Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/384

B K I D G E 8 EXAMPLES. which on more than one occasion did actually threaten their entire destruction. The stiffening frames and gussets, which in an ordinary girder would have only been neces sary at the cuds, became therefore necessary throughout the whole length, and even the top and bottom were consider ably modified, as it is evident that while overhanging the pontoons on each end to the extent of 70 feet, the top, instead of being in compression was thrown into extension ; the weight of the tubes was consequently much increased by these arrangements. Again, they had to be raised by being suspended freely from four chains. Provision for this suspension from such limited attachment had also to bo made of a totally opposite character from that made for their vertical support when on their bed ; and, ultimately, when raised to their place, they remained no longer inde pendent beams, but were converted into continuous beams, parts before in tension being now thrown into compression, and vice versa; while the ends which were before subject to no horizontal strain were now exposed to greater strain than even the centre of the span. And, last of all, during the act of raising one of thess enormous masses, the press from which it was suspended burst, and one end of the beam fell through a space of no less than 9 inches on to a loose uneven heap of planks beneath it, bulging in the bottom plates, breaking all the castings, distorting seriously the sides and stiffening frames ; while the broken press itself, which descended from a height of about 100 feet above, broke through the top plates and completed the crippling of the whole section of support. It may surely be doubted whether anything but a tube could have stood such unexampled violence ; and in proportioning the parts of a structure destined for such usage, the mere considera tion of the strain to which as an ordinary beam it would be subjected, formed but a part of the problem ; no direct comparison can therefore be made between the weight of this bridge and an ordinary beam. If this were the case with the large spans, it is still more so with the small spans of 230 feet, which as simple beams would weigh only 230 tons each, whereas their actual weight is G50 tons. But it must be borne in mind that as regards the bridge itself these small spans were not required at all, and that they were merely designed and used as counterpoises for Fca. 121. Elevation of Bowstring Arcli, High-Levcl Bridge, Newcastle FIG. 125. Plan of Higli-Level Bridge, Newcastle. the large tubes, for the important purpose of converting them into continuous beams by their overhanging weight. By examining their detail, it will be found they are designed solely for this special purpose, their use as beams being made entirely subsidiary. " Some misapprehension exists on the object and import ance of the cells of which the top and bottom of these tubes is composed. These cells are rectangular, there being eight of them in the top and six of them in the bottom, and they run throughout the bridge. With respect to their import ance, it must ba observed that the whole section of the top of the Britannia tube at the centre is G4S 25 square inches, and of the bottom 5 85 43 square inches, and that the tube is 15 feet wide; the thickness of a single plate to ensure this section would therefore have been 2 7 inches for the top, and 2 - 3 inches for the bottom ; and had such a plate been procurable, nothing better could have been desired, and the cells would be unnecessary. Such a thing, however, is evidently impossible, and the engineer in this, as in numberless other details, had to adopt what he could obtain ; now the arrangement of the plates in cells is almost the only conceivable arrangement possible for obtaining the required section, allowing access, at the same time, to every part for construction and future maintenance. This alone led to their use in the bottom of the tube, where their form was totally unimportant. With respect to the top, however, it was of great importance, since thick plates could not be had, to ascertain the best form of cell for resistance to compression that could be devised with thin plates. A series of valuable experiments by Mr Eaton Hodgkinson led to the use of the rectangular cells as actually used, not because such form presented any peculiar advantage over any other form, as some have imagined, but because these experiments demonstrated that cells of that magnitude and thickness were independent of form, and arc crushed only by the actual crushing of the iron itself ; under these circumstances, the square cells were used as the best practical, method of obtaining the sectional area required. " Similar misapprehension also exists as to the considera

tions which led to the rectangular form of the tubes