SLABS, CROSS-BEAMS, AND GIRDERS 161 



formula as is employed for the tension rods at the end of a simply 

 supported beam. However, if bent up rods are employed for 

 web reinforcement and if these same rods are employed to take 

 the tension over supports, the beam is greatly stiffened and 

 the bond stress along the top rods is reduced appreciably below 

 that given by the theoretical formula. This bond stress is 

 affected by the amount of web reinforcement used in a some- 

 what similar manner to the way the bond stress is affected 

 along the rods at the end of a simply supported beam. Art. 

 38 should be reviewed so that the student will have some guide 



r 



rH 



Point of 

 zero bending 

 moment at a 

 distance of , 

 approximately 4 from 

 section A-A J 5 



Section where 

 haunch is not 

 necessary Moment - M, 



FIG. 77. 



moment M* 

 momer M 2 



as to how much the allowable bond stress may be increased for 

 different cases. A careful study of the problems following Art. 

 50 will also help to make the matter clear. 



In beams considered uniformly loaded, the rods which are 

 bent should extend beyond the center of support at least to 

 about the third point (point of zero moment varies for different 

 loadings) to provide thoroughly for negative moment, and this 

 length should be increased if it is not sufficient to transfer to 

 the concrete through bond, the greatest allowable tensile stress 

 in the rods. Some designers consider the negative moment 

 properly looked after if the top rods are extended only to the 

 fourth point. The matter admits of much difference of opinion 

 but it would seem well to be conservative in this part of the 

 design. 



If half of the rods from each span are used over the support, 

 then half of the total number will extend to about the third 

 point where the tension due to negative moment becomes zero. 

 At this point the shear is only 1/3 of what it is at the end of 

 span, if the beam is considered uniformly loaded. Since bond 

 stress due to increment (or decrement) tension varies as shear, 



