Standard nickel steel having an elastic limit of 60,000 lbs. per square inch is worth 7.3 ¢ per lb. while carbon steel costs 5.1 ¢. What are the economics of employing it for building a double-track, simple-truss span of 550 feet? Here we have

r = 7.3 ÷ 5.1

r' = 35 ÷ 60

and r r' = 7.3/5.1 X 35/60 = 0.835

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Fig. 5a gives 1.01 as the limit; hence the use of the alloy would effect considerable saving.

Example No. 4

In the investigation for a proposed three-span, cantilever, railway bridge having a main opening of 1,550 feet, it was found that an alloy steel of 75,000 lbs. elastic limit would cost 13.8 ¢ per lb. erected, while standard carbon