574 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 31 



QUALITY OF MATERIALS 



An important phase in the development of long span bridge build- 

 ing is the improvement of the materials, more particularly the intro- 

 duction of so-called high strength alloy steels, for high strength 

 material does not only effect a reduction in the dead weight which in 

 large bridges may mean a saving of millions of dollars, but it makes 

 possible certain structural members and connections of large propor- 

 tions which would be impracticable with ordinary steel. For riveted 

 members and connections a medium hard structural steel of from 

 55,000 to 70,000 pounds per square inch is still generally used for 

 ordinary bridges. 



About 25 years ago nickel steel, which has a strength of about 50 

 per cent greater than the ordinary steel, was introduced and found 

 increasing application in large bridges, as for instance in the Quebec 

 Cantilever Bridge, in the stiffening trusses of the Manhattan Bridge, 

 and also in a number of long simple span trusses. 



During and after the World War silicon steel entered the field in 

 sharp competition with nickel steel. Its strength is about 40 per cent 

 greater than ordinary steel or about 7 per cent less than nickel steel, 

 but it can be manufactured at a materially smaller cost than the 

 latter. In fact, its cost has now been so reduced that it can be used 

 economically even in bridges of medium size in place of ordinary 

 steel. The towers and floor structure of the George Washington 

 Bridge are built almost entirely of silicon steel. 



In the case of the Bayonne Bridge with its exceptional span of 

 1,675 feet, manganese steel was introduced for the heavy main arch 

 ribs. Its strength is equivalent to that of nickel steel, but its price 

 was slightly less. In this same bridge there was also used for the 

 first time manganese steel for the rivets with a strength of about 60 

 per cent in excess of that of ordinary steel rivets. 



For suspension bridges in particular the marked improvement in 

 quality of wire steel was of importance. Since the construction of 

 the Brooklyn Bridge in which steel wire of 160,000 pounds per square 

 inch strength was used for the first time in place of the earlier 

 wrought-iron wire, the strength of wire successively stepped up to a 

 new record in almost each new large bridge until it has now reached 

 a strength of nearly 240,000 pounds per square inch in the cables of 

 the George Washington Bridge. 



The question is frequently asked. What would be the maximum 

 practical length of span? The answer to this depends essentially 

 upon the quality of steel wire. With the quality now available it 

 would be structurally feasible to build suspension spans of up to 

 about 10,000 feet in length. Such a span, of course, would be ex- 

 tremely costly and probably nowhere justified financially. 



