154 REINFORCED CONCRETE CONSTRUCTION 



To give good space for the steel and still obtain satisfactory proportions 

 for the cross-section, we will select b' = 18 in. and d = 47 in. for a preliminary 

 value. A thickness of flange of 12 in. will be tried. For this thickness 



/ 12 



j- = Ty =0.256. Then, by means of formulas (1), (4), and (5) of Art. 59, 



we find that k = 0.379 and /d = 42.0 in. Also, from formulas (7), a, = 15.7 

 sq. in. and 6=48 in. 



To illustrate the effect of varying proportions, calculations will also be 

 made for a flange thickness of 8 in., 10 in., 14 in., and 16 in. The results 

 are as follows: 



It should be observed that the amount of concrete is less the thinner 

 the slab and that the effect of variation of t upon the amount of steel is 

 very small. However, a relatively thick flange is desirable considering 

 the fact that the girder is not a part of a floor system and that the flanges 



FIG. 74. 



are unsupported at their outer edges. The 12-in. flange will be adopted. 

 The dead load assumed is considerably on the safe side, but the design 

 need not be changed. 



The steel area required is 15.7 sq. in. Five round rods 1 3/8 in. diameter 

 and seven round rods 11/4 in. diameter, giving a total area of 16.0 sq. in., 

 will be used. To provide sufficient spacing between rods, the rods will 

 be placed in three rows as shown in Fig. 74. The five 1 3/8-in. rods will 



