RECTANGULAR BEAMS 77 



takes place just sufficient to bring the maximum tensile stress in 

 the concrete to its ultimate value. When loads are applied which 

 cause a greater bending moment, the concrete in tension becomes 

 broken by fine cracks, and the steel takes a greater part of the 

 tensile stress. The elongation at b now increases faster than at a, 

 and the neutral axis rises rapidly. When working loads are 

 reached, the position of the neutral axis moves but little, and 

 the steel takes all the tension. 



i 



T t 



FIG. 42. 



Fig. 42 illustrates a typical diagonal tension failure where 

 only horizontal bars are used. The initial crack forms at a 

 and branches in a diagonal direction, running in a direction away 

 from the support. A little later the concrete begins to fail in a 

 horizontal tension crack just above the rods, progressing from a 

 toward the end of the beam. This horizontal crack is brought 

 about by the new conditions which exist after the concrete has 

 become cracked along the diagonal line and the normal diagonal 

 tension has thus ceased to act. Sometimes this horizontal 

 crack does not extend to the end of the beam the final failure 

 occurring either by the diagonal crack extending to the top of the 

 beam or the horizontal rods pulling out. Thus final failure often 

 occurs from stresses which are developed after initial failure has 

 occurred. However, the initial failure and its cause is what 

 is of importance in design. 



In regard to tests of beams having web reinforcement, it is 

 found that vertical stirrups spaced a distance apart equal to, 

 or greater than, the depth of the beam, will give little aid in 

 the prevention of diagonal cracks between successive stirrups, 

 although they may prevent final failure by preventing the exten- 

 sion of a crack horizontally along the reinforcing rods. 



Using stirrups with part of the horizontal rods bent up, the 

 diagonal tension cracks appear with an inclination about the 



