86 SILOS: CONSTRUCTION AND SERVICE 



mentioned, where only property value is concerned, greater 

 loads naturally are imposed than where human life is at stake. 

 Causes of Failure. As to their strength, materials may 

 fail in any one of three different ways, or by combinations of 

 two or all three of them. 



First, pressure may be brought to bear which will crush the 

 material; for instance, a block of wood may be put between 

 the jaws of a vice and enough pressure brought to bear upon 

 it to crush the fibers together. This is spoken of as failing 

 in " compression." An example of this is a wall that is poorly 

 built, of made too thin, and subjected to a greater weight 

 than it will bear. In failing in this manner it simply means 

 that the fibers or fine particles of the material have been 

 crushed. 



Second, the force known as " tension" may cause failure. 

 This is a force which tends to pull materials in two. The 

 force tending to separate the ends of a straight piece of mate- 

 rial may be sufficient actually to separate the fibers or parti- 

 cles of the material. The intensity of this force is also given 

 in pounds per square inch of cross section of the material. 



Third, a force known as shear may cause failure. As the 

 name infers, portions of the material which fail in this respect 

 are simply forced past each other sidewise. An example of 

 failure in this respect is the unequal settling of a foundation. 

 This simply means that the foundation is more thoroughly 

 supported in one place than in another, and the rigidity of the 

 material is not sufficient to support the pressure without 

 breaking. Another illustration of this method of failure is 

 in case of blocks pushed out of the wall, the shear occurring 

 between the mortar and the block, causing the mortar to 

 slide upon the block. The intensity of this force is generally 

 spoken of as so many pounds per square inch, and indicates 



