ON THE DESIGN OF GIKDEB BRIDGES. 473 



been more or less discarded in modern practice, and is replaced by lighter 

 structures, designed with a more scientific disposition of the material, 

 and constructed chiefly of angle and bar iron. 



Both in Germany and in America the modern practice appears to be to 

 adopt large panels and great depth — a system of construction which has 

 been greatly facilitated by the use of steel, owing to the greater length of 

 the bars and plates procurable in that metal, and the consequent diminu- 

 tion in the number of joints. 



In America, also, pin-jointed bridges are the rule, and riveted joints 

 the exception — exactly the reverse, in fact, of the English practice — and 

 the deep pin and link truss, with a straight top boom and long panels, 

 there almost the universal method of construction for spans over 75 feet, 

 •differs in a striking manner from both English and German bridges. Its 

 construction has been brought to considerable perfection, and appears to 

 be eminently suitable for a country where distances are great, labour 

 scarce and expensive, and rapid construction of the utmost importance. 

 By this system the Americans are able to turn out a bridge with the 

 greatest accuracy and expedition, and can erect it without previous erec- 

 tion in the shops, and with little staging, in an incredibly short space of 

 time. 



In England, on the other hand, a strong bias among engineers in 

 favour of riveted joints has led to the absence of special appliances for 

 the manufacture of pin and link bridges; and the practical advantages of 

 the system are less esteemed, as English engineers do not push forward 

 their railways so rapidly as the Americans. 



As most English railway engineers must now look chiefly to the 

 development of new countries for future work these general facts should 

 be sufficient ground for an examination of their practice ; but if more 

 definite reasons are sought, reference may be made to the case of Canada, 

 where English engineers, who built the first bridges, have since been 

 superseded by Americans. 



The design of a bridge of exceptional span is almost invariably the 

 subject of special study, to an extent which is inadmissible in the case of 

 bridges of ordinary size. These are usually constructed in accordance 

 with a limited number of standard types, which experience has shown to 

 be suitable ; and ic is to these only that such general considerations as 

 have been suggested properly apply. 



The economic importance of smaller bridges is also greater, and it 

 is to bridges of spans less than 200 feet, which, with reference to the 

 American system, may be termed ' merchantable ' sizes, that the scope of 

 this paper is therefore limited. 



In order to ascertain the extent to which the weight of such bridges 

 is affected by their design, seven designs were chosen for bridges of 

 140 feet span for a double line of railway. The designs were selected to 

 represent leading English and American types ; their strength was deter- 

 mined by a rolling load of If ton per lineal foot for each track : and their 

 weights were estimated, including flooring, rail bearers and cross girders, 

 and the necessary stiffening or bracing for wind pressure and lateral 

 oscillations. Particulars of the design and weight of each bridge are 

 given in the following table : — 



