90 REINFORCED CONCRETE CONSTRUCTION 



41. Vertical and Inclined Reinforcement. Thus far in the 

 course the student has been brought to realize the effectiveness 

 of inclined and vertical bars in preventing diagonal tensile cracks 

 in reinforced concrete beams. This type of failure we have seen 

 is the one to be most feared. The inclined reinforcement may 

 be separate members firmly connected with the horizontal 

 reinforcement to prevent slipping, or some of the horizontal 

 bars may be bent up near the ends of the beam where they are 

 not needed to resist bending. The vertical reinforcement may 

 be used separately or in combination with inclined reinforcement, 

 depending upon the preference of the designer and upon the 

 amount of diagonal tension to be provided for. When vertical 

 stirrups are used, they should be looped around the horizontal 

 bars so as to be firmly anchored at their lower ends where the 

 stress is a maximum. The value of a stirrup unless looped or 

 hooked at the top is limited by its strength of bond and, as its 

 length is not great, this point may need consideration. 



The proportioning of web reinforcement cannot be done with 

 any degree of exactness since very little experimental work has 

 been performed along this line. However, rough determinations 

 of what is required may be obtained on rational grounds. The 

 only information given us by tests, is the value of the shearing 

 stress which measures diagonal tension failure (1) for beams 

 with horizontal bars only, and (2) for beams having an effective 

 system of web reinforcement. Also, tests on beams, with and 

 without web reinforcement, show that when reinforcement is 

 provided for diagonal tension, the concrete may be assumed to 

 carry its full value of the shear and the steel the remainder. 

 This action of the concrete and web reinforcement is found to 

 exist at ultimate loads namely, when the beam has failed by 

 the slipping of the bars. Since this is found true at ultimate 

 loads, it undoubtedly would be even more certain at working 

 loads when the concrete at the most is only slightly cracked. 



The Joint Committee limits the shearing unit stress in a beam 

 with an effective web reinforcement to 120 Ib. per square inch for 

 a 2000 Ib. concrete namely, three times the allowable shearing 

 stress in a 2000 Ib. concrete without reinforcement. In the dis- 

 cussion which follows, we shall assume the concrete to take one- 

 third of the total shear and the stirrups the remaining two-thirds. 

 The point in the beam beyond which web reinforcement is 

 unnecessary will, however, be determined by the maximum 

 allowable shear in the concrete. 



