620 ENGINEERING MATERIALS. CHAP. XV. 



details of manufacture. It is impossible to predict the properties of an alloy from the properties 

 of the metals forming it. Many alloys are sold under trade names in which the properties depend 

 both on the proportions of the ingredients and upon the details of manufacture. The most im- 

 portant alloys used by the structural engineer are as folbws: 



Brass is an alloy of copper and zinc in which the copper varies from 60 to 89 per cent, and 

 the zinc from 40 to 1 1 per cent. A small amount of tin is sometimes added to make the brass more 

 easily worked. The tensile strength of brass is greatest (about 50,000 Ib. per sq. in.) when the 

 composition is about 62 per cent copper and 38 per cent zinc; and the ductility and malleability 

 are greatest when the composition is about 70 per cent copper and 30 per cent zinc. A widely used 

 brass has f copper and i zinc. 



Delta metal is brass with I to 2 per cent iron. The tensile strength of delta metal is about 

 45,000 Ib. per sq. in. 



Tobin bronze is brass with I to 2 per cent iron, and small amounts of lead and tin. 



Bronzes are alloys of copper and tin or of copper, zinc and tin, and usually have small quan- 

 tities of other metals. Bronzes having more than 24 per cent tin are too weak to be used. The 

 tensile strength is greatest (23,000 Ib. per sq. in.) when the composition is about 80 per cent copper 

 and 20 per cent tin. 



Phosphor bronze is an alloy of copper and tin containing i to i per cent phosphorus. It makes 

 excellent castings and is very hard. The ultimate tensile strength varies from 50,000 to 100,000 

 Ib. per sq. in. 



Aluminum bronze is an alloy having 5 to 10 per cent aluminum and 95 to 80 per cent copper. 

 The tensile strength varies from 75,000 to 100,000 Ib. per sq. in. 



Manganese-bronze as specified by the American Society for Testing Materials contains, 

 copper 55 to 65 per cent, zinc 39 to 45 per cent, iron not over 2 per cent, tin not over 2 per cent, 

 aluminum not over 0.5 per cent, manganese not over 0.5 per cent. The ultimate tensile strength 

 of standard test pieces cut from manganese-bronze ingots shall not be less than 70,000 Ib. per sq. in., 

 with an elongation in 2 in. of not less than 20 per cent. 



TIMBER. For definitions of terms, standard def cts, specifications and allowable stresses 

 in timber, see Chapter VII. 



STONE MASONRY. For definitions of terms used in masonry construction and for speci- 

 fications for different classes of stone masonry, see Chapter VI. 



For the allowable pressure on masonry, see Table IV, Chapter V, and for the weight, specific 

 gravity and crushing strength of masonry, see Table V, Chapter V; also see Table VIII, Chapter 

 II. For an exhaustive treatise on brick and stone masonry see Baker's " Masonry Construction." 



CONCRETE. The average strengths of different mixtures of Portland cement concrete as 

 given in Report of the Committee on Reinforced Concrete of the American Society of Civil 

 Engineers, 1913, are given in Table II. 



TABLE II. 



STRENGTH OF PORTLAND CEMENT CONCRETE. 



Aggregate 1:1:2 i:ij:3 1:2:4 1:2^:5 1:3:6 



Granite, trap rock 3300 2800 2200 1800 1400 



Gravel, hard limestone and hard sandstone 3000 2500 2000 1600 1300 



Soft limestone and sandstone 2200 1800 1500 1200 1000 



Cinders 800 700 600 5 4 



Specifications for concrete are given in Chapter V, and specifications for reinforced concrete 

 are given in Chapter VI. 



Working Stresses. The following working stresses have been recommended by the American 

 Railway Engineering Association for concrete that will develop an average compressive strength 

 of at least 2000 Ib. per sq. in. when tested in cylinders 8 in. in diameter and 16 in. long and 28 days 



