RECTANGULAR BEAMS 91 



y 



When the unit shear, which equals r^> is found to only slightly 



exceed 40 Ib. per square inch, it would seem that the beam de- 

 signed according to the above assumption would be unnecessarily 

 strong as regards web reinforcement. This is true, but on ac- 

 count of the small amount of reinforcement which is usually 



needed for such cases and the importance of such reinforcement 



2 

 in reinforced concrete beams, the value ~ is proposed for all 



o 



ordinary cases. The student should always remember this 

 element of safety when using the following formulas in design. 



2 

 If desired, the coefficient = in the formulas may be changed 



o 



as required by the problem under consideration. 



Using Taylor and Thompson's method of analysis, 1 let 

 V = total shear. 



s = horizontal spacing of stirrups. 

 a a = area of section of stirrup. (In a U-shaped 

 stirrup, a s is the sum of the areas of the two 

 legs.) 

 f s = unit stress in steel. 



The maximum intensity of vertical shear v at a vertical section 



Y 

 AA, Fig. 44, is - The maximum intensity of horizontal shear 



i 



vj v| vJ 



U J 



FIG. 44. 



at a section A A is equal to the maximum intensity of vertical 

 shear v at the same section. Multiplying this shear by 6, -the 



breadth of the beam, gives the maximum horizontal shear per 



Y 

 unit of length of the beam = Consider now a horizontal 



plane of length s at, or below, the neutral plane. The intensity 



1 From Taylor and Thompson's "Concrete, Plain and Reinforced," 2nd edition, page 448. 



