | ||
| Elm | 4,480 | |||
| Ash | 4,480 | |||
| Oak | 6,720 | |||
The working load should not exceed one-fifth of the weight that would cause rupture.
Beams subject to a Transverse Strain.—The loads that act upon a beam may be concentrated, that is, acting at one point, or distributed, which means that the load is evenly placed over the entire length of the beam or a portion of the beam.
An evenly distributed load is considered to act at a point immediately below its centre of gravity.
If a beam carries several loads they are considered to act at a point immediately below the resultant centre of gravity of the whole.