220 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



fJlLY, 



the base, tapering to 55 feet by 32 feet at the level of the bottom 

 of the tubes; their liei^Iit is 190 feet from high water; they con- 

 tain '210 tons of cast-iron in beams and girders. Tlie chief dimen- 

 sions of the masonry, as well as of tlie tubes, will be seen upon 

 the engraving, which shows an elevation of the Carnarvon half of 

 the bridge. The scaffolding, both for tlie erection of the masonry 

 and for the support of the small tubes during their construction, 

 is of the same description as that employed at the Nelson Column, 

 TrafViIgar-square, London, and at the new Houses of Parliament. 

 It is that known to builders as "whole timber scaffolding;" its 

 name distinguishing it from the old kind, which was constructed 

 in a most cumbrous manner, with round poles and cords. The 

 timbers here are ''wliole balks" — logs of from 12 to IG inches 

 square, and some of them as long as 60 feet, and the method of 

 uniting them is such, that when taken down the timber is unin- 

 jured — nay, often increased in value from being more thoroughly 

 seasoned by exposure to the weather. Tlie scaft'old below the land 

 tube on the Anglesea side is, from the form of the shore, very lofty 

 — more than 100 feet in lii-ight at its outer end. As a piece of 

 construction it is most admirable; and from the multitude of cross 

 struts and bracing timbers, interlacing each other, its appearance 

 is very picturesque. That on the Carnarvon shore is not so lofty, 

 but is of the same e.xcellcnt construction. 



It will be seen presently tliat the wciglit which these scaifolds 

 will have to support, when the small tubes are completed, is about 

 1,300 tons. The fi-aming round the Britannia tower rises to the 

 vast height of nearly 2.50 feet, and at this elevation stones of many 

 tons weight are transported over all parts of the tower witli perfect 

 ease, by means of travelling windlasses, called gantries, which roll 

 upon rail laid on the top horizontal timbers. Notwithstanding the 

 constant and very heavy winds that it has been exposed to, this 

 scaflolding, which looks so light and slender, has stood hitherto 

 without injury. 



The four great Tubes. — The length of one of these tubes, as 

 constructed on the platform, is 472 feet; that is, 12 feet longer 

 than the clear span between the towers. This additional length is 

 intended to afford a temporary bearing of 6 feet at each end, after 

 they are raised into their places, until there is time to form the 

 connection between them across the towers. Our London i-eaders 

 will better appreciate tlie great length of these tubes by remem- 

 bering that if one of them were placed on end in St. Paul's Church- 

 yard, it would reach )07 feet higher than the top of the cross! 

 The span, in fact, is much greater than has ever before been at- 

 tempted, except in bridges on the suspension principle. The 

 length of the iron arch of Southwark Bridge, in London, the 

 largest rigid sjian in this country, is 240 feet. The height of the 

 tubes is not the same at all parts of their length. It is greatest 

 at the centre in the Britannia tower, where it is ,S0 feet outside, 

 and diminishes gradually towards tlie ends, at which, in the abut- 

 ments, the external lieight is only 22 ft. 9 in.; the top forms a 

 regular arch, (a true parabolic curve), and the bottom is quite 

 straight and horiiiontal. The clear internal height is, on account 

 of the double top and bottom, less by 4 feet than the external — 

 being 2G feet at the centre, and IS it. 9 in. at the extreme end. 

 The land tubes are outside 27 feet, and inside 23 feet high at their 

 smaller ends. The intermediate lieights will be seen on reference 

 to the figure. The internal width from side to side is 14 feet, 

 tlioiigli the clear sjiace for the passage of the trains is but 

 13 ft. .5 in.; the whole width outside is 14 ft. 8 in. The general 

 method of the construction of the tulies is readily seen. They 

 consist of sides, top and bottom — all funned of long narrow 

 wrought-iron plates, varying in length from 12 feet downwards, 

 and in width from 2 ft. 4 in. to 1 ft. 9 in. The direction in whiili 

 these plates are laid is not, as may be at first sight sup|ii)sed, arbi- 

 trary or immaterial, but is governed by the directions of the strains 

 in the different parts of the tube. Thus, in the toji and bottom, 

 where the strain is in the direction of the length of the tube, the 

 jilates are laid lengtliwise, whilst in the sides they are run up and 

 down. The plates, which are of tbe same manufacture as those 

 used for making boilers — "boiler-plate" — are of the best quality, 

 jirincipally from Staffordshire. They vary in thickness from i| to 

 ^ of an incli; some of tliem weigh nearly 7 cwt., and are among 

 tbe largest that it is jiossible to ndl with any existing machinery. 

 In the sides, tlie ]dates are alternately ft. (J in. and H ft. H in. in 

 length, 2 feet in breadth, and ^-incli thick, excejit at the ends, 

 where tliey become somcwliat thicker. They are joined together, 

 and greatly strengthened and stiffened at the joints by T-shaped 

 iron both inside and out, reaching from the top to the liottom, and 

 forming a complete pillar at every 2 feet, in the same manner as 

 the frames in a window-sash form a pillar between each two panes. 

 Jt is the projection of this T iron inside the tubes that reduces the 



clear space to 13 ft. 8 in. The longest plates are in the bottom. 

 They are 12 feet long, by 2 ft. 4 in. in w idth, arranged in double 

 layers, each layer being nine-sixteenths of an inch thick at the 

 centre of the large tube, and seven-sixteenths at the ends. Those 

 forming the top are each 6 feet in length, and 1 ft. 9 in. in breadth. 

 In the middle of the tubes they are J of an inch thick, diminishing 

 to gths at the ends. These thicknesses are all dependent on the 

 amount of the strains in the tube, as the direction of the plates 

 was before noticed to be. In the tiqj and bottom will be seen the 

 cells or flues of which slight mention has been before made. Of 

 these there are eight at the top, eacii 1 ft. 9 in. square; and six at 

 the bottom, of the same depth, but wider — viz., 2 ft. 4 in., from 

 their number being smaller. The vertical partitions which form 

 these cells are connected to the horizontal plates at top and bot- 

 tom, and the horizontal plates themselves to the sides by angle- 

 iron r", fitting into the corners, and rivetted through both. This 

 r iron neighs in the top 45 lb. per yard, and in the bottom 27 lb. 

 The connection between the top and bottom and the sides is made 

 much more substantial by triangular pieces of thick plate, which 

 are rivetted in across the corners. These are technically called 

 "gusset-pieces;" they are intended to enable the tube to resist the 

 cross or twisting strain, to which it w ill be exposed, from the heavy 

 and long-continued gales of wind that will assail it in its lofty and 

 unsheltered situation. They will be seen on the figure, showing 

 an end \iev/ of the tube. The rivets are placed in rows, at dis- 



tances of 4 inches apart in the top and bottom, and 3 inches in the 

 sides. They are rather more than an inch in diameter, and are 

 put into the holes red hot, and a head formed by beating up the 

 ]irojecting end of the hot rivet by heavy hammers, the operation 

 being finislied by a steel tool, with a cup-shaped end, which gives 

 the iiead of the rivet a round and neat appearance. In cooling 

 they contract strongly, and draw the plates together so powerfully, 

 that it reijuires a force of from 4 to G tons to each rivet to cause 

 the plates to slide over each other; this resistance being due solely 

 to the force of contraction, and independent entirely of the 

 strength of the rivet itself. Each of the large tubes contains 

 327,000 rivets, and the whole bridge about 2,000,000. In all cases 

 great care has been taken so to distribute the joints of tlie plates 

 that they may never come near together; and wherever they 

 occur, a thick plate is carefully rivetted over the joint on each 

 side, so as to maintain an equal strength on every part. The same 

 rule is observed at the joints of the angle-iron. The rivet-holes 

 in the plates are formed by a machine which punches out a piece 

 of iron the exact size of the re(piired hole. The plates are fas- 

 tened upon a sliding-table, which advances at every stroke double 

 the required space between each rivet; two punches then descend 

 through the plate and form the hides. In this way about 40 holes 

 are punched per minute. Kacli tube contains about 10 miles of 

 angle and T-iron, and the whole bridge 65 miles. The weight of 

 the wrought-iron in one of the large tubes is estimated at about 

 1,600 tons, of which 500 are in the bottom, 600 iu the sides, and 

 500 in the top. 



