98 REINFORCED CONCRETE CONSTRUCTION 

 Then 



i=(2.4-) d 



Js 



The table below gives the values of 2.4^r for different working 



Ja 



values of tension and bond, as developed by Messrs. Taylor and 

 Thompson in " Concrete, Plain and Reinforced." 1 



Allowable unit bond 



stress in pounds 

 per square inch (w) 



Vertical bars 



Allowable unit tension in stirrups in pounds 

 per square inch (/ 8 ) 



Suppose that for a given vertical stirrup the allowable / = 

 16,000 Ib. per square inch and the allowable w = 80 Ib. per 

 square inch. Also, let the depth of the beam in question be 20 in. 

 (d). Then {==0.24 in. practically 1/4 in. For deformed bars 

 the bond may be increased to 100, or even to 150 Ib. per square 

 inch, varying with the character of the bar. Using 150 for u 

 and 16,000 for/ s , a beam 20 in. deep to center of steel, making no 

 allowance for the value of a bent end (called prong or hook) , would 

 require stirrups not to exceed practically 1/2 in. in diameter. 

 Deformed bars are therefore useful for stirrups to permit larger 

 diameters, although the total quantity of stirrup steel required 

 throughout a beam is not changed except where the spacing 

 would tend to exceed the allowable. In other words, with de- 

 formed bars the stirrups do not need to be spaced as closely as 

 with plain bars. 



Recent tests show that either a right angle or a semi-circular 

 bend of 5 diameters is sufficient to stress the steel to its elastic 

 limit, provided the hook is well embedded in the concrete so that 

 it cannot kick out. To rely upon such increase in strength over 

 the straight stirrup, an embedment is required in all directions 

 equal to 8 diameters of the bar. 



1 From Taylor and Thompson's "Concrete, Plain and Reinforced," 2nd edition, page 454. 

 Copyright, 1905, 1909, by Frederick W. Taylor. 



