REENFORCED CONCRETE 175 



54 520 

 each rod will be ' = 6815 lb., and hence the required area of 



each rod will be =0.68 sq. in., giving a rod slightly less 



than 1 in. in diameter. If the fillet under the head is neglected in 

 computing the shearing strength of the concrete, a spider made up of 

 eight 1-in. rods will still give sufficient area to develop the required 

 shearing strength. 



APPLICATIONS 



267. A plain concrete beam 6 in. x 6 in. in cross section, and with a 68-in. span, 

 is supported at both ends and loaded in the middle. The load at failure is 1008 lb. 

 Find the maximum fiber stress. 



268. A concrete building block 24 in. in length and having an effective cross 

 section of 8 in. x 10 in. minus 4 in. x 6 in. is tested by being supported at points 2 in. 

 from each end and loaded in the middle. The load at failure is found to be 5000 lb. 

 Find the maximum fiber stress, the height of the block being 10 in. 



269. A reenforced concrete beam 10 in. wide and 22 in. deep has four 1^-in. 

 round bars with centers 2 in. above the lower face. The span is 16 ft. The beam 

 is simply supported at the ends. Find the safe load per linear foot for a working 

 stress in the concrete of 500 lb./in. 2 , and also find the tensile stress in the 

 reenforcement. 



270. A reenf orced-concrete beam of 16 ft. span is 12 in. wide and has to support 

 a uniform load of 1000 lb. per linear foot. Determine the total depth and amount of 

 steel reenforcement required, bars to have centers 2 in. above the lower face of beam. 



271. A reenforced concrete beam 8 in. x 10 in. in cross section, and 15 ft. long, 

 is reenforced on the tension side by six ^-in. plain steel rounds. The steel has a 

 modulus of elasticity of 30,000,000 lb./in. 2 and the center of the reenforcement is 

 placed 2 in. from the bottom of the beam. Assuming that E c = 3,000,000 lb./in. 2 , 

 and p c = 600 lb./in. 2 , find the position of the neutral axis and the moment M. 



272. For a stress p c = 2700 lb./in. 2 on the outer fiber of concrete in the beam 

 given in problem 271, find the stress p s in the steel reenforcement. 



273. A concrete beam is 10 x 16 in. in cross section and 20 ft. long. It is reen- 

 forced with four |-in. steel rods with centers 2 in. above the lower face of the 

 beam. The safe compressive strength of the concrete is 600 lb./in. 2 , and the steel 

 used has an elastic limit of 40,000 lb./in. 2 What single concentrated load will the 

 beam carry at its middle ? What tension will be developed in the steel ? What 

 shearing stress along the reenforcement ? 



274. Find what load, uniformly distributed, the beam in the preceding prob- 

 lem will carry and find the tension in the steel and bond for this case. 



275. A reenforced concrete floor is to carry a load of 200 lb./ft. 2 over panels 

 14 ft. square. Find the required thickness of the slab and the area of the reenforce- 

 ment for working stresses of 500 lb./in. 2 in the concrete and 15,000 lb./in. 2 in the 

 reenforcement. 



276. Design a floor panel 14 ft. square, to be made of reenforced concrete and 

 to sustain a total uniform load of 120 lb./ft. 2 , with a factor of safety of 4. 



