242 REINFORCED CONCRETE CONSTRUCTION 



the projection of the section is taken such that the distance x 

 represents the true lever arm of. W about the center of gravity, 

 then the force W is equivalent to an equal W at and a couple 

 whose moment is Wx . The intensity of the uniformly varying 

 stress due to this bending moment at a distance x from is 



Wx x 

 (by the common flexure formula for homogeneous beams) ^-> 



in which / is the moment of inertia of the section about an axis 



-c~ x >| 



< -x,- >U - x, -> 



-< t > 



FIG. 84. 



through at right angles to the plane of the paper. At the edges 



Wx x 

 E and F this intensity = T~^' Regarding compressive and 



tensile stresses as positive and negative respectively, the intensity 

 of stress at edge E is 



/-T. + r 51 



At edge F it is 



W Wx x, 



If the stress f c comes out minus, the value obtained is the 

 maximum tension as shown in Fig. 85. In plain concrete con- 

 struction a greater tension than about 50 Ib. per square inch 

 should not be allowed or else cracks may be expected on the 

 tension side. 



When we come to reinforced concrete, which is composed of 

 two materials (concrete and steel) with different values of E, 



