252 STEEL REINFORCEMENT 



to form the trussed bar, as shown at / /, will also assist in 

 resisting such stresses, and, besides, will provide against 

 negative bending moment where tension instead of compres- 

 sion is created at g g. The line of fracture shown at e e is 

 typical of nearly all reinforced-concrete failures. 



METALLIC REINFORCEMENT 



CHARACTERISTICS OF METALS USED FOR 

 REINFORCEMENT 



Mild, or Soft, Steel. The commercial mild, or soft, steel, 

 which is an excellent material for reinforcing rods, especially 

 in concrete structures subjected to sudden strain or shock, 

 should have a unit ultimate tensile strength of from 52,000 

 to 62,000 Ib. and an elastic limit of not less than one-half 

 this amount. It should have an elongation of at least 25% 

 in 8 in., and should be capable of being bent cold through 180 

 and hammered flat on itself without evidence of fracture on 

 the outside circumference of the bend. 



Medium Steel. The grade of steel known as medium more 

 fully meets all the requirements of reinforced-concrete con- 

 struction than any other grade. Commercial medium steel 

 should have an ultimate unit tensile stress of 60,000 to 

 70,000 Ib., and an elastic limit of not less than one-half this 

 amount. Upon testing, the elongation should be found to 

 be at least 22% in a length of 8 in., and it should withstand 

 bending through 180 around a diameter equal to the 

 diameter or the thickness of the pieces tested. Such a 

 test piece should not show fracture on the outside circum- 

 ference of the bent portion. 



High-Carbon Steel. The name high-carbon steel is applied 

 to steels rolled particularly for the reinforcement of concrete. 

 Such steels contain a higher percentage of carbon than either 

 medium or soft steel. The product as a rule is brittle, and 

 possesses a unit ultimate tensile strength of from 80,000 to 

 100,000 Ib., with an elastic limit of about one-half of its 

 ultimate strength. The amount of elongation in 8 in. is 

 sometimes as low as 5%, and seldom exceeds 15 to 18%. 

 Ordinarily, it will not bend much beyond a right angle with- 



