This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
One reason for this variation in factors of safety for different materials is that certain materials vary more than others in their internal structure; and accordingly in some cases there is a greater likelihood than in others, of an individual piece being below the average strength. Other reasons are found in the varying effects of time. Changes in internal structure are likely to occur in the lapse of years; and there is the further liability that through ignorance or carelessness the structure may be put to uses for which it was never designed.
All these conditions make it unwise from the standpoint of safety to use working stresses very near the breaking strains.
Steel is less subject to variation than other materials. Timber has knots, shakes, dry rot, and other defects not readily discerned, which may greatly reduce its strength below the average. Cast iron has blow-holes, cracks, flaws, internal strains, and unequally distributed metal, which are of frequent occurrence and very likely to escape detection. Stone has seams, crack , flaws, and a structure not uniform, all causing uncertainty and variations in the strength of individual pieces.
Basement Plan Fig 40.