First.—As beams, they may fail to resist a cross strain.
Secondly.—As pillars and struts, they may fail to resist compression in the direction of their length.
Thirdly.—As ties and braces, they may fail to resist tension; that is, a strain tending to pull a beam asunder by stretching.
The loads that act upon a scaffold may be dead, that is to say, they do not create a shock; or live, which means they are not stationary, and may cause shock and vibration.
A live load causes nearly double the strain that a dead load produces. If, therefore, both are of equal weight, the timber under strain will carry as a live load only one half of what it would carry as a dead load. The breaking weight is the load that will cause fracture in the material. The safe load is the greatest weight that should be allowed in practice. It is in proportion to the breaking load, and that proportion is termed the factor of safety.
Experiments have been made to determine the resistance of timber to fracture under the various forces that act upon it.
The result of these experiments is expressed by a given number, termed a constant, which varies with the different growths of timber and the different strains to which they are subjected.
The constants (C) for the strength of different kinds of timber under a cross strain are as follows:
Table I.
| Material | C in lbs. for rectangular beams supported at both ends | Authority | |||||
|---|---|---|---|---|---|---|---|
| Spruce | 403 | ||||||
| Larch | 392 | ||||||
| Fir, Northern | |||||||
| Fir, Dantzic | 448 | ||||||
| Fir, Memel | |||||||
| Fir, Riga | 392 | Hurst | |||||
| Elm | 336 | ||||||
| Birch | 448 | ||||||
| Ash, English | 672 | ||||||
| Oak, English | 560 | ||||||
| Oak, Baltic | 481 | ||||||