Reinforced-concrete structures being the most modern of all the general types of bridge construction, it is not to be expected that the economics of their designing should be so highly developed as in the case of any of the older types. Nevertheless much has been learned about the subject through the numerous investigations made for the author's firms by a number of young computers during the last twelve or fifteen years. Practically all that was known concerning it in 1915 was stated in Chapter LIII of "Bridge Engineering"; and an elaboration of that treatment will be given herein, supplemented by the results of a series of investigations made specially for this book.

The first topic for economic discussion is that of

Reinforcing Steel

At the present time one of the mooted points in the designing specifications for reinforced-concrete bridges is the proper intensity of working stress for the reinforcing bars. It is generally conceded that it should not exceed one-half of the elastic limit of the metal; and, in consequence, engineering practice in the past has limited it to 16,000 lbs. per square inch, but a number of manufacturers now desire to raise it to 18,000 lbs. per square inch by using a higher-carbon steel—notably re-rolled steel. Of course, the higher intensity saves in the quantity of steel, but generally increases the amount of concrete required; because the higher stress in the steel reduces the moment of resistance of the concrete about six per cent. If the amount of concrete is increased, the net saving is about two (2) per cent in slabs and three (3) per cent in beams; but if the section of the concrete is determined by shear or other considerations, so that no increase is necessary, those percentages will be increased by unity.

There are two grave objections to using the higher steel, viz.:

First. When bent cold it is liable to crack on account of its increased hardness, and

Second. It tends to open up cracks in the concrete.

For these reasons the author is not willing to use such steel in his practice; and, on general principles, he is opposed to the employment of any re-rolled metal, because of the temptation for the manufacturer thereof to run in all kinds of old materials—even worn-out Bessemer rails.

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