MECHANICS OF MATERIALS 71 



and in the very heavy loads, in addition to the compressive 

 stress which would be present in a shorter compression 

 member, there is a tension on one side and additional com- 

 pression on the other because the column has a tendency to 

 bend. The shape or method of fastening the ends of the 

 column has an influence on its power to resist this bending. 

 The square-end columns are stronger than those with round 

 ends. The diagrams show several common forms of columns. 



Columns constructed of different materials generally fail 

 in different ways : cast iron by crushing on one side ; timber 

 by crushing the fibers owing to direct compression or because 

 of a tensile stress caused by bending; concrete columns by 

 shearing off at an angle ; and structural steel by buckling. 



Factor of safety. In the designing of structures only a 

 portion of the strength of material is used, a certain portion 

 of the strength being allowed for poor materials, faulty 

 construction, and unusual strains. This extra allowance of 

 strength is known as- the factor of safety.. For example, a 

 beam which according to tests would carry 24,000 pounds, 

 if placed under a working load of 8,000 pounds would have 

 a factor of safety of 3. 



