84 REINFORCED CONCRETE CONSTRUCTION 



The increase in strength as the loads approach the supports must 

 be due largely to the decrease in moment stress and consequent 

 distortion which is essentially what occurs when large areas of 

 steel and low working stresses are used." 



Tests on beams with web reinforcement show that the ultimate 

 average shearing strength may reach 300 to 400 Ib. per square 

 inch. The latter figure may, from our present knowledge, be 

 taken as about the maximum value with ordinary, closely spaced, 

 web reinforcement. 



Diagonal tension failure in a beam occurs suddenly. It is 

 the failure to be most feared and therefore should be most care- 

 fully guarded against. 



40. Working Stresses. The following working stresses were 

 recommended by the Joint Committee on Concrete and Rein- 

 forced Concrete in its first Progress Report presented early in 

 1909. This Joint Committee is composed of members selected 

 from the American Society of Civil Engineers, the American 

 Society for Testing Materials, the American Railway Engineering 

 and Maintenance of Way Association, and the Association of 

 American Portland Cement Manufacturers, and therefore 

 represents the highest authority in the United States. The 

 report on working stresses will be given in full. 1 The student 

 need not concern himself with the stresses in columns for the 

 present. 



"General Assumptions. The following working stresses 

 are recommended for static loads. Proper allowances for 

 vibration and impact are to be added to live loads where 

 necessary to produce an equivalent static load before apply- 

 ing the unit stresses in proportioning parts. 



"In selecting the permissible working stress to be allowed 

 on concrete, we should be guided by the working stresses 

 usually allowed for other materials of construction, so that 

 all structures of the same class but composed of different 

 materials may have approximately the same degree^ of 

 safety. 



"The stresses for concrete are proposed for concrete com- 

 posed of one part Portland cement and six parts aggregate, 

 capable of developing an average compressive strength of 

 2000 Ib. per square inch at 28 days when tested in cylinders 



1 The form given corresponds essentially with the 1909 Report of the Reinforced Concrete 

 Committee of the American Society of Civil Engineers. 



