REINFORCED CONCRETE 329 



spacing of stirrups, in inches. The other letters have the same 

 meaning as previously given. 

 For T beams, 



in which bi is the width of the stem. 



If the neutral axis is in the flange, j can be found as in rect- 

 angular beams; if it is in the stem, the formulas for rectangular 

 beams will not give the correct value of j, and in place of jd 



the approximate value of d -- may be used, t being the 



thickness of the flange. 



The value of v, the unit shear in concrete, should not exceed 

 40 Ib. per sq. in., when no reinforcement is used. When web 

 reinforcement is used, it is generally assumed that the concrete 

 itself can take one-third of the shear. In this case, the allow- 

 able unit shear in the concrete is usually taken from 60 to 120 

 Ib. per sq. in. 



Bond Between Steel and Concrete. In a reinforced-concrete 

 beam the stress from the load is transmitted to the steel rein- 

 forcement by means of the adhesion, or bond, between the 

 concrete and the steel. The amount of stress H that is trans- 

 mitted to the horizontal reinforcement at the bottom at any 

 section can be found approximately by the formula, 



in which V is the external shear at the section under considera- 



tion and jd = D,as before. Let fb denote the unit bond induced 



at the same section and O the sum of the perimeters of the 



horizontal reinforcement, then O will also be the total bond 



area for one unit of length; therefore, 



f = H = V_ 



Jb O jdO 



This value should not exceed the allowable unit adhesion 

 between the steel and concrete. It is usually taken at about 

 SQ Ib. per sq. in. 



