474 BEVIEWS — EEPOET ON VICTOBIA BEIDGE. 



Bridge, .and other methods of constructing iron superstructures, is 

 exceedingly interesting : 



" At present there may be regarded as existing three methods of constructing 

 wrought-iron girders or beams for railway purposes. 



First, — The Tubular Girder, or what is sometimes called the Box Girder, when 

 employed for small spans, with which may also be named the Single-ribbed 

 Girder, — the whole belonging to the class known as ' Boiler Plate Girders' 



Seconb, — The Trellis Girder, which is simply a substitution of iron bars for the 

 wood in the trellis-bridges, w.iich have been so successfully employed in the 

 United States, where wood is cheap and iron is dear. 



Third, — The Single Triangle Girder, recently called ' Warren,' from a patent 

 having been obtained for it by a gentleman of that name. 



Now in calculating the strength of these different classes of girders, one ruling 

 principle appertains, and is common to all of them. Primarily and essentially 

 the ultimate strength is considered to exist in the top and bottom, — the former 

 being exposed to a compressive force by the action of the load, and the latter to 

 a force of tension ; therefore, whatever be the class or denomination of girders, 

 they must all be alike in amount of effective material in these members, if their 

 spans and depths are the same, and they have to sustain the same amount of load. 



Ou this point I believe there is no difference of opinion amongst those who 

 have had to deal with the subject. Hence, then, the question of comparative 

 merits, amongst the different classes of construction of beams or girders, is really 

 narrowed to the method of connecting the top and bottom webs, so called. In 

 the tubular system, this is effected by means of continuous plates riveted toge- 

 ther ; in the trellis girders, it is accomplished by the application of a trellis- 

 work, composed of bars of iron forming struts and ties, more or less numerous, 

 intersecting each other, and riveted at the intersections ; and in the girders of the 

 simple triangular, or ' Warren' system, the connection between the top and bottom 

 is made with bars, — not intersecting each other, but forming a series of equilateral 

 triangles, — these bars are alternately struts and ties. 



Now, in the consideration of these different plans for connecting the top and 

 bottom webs of a beam, there are two questions to be disposed of; one is — which 

 is the most economical? and the other — which is the most effective mode of so 

 doing? But while thus reducing the subject to simplicity, it is of the u'most im- 

 portance to keep constantly in mind that any saving that the one system may 

 present over the other is actually limited to a portion, or per centage, of a subor- 

 dinate part of the total amount of the material employed. 



In the case now under consideration, namely, that of the Victoria tubes, the 



total weight of the material between the bearings is 242 t$>ns, which weight is 



disposed of in the following manner: 



Tons. 



Top of Tube 18 



Bottom of Tube 92 



—168 



Sides of Tube 84 



Total tons 252 



Assuming that the strain per square inch, in the top and bottom, is the same 

 for every kind of beam, — say four tons of compression in the top, and five toii.s ol 



