REINFORCED CONCRETE 331 



at the top is found by formula 2. As the compressive steel is 

 2 in. from the top of the beam, t = kd 2, and taking the value 

 fe = .32 from the table of properties of reinforced-concrete 

 beams for = .00499 and n=15, t = .32X18 - 2 = 3.76, and 

 y = d-kd= 18- 5.76 =12.24. Then, 



12.24 



A t = X1.93 = 6.28sq. in. 



3.76 



FORMULAS FOR T-SHAPED BEAMS 



When a slab and the beam supporting it are so constructed 

 as to form a monolith, the slab may be considered as a part of 

 the beam. Conservative practice requires that the width of 

 the slab that may be considered as 



acting with the beam should not ^ ? 



exceed one-fourth the span of the 

 beam; it should also not exceed 

 four times the thickness of the slab. 



When the neutral axis does not 

 fall below the bottom of the slab, 

 the beam may be designed as a 



rectangular beam, having a section abed, as in the accompany- 

 ing illustration. 



When the neutral axis falls below the bottom of the slab, 

 the following approximate formula may be used: 



In this formula, t is the thickness of the slab, and the other 

 letters have the same significance as before. From it the area 

 of steel required may be determined. To insure that the con- 

 crete is not overstressed, the maximum allowable unit stress 

 should not exceed 



p.. 2M 



In these two formulas, the compressional area of the stem is 

 neglected. They should, therefore, not be used when the stem 



