46 REINFORCED CONCRETE CONSTRUCTION 



a part of these upward as they approach the end, where they 

 are not needed to resist bending stresses. This matter will be 

 taken up in detail later. The concrete is depended upon for 

 the compressive and shearing stresses, its resistance to such 

 stresses being large. 



It cannot truly be said that the above remarks refer directly 

 to homogeneous beams of concrete. As we have already seen, 



FIG. 19. 



the elastic property of concrete is very much inferior to that of 

 steel, consequently the formula / = -, does not exactly apply 



except for very low stresses. In fact, in the previous inves- 

 tigation we have erred slightly by representing the direction of 

 the maximum internal stresses in a concrete beam to be iden- 

 tical with the direction of the maximum stresses in a steel beam. 

 The difference, however, is inappreciable, especially for low 

 stresses. The student should appreciate the fact that the 



FIG. 20. 



directions of the internal stresses at the principal points in con- 

 crete beams are the same as in steel beams namely, at the upper 

 and lower fibers and at the neutral plane. This statement 

 should be understood when it is considered that the direction 



2v 

 at these points, as determined by the formula tan 2K=, 



depends only upon a zero value for either v or /. For example, 

 at the lower and upper fibers, the value of v becomes zero and 

 the magnitude of / does not control. Likewise, at the neutral 

 plane, / becomes zero and the value of the angle K is not affected 

 by the magnitude of v. 



