FARM STRUCTURES 403 



Equilibrium of Moments. The forces acting upon a body 

 are in equilibrium when the algebraic sum of their moments 

 about any one point is equal to zero. 



Stress. A stress is the resistance offered by a rigid body 

 to an external force tending to change its 

 form. A rope suspending a weight is under 

 stress. If a section of the rope be taken at 

 any point, the force exerted by the part of the 

 rope on one side of the section on the part on 

 the other side to prevent the rope from part- 

 ing or breaking, is termed the stress at a section. p A 



The word strain is often used incorrectly for sketch illustrat- 

 ing a tensi'e 



stress, but strain is the change of form pro- stress. 

 duced by a stress. Simple stresses are of three kinds, ten- 

 sile, compressive, and shearing. 



Stresses are measured in pounds or tons in countries using 

 English units. The pound is the more often used. 

 s/s//,ss\\\\ ///,;/// Tensile stresses are those tending to pull 

 the object or material in two, or to stretch it. 

 A rope suspending a weight is under a tensile 

 stress. A tie rod in a truss is subjected to 

 tensile stress. 



Compressive Stresses. Compressive stresses 

 are those tending to crush the object or ma- 

 Fig. 256. A terial, as the load that is placed on a column 



sketch illustrat- /. j ,. 



ing a compres- or on a loundation. 



Shearing Stresses. Shearing stresses are 

 those tending to slide one portion r . f f ' -t " H i^ ^ .- 

 of the material over another, or Fig 257 A sketch illustrat . 

 when there is a tendency to cut. ing a shearing stress. 

 The stress on riveted joint is a good example. 



Complex Stresses. Complex stresses are those formed 

 by a combination of simple stresses. The stresses in beams 

 are usually complex. 



