38 REINFORCED CONCRETE CONSTRUCTION 



PROBLEMS 



6. A steel bar of 20-in. length and a cross-section of 2 1/2 sq. in. is put in 

 tension. The force applied is 40,000 Ib. and the extension under this 

 load is 0.011 in. Determine the modulus of elasticity of this steel in 

 pounds per square inch. 



7. If steel costs 40 per pound with a working compressive strength of 

 16,000 Ib. per square inch, and concrete costs 300 per cubic foot with a 

 working compressive strength of 450 Ib. per square inch, what will be 

 the relative cost of the two materials in members sustaining purely 

 compressive stresses? 



8. A plain concrete retaining wall has a height of 20 ft. and a width of 8 ft. 

 at the base. The front face has a batter of an inch to the foot or 1 : 12. 

 (a) Compute the number of cubic yards in 1 ft. length of wall, (b) 

 Compute the approximate amount of cement, sand, and broken stone 

 required the mixture being 1:21/2:5. (c) What is the approxi- 

 mate weight of wall? 



9. What change of length will tend to take place in a long plain concrete 

 wall by a drop of temperature of 50 Fahr.? How do you think the 

 disfiguration of such a wall by irregular cracks may be prevented? 



10. The allowable tensile stress in a 1-in. plain round steel rod is 16,000 Ib. 

 per square inch and the working bond strength is 80 Ib. per square inch. 

 What length of embedment is necessary to develop the allowable stress 

 in the steel? What would be the length of embedment if a 1/2-in. 

 plain round rod had been employed? 



11. Explain how it is that a constant value of n, equal to 15, may be used in 

 the computations for the design of beams and columns. 



12. At a given point in the tensile part of a reinforced concrete beam, the 

 ultimate tensile strength in the concrete of 200 Ib. per square inch is 

 reached. Approximately what is the corresponding tensile stress in 

 steel located at about this point? 



13. A load is a lowed to pass over a reinforced concrete beam which is in 

 amount about 65 per cent of the ultimate load. About how many 

 repetitions of this load will cause failure? 



14. A long reinforced concrete wall is subjected to a drop of temperature of 

 50 Fahr. Consider 160 Ib. per square inch the ultimate strength of 

 the concrete and 40,000 Ib. per square inch the elastic limit of the steel, 

 (a) What stress is caused in the steel by the drop of temperature in the 

 steel itself considering the ends of the steel as fixed? (b) How much 

 more stress can be put into the steel before stressing it beyond its 

 elastic limit? (c) If the steel is to be stressed just to the elastic limit, 

 what percentage of steel is necessary to prevent large contraction cracks 

 in the concrete? 



15. Assuming the steel rods in Problem 14 to be 1/4 in. in diameter with an 

 ultimate bond strength of 400 Ib. per square inch, how far apart would 

 the minute cracks occur? 



