228 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[July, 



thickness of 4} in., and 32.i ft. long between bearings. This girder broke 

 soon after the bridge was completed, but was replaced again without increase 

 of weight. On the Midland Counties line the parallel girders were assisted 

 with side rods of wrought iron, their ends being passed through cast iron 

 lugs, on the upper edge of the girder, aud the middle bent over a stud cast 

 on the lower edge in the centre, the superstructure or roadway being gene- 

 rally planking, but where the girders were elliptic, the planking was placed 

 on the lower flanch, and the back of the girder stood above the roadway. 

 On the Great Western line the space between the girders was filled in with 

 9 in. arched brickwork, the girders being placed about 4i ft. apart. On the 

 Pollack and Govan Railway, the Glasgow and Edinburgh, Grand Junction, 

 and Leeds and Selby, the bridges were of the ordinary form. 



The weight to be carried over a bridge or lailway should be calculated for 

 two trains passing over at the same time, and for a turnpike road the passage 

 of troops is considered the most severe test, but in practice one of the ribs 

 should be calculated to hear the estimated weight which it is presumed will 

 pass over the bridge. In the Park-street bridge previously described, of 

 95 ft. span, an area of 50 superficial inches is given to the rib ; and in No. 7 

 of the first table of experiments, for a span of 28 ft., with 10 tous the 

 deflection was J of an inch, the sectional area being 38 in., evidently showing 

 the Park-street bridge abundantly strong. By a reference to the unpublished 

 experiments on cast iron rails, you will see that the (perceptible) deflection 

 of cast iron commences at about one-fourth of its breaking weight, so that 

 the girder No. 7, with an area in section of 38 in. would be capable of sus- 

 taining a weight of 40 tons. The girder of the bridge that broke on the 

 Great Western Railway at the Uxbridge road had an area of nearly 176 in., 

 with a span of only 32 ft. 



By reference to the paper in the Transactions of the Royal Society it will 

 be seen that wrought iron will bear nearly as much again as cast iron before 

 crushing, and by reference to Wood's experiments on rails, it will be observed 

 that it remained elastic up to five-sixths of the weight that destroyed its 

 elasticity, and the deflection commenced with one sixth part of that weight ; 



therefore, notwithstanding the greater power of its resistance to a crushing 

 force, the greater elasticity renders it inapplicable to the purposes of bridge 

 building on the plan of an arch. 



I have been induced to extract the table from Wood's work on railways, 

 to see whether wrought iron was ap])licable for bridges constructed on the 

 common plan, having lately seen curved ribs of that material formed of rail- 

 way bars acting as the suppott of the "lagging" of centreing, aud as Mr. 

 Pritchard (who introduced cast iron), in one of his earliest propositions on 

 the subject, suggested cast iron ribs as a permanent centre to a back arch of 

 masonry (Weale's Bridges, page 100). 



From what has been said, I do not think wrought iron can be applied as 

 I thought it could to large bridges. In the preceding examples of construc- 

 tion are the following different plans. The principle of the arch fully de- 

 veloped, as in Sunderland Bridge ; arched ribs on the common plan, as Vaux- 

 hall and Southwark Bridges ; the French tubular rib ; the bow and string, as 

 Monk Bridge, Leeds ; and four descriptions of girders, viz. parallel, para- 

 bolic, parabolic curved, or one whole elevation as a phase, and the parallel 

 assisted with tension rods ; also an adaptation of the bow and string of three 

 varieties, viz. the Ijridge over Regent's Canal, on the road from Baubuiy to 

 Lutterworth, on the Birmingham railway, and the Cheltenham bridge on the 

 Gloucester and Birmingham railway, being a total number of thirteen diffe- 

 rent constructions, many being used to suit localities and confined situations; 

 but for situations where no restrictions exist, the girder is the best form up 

 to 30 ft. span, the arched bow and string to 60 ft., the common arch to 150 

 feet, the framed voussoir, as Sunderland bridge, to 250 ft. and all spans 

 larger. 



As to what sliould be the sectional area when the common method is 

 practised, the following particulars, collected from executed examples, may 

 be found useful : say 25 ft. span, 30 superficial inches ; 50 ft., 40 ; 100 ft., 

 110 ; 130 ft., 165 ; and the proportion of the breadth of the rib to its depth, 

 one-fifteenth, say 2^ inches breadth for 36 inches depth. 



With the first weight the elasticity remainecl, with the second weight it was destroyed. 



