TRANSACTIONS OP SECTION J. 445 



cut from the axle when new. Eetestnig specimens cut from 

 old structures does not show any measurable change in the 

 material. It is known, however, that the fracture of speci- 

 mens by repeated stresses in Wohler's machines, and that of 

 railway-axles from the fatigue occurring in ordinary work, are 

 short and crystalline, showing no signs of ductility ; hence the 

 reason given by Professor Unwin that fatigue is primarily a 

 loss of power of yielding in the particles near the plane of 

 weakness at which fracture occurs. Whatever the cause, the 

 facts shown in Table YIII. remain, and must be accepted as the 

 true basis of the determination of the safe working- stresses in 

 materials. 



The rule which still exists in the regulations of the British 

 Board of Trade for iron I'ailway-bridges is simply to limit the 

 working-stresses to 5 tons for tension and 4 tons for com- 

 pression. For steel the limiting stress is 6^ tons. No account 

 is taken of the variable range of stress. The rule obviously 

 giv^es excessive strength for those parts of bridges where the 

 range of stress is small, as in plate web box-girders of 150ft. 

 span, while it would be dangerous if applied to those parts of 

 structures where the range of stress is large, as in the longi- 

 tudinals and bracing of modern bridges. 



It may be stated, without fear of contradiction, that no 

 properly-trained engineer would use the British Board of Trade 

 rules in railway-bridges. In America and in Europe the prac- 

 tice has long been to make the working-stress depend upon the 

 range of stress. Thus, in the elevated railway at New York, 

 the flanges of the girders were designed for a stress of 3-6 tons 

 per square inch, the web-bracing for 3-4 tons per square inch, 

 and for members subjected to alternating stress 2 tons per 

 square inch. 



In the numerous elaborate specifications which have been 

 written by American engineers to govern the design of im- 

 portant railway-bridges, rules are given for limiting the in- 

 tensity of working-stresses in various parts of the structure 

 more or less in accordance with the results given in Table VIII. 

 In the Eoads and Bridges Department, New South Wales, 

 Mr. J. A. McDonald, M.Inst. C.E., M.Am.Soc.C.E.,. Engineer 

 for Bridges, has for some years limited the intensity of working- 

 stress in the various parts of the structures designed by him 

 in strict accordance with the results recorded in Table VIII. 

 Mr. McDonald has prepared Table VII. for general office use, 

 which gives the ultimate breaking-strength for various ranges 

 of stress as given by Launhardt's and Weyrauch's formulas. 

 Plate XIV. has been prepared to show the practice of the 

 Eoads and Bridges Department for fixing the intensity of 

 working-stress, compared with that sanctioned by the Board 

 of Trade. The stresses given on Plate XIV. are found from 

 TableVII. by dividing by the factor of safety, which is 3. 



