476 EEPORT — 1886. 



The Board of Trade have exercised since 1840 a statutory power of 

 inspecting arid testing bridges on railways for passenger traffic, and 

 have issued rules for the guidance of engineers in designing these struc- 

 tures, which have been generally accepted as applicable to all bridges, 

 and exercise a strong influence over English engineers in their use of 

 iron and steel in all permanent engineering structures either at home or 

 abroad. 



It is pi'ovided by these rules that the greatest strain produced by the 

 combined moving and dead load on any part of a structure shall not 

 exceed, for wrought iron five tons, and for steel six and a half tons, per 

 square inch. 



No defined quality or strength is required in order that either material 

 may be subjected to these stresses, but in the case of steel the engineer 

 must certify that it is possessed of considerable toughness and ductility, 

 and state the tests to which it has been subjected. 



It is also provided that the heaviest engines, boiler trucks, and travel- 

 ling cranes in use on railways shall be a measure of the load to which 

 bridges may be subjected. 



For convenience the strength of bridges is usually measured by an 

 assumed uniform load per foot run, intended to cover the weights of the 

 heaviest engines and trucks. [The manner in which this equivalent — 

 for the same actual load — varies with the span of the bridge, was shown 

 by a diagram.'] 



This diagram showed how important the rolling load becomes in small 

 girder bridges, and in the floors of large bridges — matters which have 

 certainly not been properly considered in numerous existing examples in 

 England. The determination of the greatest load to which a bridge may 

 be subjected, also afibrds an excellent example of the insufficiency of any 

 rules, however perfect, to relieve the engineer of a large amount of 

 responsibility. 



The origin of the rules may be traced in the Parliamentary Reports 

 of the Board of Trade, and of the Railway Commissioners who from 

 1846 to 1851 exercised the powers of the Board of Trade with respect to 

 railways. 



From these reports it does not appear that previous to 1849 the 

 inspecting officers had a defined rule for the strength of bridges either 

 of wrought or cast iron. 



In 1847, however, a Royal Commission was appointed in the following 

 terms : — 



' To enquire into the conditions to be observed by engineers in the ap- 

 plication of iron to structures subject to violent concussions and vibra- 

 tions ' and ' to endeavour to ascertain such principles, and to form such 

 rules, as may enable the engineer and mechanic to apply the metal with 

 confidence, and to illustrate by theory and experiment the action which 

 takes place under varying circumstances in iron railway bridges which 

 have been constructed.' 



The terms of this Commission clearly indicate that the interest of the 

 engineer was to be studied as well as the safety of the public. 



The Commissioners were Lord Wrottesley, then President of the 

 Royal Society ; the Rev. Robert Willis, M.A., an eminent mathematician ; 



' The results deduced from this diagram are given in the Appendix in Schedules 

 A and B. 



