RECTANGULAR BEAMS 89 



BO that the factors of safety already mentioned with regard to 

 concrete are by no means the ultimate. 



The standard compression specimen adopted by the Joint 

 Committee is a cylinder 8 in. in diameter by 16 in. long. Most 

 of the important tests of concrete up to the present time have 

 been made on either 6 by 6 in. or 12 by 12 in. cubes. It is often 

 important to compare the strength of cylinders and cubes and 

 this can be done by means of the formula given in Art. 10. 



PROBLEMS 



22. A simply-supported reinforced concrete beam having 6 = 8 in. and d = 12 

 in. has a span of 15 ft. and sustains a uniform load (live plus dead) of 

 350 Ib. per foot. The beam contains four 1/2-in. square rods straight 

 throughout and properly spaced. What is the maximum bond stress 

 on these rods in pounds per square inch? 



23. Determine whether or not the beam of Problem 22 is of equal strength 

 in tension and compression, assuming the allowable working stresses to 

 be / = 16, 000 Ib. per square inch and f c = 650 Ib. per square inch. 

 Taken = 15. 



24. (a) What would be the maximum bond stress in Problem 22 if two 3/4-in. 

 square rods were employed? (b) If one 1-in. square rod was used? 

 (c) What fact is brought out by (a) and (b)? 



25. What is the maximum shearing stress in the beam of Problem 22? 

 Where does this maximum occur? 



26. Is web reinforcement needed in the beam of Problem 22 according to the 

 recommendations of the Joint Committee? 



27. If two of the rods in Problem 22 were bent up near the end of beam, 

 approximately what would be the maximum bond stress along the two 

 remaining rods? 



28. If you were designing a beam under the conditions of Problem 22 and 

 the depth (d) was limited to 12 in., what breadth of beam and amount 

 of steel would you consider satisfactory? 



29. A reinforced concrete structure is subjected to a constant repetition of 

 a live load which is large in comparison with the dead load. Explain 

 what you know pertaining to the proper working stresses for the same. 



30. A 1 : 2 1/2 : 5 concrete (see table on page 17) is to be used under average 

 conditions in a reinforced concrete beam. Medium steel. Determine 

 the working stresses to use. 



31. A 12-in. by 12-in. cube of a concrete, which is to be used in a reinforced 

 beam, has an ultimate strength of 3000 Ib. per square inch. High car- 

 bon steel. Average conditions. What working stresses would you 

 employ in design? 



32. A simply-supported beam is loaded with a heavy concentrated load 

 at a point one-fourth the span length from the left support. Con- 

 sidering horizontal steel, show the general directions of the diagonal 

 tensile stresses throughout the beam. 



