IS* 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



L-Mat, 



Hence the great advantage in cast-iron, of using hollow pillars or tubes in 

 place of solid metal, whereby, with the same area or section of fracture the 

 diameter of the pillar is incr'easeil, and with it the resistance to flexure, and 

 an increase of strength in proportion to the length. A solid pillar, for in- 

 stance, G inches in diameter, if extended to 7^ feet in 'length, would be 

 weakened one-half, but if cast hollow, 10 inches in diameter and ;} inch 

 thicli, giving the same weight of metal per foot in length, it might then be 

 extended to 12.] feet, and still possess the same strength as the other. In 

 all these cases a remarkable circumstance was observed in regard to the 

 mode of applying the strain. M'ith the ends of the pillar turned flat, and a 

 flat plate interposed at top and bottom, which is the case in supporting 

 buildings, this was found to sustain nearly three times as much as when the 

 pillar was rounded on the ends, so as make the force pass directly through 

 the axis, as occurs so frequently in machinery with the connecting rods of 

 steam-engines, and in other cases. The effect of the length of pillars in 

 weakening the strength was illustrated by a striking experiment with a spiral 

 wire, quite flexible, yet, when set up as a pillar, and tied in the middle 

 laterally, with slender threads, carried a weight of 50 lb., and would have 

 carried'much more, but the moment the threads were cut, the wire gave way 

 by flexure, and oversetting the balance, the weight immediately sunk. 



In regard to the Transverse Strain, he had already explained the nature of 

 this compound action, and particularly the manner in which, under it, the 

 beam becomes exposed at once to the effects of tension and compression, 

 the one side being distended and the other compressed. On this most in- 

 teresting and important subject he had still much to «ay, but would defer it 

 to another evening, as the time was short, and he was anxious to proceed 

 with another part of the paper which had been particularly referred to, 

 namelv, the subject of the tubular bridges. 



The application of malleable iron had been already used in the shape 

 of tension-rods in cast-iron girders, and was applied, as we have seen, in 

 the high level bridge at Newcastle ; but the application of girders con- 

 structed of malleable iron alone is a new idea. It Las been applied on 

 railways in the case of skew bridges of wide opening and limited depth 

 between the railway and the road ; in these cases the girder consists of a 

 rectangular hollow tube or square box, extending over the whole span, 

 and of such depth as can be attained. These have hence received the 

 name of Tubular Bridges, and have excited much attention since the grand 

 experiment has been determined on, of trying these structures on such a 

 magnificent scale as is now in progress of execution in the crossing of the 

 Straits of Menai by the Britannia Bridge, and the estuary of the Conway 

 by the Conway Bridge, and which form, without doubt, the most remark- 

 able engineering enterprises of the present day. These spots, as is well 

 known, had already been the scenes of vast engineering operations con- 

 nected with the suspension bridges of Telford to form the great turnpike 

 road communication from the metropolis to Holyhead, and thence across 

 the channel to Dublin; and when it was determined that this communica- 

 lion should be superseded by railway, it became a matter of most serious 

 consideration how these two openings were to be spanned, keeping in view 

 tlie new conditions of stability required for railway traflic ; and the subject 

 having been remitted to Mr. .Stephenson, the engineer of the line of rail- 

 way, namely, the Chester and Holyhead, he at once rejected the principle 

 of the suspension bridge as inapplicable, owing to the undulations to which 

 it was liable, and which bad been proved by practice in a similar bridge 

 for a railway across the Tees, to be both inconvenient and dangerous. 

 How far the principle might have been modified by the introduction of 

 proper ties and braces may be a question ; but iu a case of such vast mag- 

 nitude and importance there might still have been risk, and, on the 

 maturest consideration, Mr. Stephenson determined to recommend the 

 simple and bold design of a hollow rectangular tube of malleable iron, 

 consisting of thin plates rivetted together, such as he had already tried 

 with success on a smaller scale upon railway bridges, and which he con- 

 ceived was the best form for securing not only strength, but sufficient sta- 

 bility and stifl'uess to prevent any undue oscillations or vibrations. To 

 carry out this plan, the assistance of the first authorities, scientific and 

 practical, on the strength of materials was called in, and to Messrs. 

 Hodgkinson and Fairbairn the duty was remitted of trying the efl'ect with 

 experimental tubes on a small scale, and finally on a model one-sixth of 

 the dimensions of the bridge, being 75 feet long. Much valuable informa- 

 tion was obtained during the progress of these experiments. The first 

 thing observed was the uniform tension of the under side of the tube when 

 loaded, and the violent compression of the upper side, forming a beautiful 

 illustration of tiie nature of the tensile and compressive forces already laid 

 down. The former, by its uniform tendency to produce the stable equi- 

 librium, bringing the thin masses into a straight line, the line and position 

 of repose ; but the latter, on the contrary, tending to produce flexure in 

 the plates, to push them out of the straight line, and push everything out 

 of joint; so that when the bottom plates remained Arm, and retained their 

 form, the top plates became bagged up and puckered like a loose web of 

 cloth. The top plates were, therefore, strengthened, and the addition of 

 another plate to the top increased the breaking weight from 3,700 lb. to 

 4,500 lb. 



As it was not so much strength that was wanted on the top plate as 

 stillness, in place of adding layer upon layer of plates, the idea naturally 

 occurred of forming the top plate into a series of little hollow square tubes 

 running longitudinally the whole length of the bridge, having the appear- 

 ance, looking endways, of little cells, the effect of which was such, that 

 while the top plates remained firm, the bottom ones now appeared to give ' 



\^'ay. These being next strengthened, an extraordinary elTect was then ex- 

 hibited when tlie lube broke, the sides collapsing together, and twisting 

 and distorting the whole fabric iu a singular manner, showing that the 

 sides formed now the weak point. These, then, were strengthened and 

 stiffened by numerous ribs of angle-iron running vertically from top to 

 bottom, and at last, by these repeated trials, the strength and proportions 

 of the iliflerent parts of the structure appeared to have attained a fair and 

 proper distribution. The strength of the tube, which at first only carried 

 seven times its own weight, was then increased to eleven times, and from 

 these experiments the strength and proportions of the real design have 

 been calculated, and one of these tubes, as is known, has now been actually 

 constructed on the shore of the Conway, floated by water to its place, and 

 raised to its proper height by the power of two enormous hydraulic rams, 

 one at each end, lifting the gigantic mass, which is 412 feet in length, 15 

 feet wide, 25^ feet high, and weigiiing no less than 1,300 tons. This is in- 

 tended for one set of rails, and there is another tube of the same dimen- 

 tions in preparation to be set parallel to it for the other. 



The situation of the structure close to the suspension bridge, and close 

 to the base of the magnificent Castle of Conway, and the effect of spanning 

 the wide estuary of the Conway, were all illustrated by a beautiful draw- 

 ing, and the nature and construction of the tube or bridge itself, was illus- 

 trated by a model which he had himself constructed. The model was 

 composed of only three thicknesses of paper and one of cloth, and the sides 

 were strengthened by thin slips of wood to represent the angle-iron ; it was 

 8 ft. 6 in. long, GJ inches deep, and 3.j broad, and although weighing only 

 4 lb. it carried a weight of 32 lb. in the centre, without visible deflection. 



The dimensions and structure of the bridge he would now describe, 

 from information for which he was indebted to Mr. Fairbairn of Manches- 

 ter, and, through Mr. Stephenson, to Mr. Edwin Clarke, the resident en- 

 gineer under him. 



The sides of the tube, vvhich are 25J feet deep at the centre, consist of 

 malleable iron plates, only i inch in thickness, rivetted together in plates 

 2 feet broad and from 4 to 8 feet long (as was shown in an enlarged view 

 or elevation with cross sections), adjusted so as that the joints may break 

 band. At the joints, however, the strength and stilTaess of these plates is 

 greatly increased by slips of angle or T iron, one of which is laid on the 

 outside of the plate and the other opposite to it on the inside, face to face, 

 and all the four surfaces strongly rivetted together. The top of the lube, 

 again, consists of two separate horizontal plates, running parallel to one 

 another, 1 ft. 9 in. apart, forming together as it were a ceiling to the tube 

 or tunnel and an external flooring on the top. These plates are | inch 

 thick, rivetted together in breadths of 2 ft. 9 in. thick, and in lengths of 

 G feet, and between them there runs seven vertical plates longitudinally, 

 from end to end of the bridge, 1 ft. 9 in. high and ^ inch thick, separating 

 the ceiling from the floor or upper platform, and at the same time uniting 

 them strongly together by rivets and joints, each vertical plate having a rib 

 of angle-iron on each angle, running longitudinally the whole length, by 

 which it is united into one vast cellular mass, consisting of eight separate 

 cells or tubes, 1 ft. 9 in. square. The object of all this strength and dis- 

 tribution of materials is to give the necessary stiffness and strength where 

 the compressive force acts. And on this account the top and bottom plates 

 are merely united by butt joints with covering plates. The whole sec- 

 tional area of this cellular frame consists of 608 square inches. Lastly, 

 the bottom of the tube consists of a similar frame of cells, but only six ia 

 number. The upper plate consists of two layers of plates, each 5 inch 

 thick, and the under one the same ; but as these plates are intended to 

 resist tension, and ought to be formed, if it were possible, like a chain, 

 besides being laid in two layers, the plates are arranged so as to break 

 joint, and a covering plate 3 feet long and as thick as the plate is placed 

 over every joint with sufiicieut rivets, such that the tearing strain is equal 

 to the tensile strength of the plates they connect. The plates are 12 feet 

 long and 2 ft. 4 in. broad, being the whole breadth of the cell. The angle 

 iron in the bottom cells and plates is rendered continuous by covers. 



The top and bottom are united to the cells by strips of angle-iron run- 

 ning the whole length, inside and out ; the interior vertical angle-irons at 

 top and bottom are curved round to increase the strength of attachment, 

 and there are also gusset or angle pieces rivetted on for additional 

 strength. The rivets used vary from 1 inch to 1| inch diameter, and there 

 are about a quarter of a million iu each tube. The holes were made so as 

 to make the rivets fit well, and they were all put in red hot. The sectional 

 area of the bottom frame of cells is 508 square inches. 



These are the dimensions in the centre of the tube, but the top plates 

 become thinner towards the euds, where they are only :J-inch thick, and 

 also the bottom plates, where they are reduced to ^-inch each. The side 

 plates again get thicker towards the euds, where they are -jtt'''^ thick. 

 The ends of the tube are slifl'eued with cast-iron frames, and there are 

 also castings in the cells for 8 feet at the ends, and the sides are also 

 "leatly strengthened at the ends. The tube was originally curved on the 

 top 7 inches, and was brought to the straight line by the elasticity of the 

 material as calculated on; showing that with its own weight, 1300 tons, it 

 only sunk 7 inches. The one end of the tube is to be fast in the stone 

 pier or abutment, the other is to be loose to allow of expansion, which has 

 been found quite visible in diff'erent states of the atmosphere. Mr. Clarke 

 says that the lube is a sensible thermometer, — half-an-hour's sunshine at 

 one end, or on the top, will move it laterally an inch and a half, and 

 vertically two inches, and this when the lube is loaded mUh 200 tons ia 

 the centre. 



