CORRUGATED BAR COMPANY, INC. 



The lateral spacing of parallel bars should be not less than three diameters from 

 center to center, nor should the distance from the side of the beam to the center of the 

 nearest bar be less than two diameters. The clear spacing between two layers of bars 

 should be not less than one inch. The use of more than two layers is not recommended, 

 unless the layers are tied together by adequate metal connections, particularly at and 

 near points where bars are bent up or bent down. Where more than one layer is used 

 at least all bars above the lower layer should be bent up and anchored beyond the 

 edge of the support. 



Diagfonal Tension and Shear. When a reinforced concrete beam is subjected 

 to flexural action, diagonal tensile stresses are set up. A beam without web reinforce- 

 ment will fail if these stresses exceed the tensile strength of the concrete. When web 

 reinforcement, made up of stirrups or of diagonal bars secured to the longitudinal 

 reinforcement, or of longitudinal reinforcing bars bent up at several points, is used, 

 new conditions prevail, but even in this case at the beginning of loading the diagonal 

 tension developed is taken principally by the concrete, the deformations which are 

 developed in the concrete permitting but little stress to be taken by the web rein- 

 forcement. When the resistance of the concrete to the diagonal tension is overcome 

 at any point in the depth of the beam, greater stress is at once set up in the web rein- 

 forcement. 



For homogeneous beams the analytical treatment of diagonal tension is not very 

 complex, the diagonal tensile stress is a function of the horizontal and vertical shear- 

 ing stresses and of the horizontal tensile stress at the point considered, and as the 

 intensity of these three stresses varies from the neutral axis to the remotest fibre, the 

 intensity of the diagonal tension will be diflferent at different points in the section, 

 and will change with different proportionate dimensions of length to depth of beam. 

 For the composite structure of reinforced concrete beams, an analysis of the web 

 stresses, and particularly of the diagonal tensile stresses, is very complex; and when 

 the variations due to a change from no horizontal tensile stress in the concrete at 

 remotest fibre to the presence of horizontal tensile stress at some point below the 

 neutral axis are considered, the problem becomes more complex and indefinite. Under 

 these circumstances, in designing recourse is had to the use of the calculated vertical 

 shearing stress as a means of comparing or measuring the diagonal tensile stresses 

 developed, it being understood that the vertical shearing stress is not the numerical 

 equivalent of the diagonal tensile stress, and that there is not even a constant ratio 

 between them. It is here recommended that the maximum vertical shearing 

 stress in a section be used as the means of comparison of the resistance to diagonal 

 tensile stress developed in the concrete in beams not having web reinforce- 

 ment. 



Even after the concrete has reached its limit of resistance to diagonal tension, if 

 the beam has web reinforcement, conditions of beam action will continue to prevail, 

 at least through the compression area, and the web reinforcement will be called on to 

 resist only a part of the web stresses. From experiments with beams it is concluded 

 that it is safe practice to use only two-thirds of the external vertical shear in making 

 calculations of the stresses that come on stirrups, diagonal web pieces, and bent-up 

 bars, and it is here recommended for calculations in designing that two-thirds of the 

 external vertical shear be taken as producing stresses in web reinforcement. 



198 



