M 



SCIENCE. 



[N.S. Vol. XVI. No. 392. 



44,000 pounds in 1893 to 60,000 pounds in 

 1901. 



The unusual amount of new bridge con- 

 struction required caused a general re- 

 vision of the standard specifications for 

 bridges, the effect of which was to in- 

 clude the results of recent studies and ex- 

 periment, and to eliminate some of the 

 minor and unessential items formerly pre- 

 scribed. 



Meanwhile another movement was in 

 progress. Experience having shown the 

 great advantage of more uniformity in 

 various details and standards relating 

 to the manufacture of bridges both in 

 reducing the cost and the time reqiiired 

 for the shop work, an effort was begun 

 to secure more uniformity in the require- 

 ments for the production and tests of steel, 

 which is the metal now exclusively em- 

 ployed in bridges. 



The American Section of the Interna- 

 tional Association of Testing Materials is 

 bringing together through its investiga- 

 tions and discussions a mass of selected 

 information on the relations of chemical 

 composition, heat treatment, mechanical 

 work, etc., to the physical properties of 

 steel as well as of other metals used in 

 structures and for mechanical purposes. 



The thorough digest of these results of 

 scientific research and practical tests, and 

 the preparation and adoption of standard 

 specifications for different classes of ma- 

 terial, are confidently expected to eliminate 

 many old requirements which are proved 

 inefficient in securing the results for which 

 they were originally intended, and to in- 

 corporate in the specifications only the 

 essential requirements by which the char- 

 acter of the product may be determined 

 with sufficient precision for its actual 

 duty. By making these requirements rea- 

 sonable and fair, on the one hand as sim- 

 ple and definite as possible without im- 

 pairing their real value, and on the other 



hand flexible enough to avoid imposing 

 undue hardship upon the manufacturer 

 who keeps in touch with the best methods 

 available, the result is confidently expected 

 to be a high degree of interested coopera- 

 tion on the part of both engineer and man- 

 ufacturer in securing the best grade of ma- 

 terial which the present state of science 

 makes practicable. 



The American section of that Associa- 

 tion in 1901 adopted a series of proposed 

 standard specifications, one of which re- 

 lates to steel for bridges and buildings and 

 which is recommended for adoption by 

 those who buy such structures. A commi1> 

 tee of the Railway Engineering and Main- 

 tenance of Way Association is now at work 

 on the same problem, a full agreement 

 having not yet been reached. 



With greater uniformity in the physical, 

 chemical and other requirements for steel, 

 as determined by standard tests, the unit 

 stresses to be prescribed for the design of 

 bridges will naturally approach to a corre- 

 sponding iiniformity. To what extent this 

 is desirable may be inferred from the fact 

 that the application of several of the lead- 

 ing specifications to the design of a railroad 

 bridge under a given live load yields 

 results which may vary by an • amount 

 ranging from zero to twenty-five per cent, 

 of the total weight. 



In the revision of specifications a decided 

 tendency is observed to simplify the de- 

 sign by making an allowance for impact, 

 vibration, etc., by adding certain percent- 

 ages to the live load according to some well- 

 defined system. It needs but relatively 

 little experience in making comparative 

 designs of bridges under the same load- 

 ing, to show the advantage of this method 

 over that in which the allowance is made 

 in the vinit stresses according to any of the 

 systems usually adopted in such a case. 

 Not only are the necessary computations 

 greatly simplified but the same degree of 



