88 REINFORCED CONCRETE CONSTRUCTION 



It must be plain to the student that with the values recom- 

 mended by the Joint Committee for the compression in concrete 

 and the tension in steel, that a beam will reach its elastic limit 

 (determined by the concrete) before it fails (determined by the 

 steel). Of course, the preceding statement applies only when 

 the beam is adequately reinforced against diagonal tension and 

 bond stress. In choosing the above factors, the margin of safety 

 between the elastic limit and ultimate strength has received 

 consideration. 



The student should note that a working tensile stress of 16,000 

 Ib. per square inch is recommended by the Joint Committee 

 for all grades of steel. High working stresses in the steel involve 

 large distortions in the concrete, not only at the center of beam 

 but also diagonally near the end of beam. Low unit stresses in 

 the steel are greatly to be preferred on this account. It can also 

 be shown that very little is to be gained in economy by using 

 high stresses such as is often done with high elastic limit steel. 



In determining the relative working stresses in steel and concrete 

 some attention should be given to the question of repeated loads. 

 If the live load is a large percentage of the total load and subject 

 to frequent repetitions, a relatively low working stress in the con- 

 crete may well be employed in order to maintain elastic condi- 

 tions. As regards the steel, more perfect elasticity exists up to 

 a definite point, and hence repetition of load need not be con- 

 sidered in the selection of its working stresses. Low unit stresses 

 in the steel are to be preferred, in order to prevent excessive 

 deformations and reduce the liability of diagonal tension cracks. 

 The working stress of 16,000 should be the maximum for medium 

 steel. 



The "General Assumptions" of the Joint Committee's report 

 should be studied until the student feels that he can use his 

 judgment correctly in the selection of working stresses for any 

 given concrete and steel. The table of crushing strengths of 

 concrete given in Art. 10 should be employed in this connection. 

 A 1:2:4 concrete is considered to have an average ultimate 

 strength of 2000 Ib. per square inch, and the strengths of other 

 concretes should be proportional to this. If high carbon steel 

 is used a stress of 20,000 Ib. is frequently permitted. 



Age of concrete is another point frequently overlooked when 

 choosing working stresses. The stresses recommended by the 

 Joint Committee refer to the strength at the end of one month, 



