474 



EEPOET 1886. 



(a) includes wind-bracing ; (J) at centre ; (<?) inseparable. 



Plate floors were adopted in all these bridges in order to enable a 

 comparison to be made between their weights, although in America 

 they would be replaced by heavy timber decks with diagonal bracing of 

 iron rods. Nos. 1 and 2 derive lateral stiffness entirely from the plate 

 floor, and owe their transverse stability to the connection of the verticals 

 to the cross girders by knee pieces, aided in the case of the bowstring 

 girder by light overhead stays at the centre. In Nos. 3 and 4 the depth 

 (18 feet) admits of a complete system of lateral bracing between the top 

 booms, so that the transverse stiffening is confined to the ends ; their 

 relative advantages are discussed at length by Mr. C. Bender in his recent 

 book on the Economy of Metallic Bridges,' and preference is given by 

 him to the Whipple truss. No. 5 is an ordinary type of American bridge ; 

 the lateral stiffness of the bottom is derived from the plated floor, and it 

 has top lateral bracing and transverse stiffening at the ends, as in Nos. 3 

 and 4. No. 6 represents an attempt to apply to the conditions of the 

 problem the principles enunciated by Mr. Max am Ende in a paper 

 recently read before the Institution of Civil Engineers.^ The girders are 

 polygonal ; lateral stiffness is derived entirely from the floor ; and trans- 

 verse stiffness is obtained by combining each of the cross girders with the 

 verticals and overhead stays into a rigid frame. No. 7 was suggested by 

 the central span of the Niagara Cantilever bridge, and has a wind system 

 similar to that of Nos. 3, 4, and 5. 



From these examples it will be seen that the difference in weight of 

 good designs of the same depth is comparatively trifling, and is not 

 greater than might be compensated by local considerations ; such as the 

 relative cost of labour and material ; facilities for erection ; and the differ- 

 ence in cost of various methods of construction ; while the fundamental 

 principle that the weight of a girder decreases as the depth increases is 

 generally applicable to an extent which, if recognised in theory by English 

 engineers, has not hitherto found general expression in their practice. 



The extra depth required for the rail-bearers in bridges with long 

 panels is, however, in England, where the headway is frequently very 



' The Principles of Ecmiomy in the Design of Metallic Bridges. 

 Bender. New York, 1885. 



* Minutes of Proceedings Inst.C.E., Ixiv. 243. 



By Charles B. 



