44 TOWERS AND TANKS FOR WATER-WORKS. 



placed upon its presence, is as follows: "No line can be 

 drawn that shall be called the limit of safety, since no practi- 

 cal test has ever been devised which completely represents the 

 effect of incessant tremor. For common structural materials 

 the critical content has been placed at . 10 per cent, by general 

 consent, but this is altogether too high for railroad-bridge 

 work. All that can be said is that safety increases as 

 phosphorus decreases, and the engineer may calculate just 

 how much he is willing to pay for greater protection from 

 accident." 



To what extent specifications calling for reduction cf this 

 element affect the market price of materials is shown from 

 the following, taken from Prof. Pence's " Stand-pipe Acci- 

 dents and Failures": 



''A recent proposal for the construction of an important 

 stand-pipe in a Western city included bids according to five 

 limitations for phosphorus, running from 0.08 to 0.04 per 

 cent, inclusive. The relative bids on the superstructure for 

 the several grades of steel, taking that for the highest phos- 

 phorus limit as unity, were as follows: 



Phosphorus Limit. Relative Bid. 



O. O8 I -OO 



O.O/ 1.03 



0.06 1. 08 



0.05 I.I7 



0.04 1.23 



''The plates were to be 'soft, acid, open-hearth steel/ of 

 54,000 to 62,000 Ibs. per sq. in. in tensile strength; elastic 

 limit, 31,000 Ibs. persq. in. ; minimum elongation in 8 inches, 

 26$; minimum reduction of area, 50$; cold bent flat; and 

 not more than 0.08$ phosphorus, and less per cent, as per 

 detailed bid." 



Standard specifications for structural steel have been 

 adopted in the United States as follows: 



