216 REINFORCED CONCRETE CONSTRUCTION 



the above dimensions and with 0.7 per cent of steel, to be subjected to a 



1 200 000 

 bending moment of 1,200,000 in.-lb., or K = 77\3 = SS ' 2 The inter- 



section of the vertical and horizontal lines through these values respectively, 

 gives f c = 550 and/ 8 = 14,400. This procedure is followed in reviewing beam 

 design. 



Diagram 1 may be also employed to find minimum allowable depth of 

 beam for a given percentage of steel and various assumed widths, also to 

 find the amount of steel for a beam with given loading. The preceding 

 discussion and the discussion under Problem 1 of the preceding article should 

 make clear the method of procedure. 



2. Design a beam to span 10 ft. and to support a load of 4900 Ib. per foot. 

 Beam is assumed to be simply supported. 



We shall use Diagram 7 in the design of this beam. The weight of beam 

 will be assumed at 400 Ib. per foot. 



Assuming a width of beam of 14 in., the bending moment to use for one inch 

 in width is 



795,000 



=56,800 in.-lb. 

 14 



Selecting this value on the left-hand margin of Diagram 7, Part 3, and follow- 

 ing the horizontal line to the right, a depth (d) of 23 in. will give maximum 

 efficiency. This is shown by the fact that the horizontal line mentioned 

 above meets a line half way between d = 22 in. and d = 24 in. much closer to 

 the break in the curves than it does any other line representing depth. The 

 area of steel required, shown by the curved lines crossing the d lines, is found 

 to be 0.178 sq. in. per inch width or (0.178) (14) =2.49 sq. in. for the beam. 

 We shall select ten 9/16-in. round rods = 2.485 sq. in. The spacing of the 

 rods at the center of beam is shown in Fig. 56. This beam as designed con- 

 tains two rows of steel and the computed weight per foot is 

 (26KH)(150) = 3791b. 



JL44 



The assumed and calculated weights do not differ materially and the beam 

 as designed will be considered satisfactory. 



It should be noted that Diagram 7 may be employed for all cases cited 

 under Problem 1 except the case of finding the greatest unit stresses in the 

 steel and concrete due to a given bending moment. The student should 

 have no difficulty in determining the method of procedure. 



Diagram 1 may also be used in the above design. 



3. What safe load per square foot (including dead weight) can be sup- 

 ported by a slab 6 in. deep (d = 4 3/4 in.) and 10-ft. span reinforced with 

 1/2-in. round rods placed 8 in. apart? The slab is simply supported and 

 reinforced in only one direction. 



Referring to Diagram 6, Part 2, and tracing vertically from this value of p 

 on the lower margin to an intersection with the curve of d = 4 3/4 in., and 



