124 STRESSES AND STRAINS 



From this equation, when 5 and E are known, q may be 

 found; that is, if the modulus of elasticity of a certain sub- 

 stance and the unit stress are known, the unit strain can 

 readily be found. 



STRENGTH OF BUILDING MATERIALS 



The ultimate strength of any material is that unit stress 

 which is just sufficient to break it. 



The ultimate elongation is the total elongation produced 

 in a unit of length of the material by a unit stress equal to 

 the ultimate strength of the material. 



The modulus of rupture differs from the ultimate tensile 

 or compressive stress, but is a quantity, something like it, 

 that is used in calculating the strength of beams. Its use 

 is discussed under the heading Homogeneous Beams. 



FACTOR OF SAFETY 



A value that is taken for the ultimate strength of any 

 material is an average value of a number of experiments 

 made on the material. As it is impossible to get two sam- 

 ples of the same material exactly alike, so is it also impos- 

 sible to get two samples with the same ultimate strength. 

 It is therefore customary in design to avoid stressing a 

 material up to its ultimate strength or even up to its 

 elastic limit. 



The factor of safety, or, as it is sometimes called, the 

 safety factor, is the ratio of the ultimate strength of the 

 material to the load that, under usual conditions, the material 

 is called on to carry. Suppose that the load required to 

 rupture a piece of steel is 5,000 lb., and that the load it is 

 called on to carry is 1,000 lb. ; then, the factor of safety 

 may be obtained by dividing the 5,000 lb. by the 1,000 lb. 

 Thus, 5,000 -5-1, 000 = 5, which is the factor of safety of this 

 material. 



Another factor to be considered in selecting materials is 

 deterioration, which is due to various causes. In metals, 

 there is corrosion on account of moisture and gases in the 

 atmosphere. 



Wood is subject to decay from either dry or wet rot, 

 caused by local conditions; it may, like iron and steel, be 





