134 REINFORCED CONCRETE CONSTRUCTION 



be specially computed. We will not consider such loading for 

 the present. 



The negative moment at the supports of continuous beams is 

 provided for by bending up a sufficient amount of horizontal 

 steel in the beams on each side of the support, and carrying it 

 across over the support to about the third point of the span. The 

 rods that are bent up for negative moment can be figured to take 

 diagonal tension. 



The moment at the supports being negative, the tendency is 

 for diagonal cracks to start at the top while farther along the 

 cracks tend to start at the bottom, as shown in Fig. 65. Stirrups 

 at points of negative moment should loop about the upper bars, 

 and at points of positive moment should loop about the lower 

 bars. The student should satisfy himself with regard to the 

 direction of the diagonal stress lines in such beams. 



PROBLEMS 



Make the best design of beam possible for the conditions given below. 

 Use / c = 600, / = 16,000, and the allowable unit stresses for shear and 

 bond as recommended by the Joint Committee. Beams are to be considered 

 as simply supported. p = 0.00675. k = 0.360. 7 = 0.880. 



40. Span 30 ft., load 800 Ib. per ft. (including wt. of beam). 



Take 6 =16 in. Use 6 plain round rods. Determine also the maxi- 

 mum deflection. 



41. Span 20 ft., load 1800 Ib. per ft. (including wt. of beam). 

 Take b = 16 in. Use 6 plain round rods. 



42. Span 15 ft., load 3200 Ib. per ft. (including wt. of beam). 

 Take 6= 16 in. Uss 7 plain square rods. 



43. Span 12 ft., load 5000 Ib. per ft. (including wt. of beam). 

 Take 6= 16 in. Use 10 plain round rods in two rows. 



44. Span 10 ft., load 8000 Ib. per ft. (not including wt. of beam). 

 Take 6= 16 in. Use 14 plain round rods and bend up six. 



45. Change the concentrated loads shown in Fig. 58 to 25,000 Ib. each, and . 

 design a beam for the conditions shown, but with the allowable unit 

 stresses as specified above. Consider the concrete to take its allowable 

 value of shear. Take 6 = 20 in. Include weight of beam in given loading. 



Note. In Problem 44 given above and in all slab and beam problems 

 which follow in this course, the student is required to make his designs so 

 that the assumed weight of beam checks within 10 per cent of the actual 

 weight. All designs must be submitted neatly drawn in ink with the ac- 

 companying computations. 



