20J< REPORT — 1873. 



vary directly as the strength of the material employed in their consti'uction -when 

 the proportion of depth to span and all other circumstances remain the same. 

 We know also that, taking an ordinary form of open wrought-iron detached girder 

 (as, for example, -when the depth is one fourteenth of the span), the limiting span 

 in iron, with a strain of five tons to the inch upon the metal, is about GOO feet ; 

 and it follows that a steel girder of like proportions, capable of bearing eight tons 

 to the inch, would have theoretically a limiting span of 9C0 feet. 



This theoretical limiting span of 960 feet would, however, be reduced by some 

 practical considerations connected with the minimum thickness of metal employed 

 in certain parts, and it would, in effect, become about 900 feet for a girder of the 

 before-mentioned construction and proportions. 



The knowledge of the limiting span of a structure, as has been explained else- 

 where, enables us to estimate very quickly, and with close approximation to the 

 truth, the weight of girders required to carry given loads over given spans ; and 

 although the limiting spans vary with ever}' form of structure, we can obtain an 

 idea of the effect of introducing steel by the relative weights of steel and iron 

 required in girders of the kind above mentioned. 



v^ssuming a load, in addition to the weight of the girder, of one ton to the foot, 

 the relative weights under these conditions would be as follows : — 



Again, taking such a case as that of the Menai Bridge, which consists of two 

 spans of 500 feet over the navigable waterway. 



This structure is composed of four ^vl■cught-iron tubular girders, each weighing 

 about ]500 tons, or 6000 tons in all; and in order to avoid the difficulties of 

 scaffolding, each of these tubes was built on the shore, floated oft' on pontoons, and 

 lifted bodily into its place by hydraulic machineiy. 



This great work was erected when the application of ^\TOUght iron to engineering 

 works was in its infancy, and when wrought iron was the only available material 

 for such a purpose. 



With such materials only at command, and in the then state of knowledge of 

 structures, the aceompliahment of this bridge, capable as it is of carrying railway 

 trains across clear spans of 500 feet, was an achievement far in advance of the 

 time in which it was done, and worthy of the nanre of its great designer, Robert 

 Stephenson. 



But if this work liad to be constructed now, and were made an open girder of 

 steel instead of plate iron, the weight of metal required would be little more than 

 one third of that used, and the cost of erection, the time required for its execution, 

 and the total cost of its construction would be most materially reduced. 



It is not alone in tlie relative weight or in the relative cost that the advantage 

 of the stronger material is important, but with steel we shall be enabled to cross 

 openings which are absolutely impracticable in iron. 



It will naturally be asked why it is that steel is not used in these structures, if 

 such manifest advantages would result from its employment. 



The reason is twofold : — 



1st. There is a want of confidence as to the reliability of steel in regard to its 

 toughness and its power to resist fracture from sudden strain. 



2nd. Steel is produced of various qualities, and we do not possess the means, 

 without elaborate testing, of knowing whether the article presented to us is of the 

 required quality for structural purposes. A third reason, arising probably out of 

 those before mentioned, is found in the fact that in the regulations of the Board of 

 Trade relative to railway structures, although rules are given for the employment 

 of cast iron and wrought iron, steel has not, up to the pi'esent time been recog- 

 nized or provided for. 



