STRESSES AND STRAINS 121 



and the loads per foot, the shear at any section may be found. 



The coefficient from the second table multiplied by Wl, 

 which represents the load per span and the length of span, 

 will give the negative bending moment over each support. 

 As the maximum moment occurs over a support, it is seen, 

 by referring to the table, where this moment will be. If 

 the coefficient given in this table is multiplied by the load in 

 pounds and the span in feet, the result will be in foot-pounds; 

 but if the span is taken in inches, the result will be in inch- 

 pounds. 



To illustrate the method of using the tables, the following 

 example will be assumed: 



EXAMPLE. A beam of three spans is supported by four 

 reactions. Each span is 11 ft. The beam carries a load of 

 2,000 Ib. per ft. (a) What will be the amount of the end 

 reactions? (6) What is the greatest bending moment pro- 

 duced? 



SOLUTION. (a) The load W supported by each span is 

 2,000 X 11 =22,000 Ib. From the first table, the reaction at 

 each end support is equal to -jV W. Substituting these 

 values, the reaction at each end is equal to iijX 22,000 

 = 8,800 Ib. 



(6) The greatest bending moment occurs at the inter- 

 mediate supports; from the second table, this moment is 

 equal to -& Wl, or -&X 22,00X11 =24,200 ft.-lb. 



STRESSES AND STRAINS 



STRESS 



It is evident that the weight of the materials composing 

 a building and its contents produces forces that must be 

 resisted by the different members of the structure. The 

 action of these forces has a tendency to change the relative 

 position of the particles composing the members, and this 

 tendency is, in turn, resisted by the cohesive force in the 

 materials, which acts to hold the particles together. 



The internal resistance with which the force of cohesion 

 opposes the tendency of an external force to change the 

 relative position of the particles of any body subjected to a 

 load is called a stress; or, stress may be defined as the load 



