Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/383

EXAMPLES. BRIDGES 335 tions, experiments and calculations on the strength and best form of tubes were made by Mr Williiun Fairbairn (Sir William Fairbairn) and Mr Eaton llodgkinson. la the course of the experiments it was found that the tube could be made self-supporting over the desired span of 4GO feet ; and in consequen.ee of this discovery the Conway and Menai tubular bridges were built, being the first great examples of properly designed girders. Some disputes arose as to the real inventor of these bridges. Sir William Fairbairn justly claimed the great merit of first perceiving that the girder might be self-supporting. Mr Hodgkinson had, perhaps, the smallest part in the design, but the shares of Fairbairn and Stephenson respectively cannot be very rigorously npportioned ; nor is this now of much conse- HALF SECTION SHEWING KEELSONS HALF SECTION SHEWING CUS2ETS FIG. 122. Britannia Bridge (Cross Section of Tubular Girder). quence. Both engineers were men of extraordinary merit, and co-operated in producing the great revolution in prac tice which has led to the adoption of the wrought iron girder as the most common type of bridge. The first train passed through the Britannia Bridge in 1850. The following description of the structure appeared in the 8th edition of the present work. It will be seen that this de scription is to some extent a defence of the design against criticisms asserting that the structure was unnecessarily heavy and costly. It is true that a considerably lighter bridge could now be built, but some prudence in intro ducing so great a novelty was certainly commendable. " The Britannia Bridge which carries the Chester and Holyhead Railway over the Menai Straits (figs. 1 22 and 1 23, and Plate XIX. iig. 1) consists of two independent continuous wrought iron tubular beams, 1511 feet in length, and weigh ing 4G80 tons each, independent of the cast-iron frames inserted at their bearings on the towers. They are 15 feet wide, and vary in depth from 23 feet at the ends to 30 feet at the centre. They rest on two abutments and three towers of masonry at a height of 100 feet above high water. The roadway is laid along the bottom, viz., one line of rails in each tube. The centre or Britannia tower, which is alto gether 230 feet high, is built on a rock in the middle of the Straits. The bridge has thus four spans, viz., two .spans of 4GO feet over the water, and two spans of 230 feet over the land. On each side the weight of a single span of 470 feet is 1587 tons, and of a span of 242 feet G30 tons. These tubes repose solidly on the centre tower, but repose on roller beds on the land towers and abutments. Now, these gigantic dimensions are by no means the only remarkable features in this work. The opponents of the Holyhead Road had imposed conditions on the Chester and Holyhead Railway which were thought insurmountable with respect to this bridge. The navigation was not to be interrupted- no scaffolding could thus be used and the clear height of 100 feet was to be retained throughout, - - " FIG. 123. Britannia Bridge (Part Elevation of Tubular Girder). arches being objected to unless the springing and not the centre was at this elevation. The tides set through this portion of the Strait with a velocity of 9 miles per hour, and the quiet water at each turn of the tide lasts but for a very short space of time. The tubes were designed to meet all these requirements ; they were so constructed at a considerable distance from their permanent site on the shores of the Straits ; they were floated upon pontoons upon these rapid tides to the base of the towers ; and they were then drawn up by hydraulic presses to their required height. They were here united through the towers by the insertion of shorter lengths, and ultimately brought into the condition of continuous beams as regards strain, by the means employed for their junction. It is evident such structures would be designed specially for such varied circumstances, for example : "As soon as they were completed on temporary platforms, these platforms were removed, and they became isolated beams ; the ends were accordingly strengthened with cast and wrought iron framing for this special object, and had they always remained there the sides might have been throughout considerably lighter than they are ; they now weigh nearly 40 per cent, of the whole weight. But in the next operation, that of floating, the tubes were liable to be supported at any point of their length, besides being sub

jected to chances of considerable distortion, and to disasters