ON THE DESIGN OF GIBDEE BRIDGES. 479 



tion of the Committee of 1877 appointed at the instance of the British 

 Association, allow in special cases the use of steel with a higher stress, 

 but exceptions of this nature are naturally ill-adapted to the design of 

 bridges of ordinary spans. 



The rules cannot be regarded as suited to the nature of the material, 

 and there can be httle doubt that they have operated to hinder the appli- 

 cation of steel to uses for which it is admirably suited, and have thus 

 exercised a prejudicial effect upon one of the leading industries of the 

 present day. 



Unless the rules which determine limiting stresses or coefficients for 

 iron and steel can be brought into conformity with modern knowledge of 

 the properties of materials, and of the laws by which their application to 

 construction should be regulated, their entire abolition would be prefer- 

 able, because it would conduce to the advancement of engineering science, 

 and the development of the bridge-building industries. The safety of the 

 public need in no respect be compromised by the abolition of the limiting 

 stresses, if the rules requiring the engineer to certify the quality of the 

 material used were retained (and extended to apply to iron as well as 

 steel) in order to provide the inspecting officer with all the information 

 requisite to enable him to judge whether the stress to which a structure 

 was subjected was within safe limits. 



Freed from the deadening influence of the fixed coefficients, private 

 enterprise would establish standard rules for the determination of the 

 stress to which different materials under varying conditions might safely 

 be subjected ; to the great advantage of the professions and trades in- 

 terested in bridge-building, and having in future to compete with the 

 Americans. 



On the other hand, there are many objections to such a course, which 

 would practically amount to a reversion, after the experience of thirty 

 years, to the conditions of 1850. It is also to be feared that during the 

 time which must necessarily elapse before any rules obtained the sanction 

 of a common assent, differences of opinion causing much inconvenience 

 would probably arise between civil engineers and the inspecting officers 

 of the Board of Trade — which is much to be deprecated. 



A course more worthy of the scientific attainments of English engineers 

 would be the amendment of the rules ; so that, while leaving to the 

 engineer the gi'eatest possible freedom in the choice of design and material, 

 and leaving in his hands the responsibility for the correct determination 

 of every effect of the loading of a structure which the most modern 

 methods render calculable, they should determine for his guidance by 

 coefficients based upon experience, or where practicable upon experimental 

 research, the proper allowance to be made severally for each of all those 

 effects which are usually understood to be covered by the present 

 arbitrary factor of safety. 



Rules so designed could not fail to exercise an elevating effect on the 

 professional knowledge and skill of engineers, by affording a more distinct 

 conception of the effects for which the factors of safety provide ; and by 

 abolishing the use of coefficients, of which neither the origin, scope, nor 

 intent is known to the user. 



The division of the factor of safety into many separate coefficients, 

 some of which would vary with the quality of the material and character 

 of the workmanship, would encourage good workmanship and the use 

 of materials of a high class, without restricting the use of materials of a 



