ON BRIDGES — BEAMS. 



149 



formly over it. If a great number of such beams were to be raised to 

 a considerable height, the small difference would probably be in favor 

 of the wrought iron. 



In cases therefore where a portion of the structure is much elevated, 

 where it is desirable to reduce the load on the foundations, and espe- 

 cially where wrought iron, in its simpler forms, as in tubes, bars, rods, 

 and plates, can be used, this material is entitled to a decided prefer- 

 ence over cast metal, and it will undoubtedly come gradually in gen- 

 eral use. 



The most interesting case of a large wrought-iron beam, in a scien- 

 tific point of view, which we have on record, is that of the Britannia 

 Tubular Bridge, built over the Menai straits, for the Chester and 

 Holyhead railway, in 1849, by Kobert Stephenson, C. E. 



Certain restrictions imposed by the Admiralty upon the construc- 

 tion of a bridge over this strait, induced Mr. Stephenson to decide 

 upon some form of beam which could be built on the shore, and then 

 raised into its place at an elevation of over one hundred feet — an ope- 

 ration which will be referred to in a succeeding lecture. The span of 

 the longest beam was to be 460 feet. 



At that time Mr. Stephenson, in common with the rest of the pro- 

 fession in England, considered the suspension bridge as a structure 

 entirely unsuited for railway purposes, and he was therefore required 

 to devise a bridge necessarily different from any existing examples. 

 After having abandoned the idea of a cast-iron arched bridge of pecu- 

 liar construction, he supposed that a wrought-iron hollow beam or 

 tube might be made, supported by chains at the central point, and he 

 called to his aid Mr. William Fairbairn, an engineer already much 

 distinguished for his various experiments on materials of construction. 

 Mr. Fairbairn undertook at once an extended series of experimental 

 investigations, beginning with the circular and elliptical tubes sug- 

 gested by Mr. Stephenson. 



Although direct experiment on small pieces had shown, as already 

 stated, that the resistance of wrought iron to compression was about 

 the same as to extension, these experiments soon showed that the 

 upper surface of the beams failed first, from a buckling or crimping of 

 the iron, owing to its flexibility, and pointed out the necessity for an 

 increase of material in the top. 



In short, a large number of experiments induced 

 Mr. Fairbairn to recommend the form of beam af- 

 terwards adopted ; a section of which is shown in 

 Fig. 42, where the material in the upper side bears 

 to that in the lower the proportion of 565 to 500, 

 and is so disposed to resist the crushing strain to 

 the best advantage. The cells or divisions shown 

 in the figure are made by introducing vertical iron 

 plates, and riveting to the horizontal plates through 

 angle irons in the corners, thus forming an upper 

 flange; which, as shown by the experiments, would 

 bear, without buckling, a strain approaching to the 

 experimental crushing strain of wrought iron. 

 The bottom of the bridge, since it resists only a 



Fig. 42. 



