STEEL REINFORCEMENT 251 



Members to Resist Lines of Failure. In Fig. 3 are illus- 

 trated a typical beam having the usual type of steel rein- 

 forcement and the several methods of failure that might 

 occur. At a are shown cracks, or lines of failure, that would 

 be caused by lack of tensile resistance in the main reinforcing 

 rods b. These cracks, although usually invisible, generally 

 extend from the bottom surface to the neutral axis. They 

 are nearly always present in concrete, but, of course, so long 

 as the steel holds, the beam will not fail. 



If the main reinforcing rods do not extend to the bearings, 

 failure by vertical shear may occur near the abutments, along 

 the line x x. Ordinarily, however, failures of this kind sel- 

 dom happen, because the main rods usually extend across all 



FIG. 3 



such lines of vertical shear, and add greatly to the shearing 

 resistance of the beam. 



If the slab concrete is not placed at the same time that 

 the concrete of the beam section is poured, failure by shearing 

 usually occurs at the junction of the beam with the slab, as 

 shown at c c. The shearing resistance at this junction may 

 be increased, however, by extending stirrups d into the slab. 

 If the crack c c opens, it usually joins with a crack like e e 

 at each end of the beam, as suggested in the preceding 

 paragraph. 



The lines of failure indicated at e e are those which usually 

 occur from diagonal tension stresses that cross these lines of 

 failure at right angles. A beam is held against failure in this 

 manner by placing stirrups in the concrete either vertically 

 or obliquely. The bending up of the main reinforcing rods 



