Proceedings, &c. } for 1884. 167 



The comparative cost of a few types of single-line railway 

 bridges is : — 



Strutted Bridges ... ... £4 10s. per foot rise 



Timber „ ... ... 6 „ „ 



Iron ,, ... ... 50 



Yankee Cobweb <^ T '"" QK n 



[ Iron ... 35 





April 30th, 1884. 



Mr. Fraser read a paper on "The Web of Plate Girders," a 

 subject which has attracted very slight attention from scientific 

 men, but which, from its theoretical interest and practical import- 

 ance, is worthy of close study. 



The function of the web is to carry the weight of the load on 

 the girder to the abutments, hence every element must be acted on 

 by two internal inclined forces ; a tension directed along a line 

 sloping upwards and outwards, and a compression downwards and 

 outwards. It has been usual to design the web as if it were a 

 column merely under the action of the compression, quite ignoring 

 (except, perhaps, by implication) the assistance which it receives 

 from the lines of tension crossing those of the compression. 



Now, in an open girder loaded on the bottom member, when a 

 tension diagonal crosses a compression diagonal the stress in the 

 tie is always greater than that in the strut. Hence, treating the 

 web girder as a limiting case of an open girder, in which the 

 number of diagonals is indefinitely increased and their distance 

 indefinitely diminished^ it follows that the same statement must 

 apply to a web girder. 



Therefore any tendency of the web to bulge to either side on 

 account of the compression will be overcome by the tendency, 

 exerted by the tension in the web, to keep it in one plane, so that 

 the web cannot fail in this way, 



But if the web tend to crumple under the compression, the 

 tendency to straighten under the tension is less than the tendency 

 to further crumple, so that the web would fail. 



In order to prevent this crumpling', stiffeners are introduced. 

 But from what we have seen above, it follows that these need 

 only be of the very lightest angle iron, say 2^ x 2^ x J, and on 

 one side only of the web. An apparent exception is over the bed 

 plate, but in this place the l irons do not act as stiffeners, but as 

 true struts, and must be made accordingly. As a rule, the 

 stiffeners should be inclined downwards, and outwards at an angle 

 of 45°. 



