356 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



[October^ 



of tlie roadway had been complelely carried away, and tlie remainder much 

 injured, lie then gives some account of the uiidulatory motion ohser\ed 

 during the storm. This motion was greatest at about midway between the 

 towers and the centre of the roadway; hut tlie waves of the platform did 

 not coincide witli those of the chains, either in magnitude or in order ; no 

 oscillatory motion was perceived either in tlie roadway or in the chains, al- 

 though particular attention was directed to them. 



It appears that the centre of the platform fell in a mass. This the author 

 attributes t the failure of the suspension rods, which , having no joints, were 

 twisted oft' close to the floor by the undulatory motion. A similar occur- 

 rence at the Menal Bridge* induced Mr. Provis to adopt the joints in the 

 suspension rods, which the author had previously introduced at the Montrose 

 Bridge. 



The author had long been convinced of the importance of giving to the 

 roadways of suspension bridges the greatest possible amount of stiffness, in 

 such a manner as to distribute the load or the effect of any violent action 

 over a considerable extent. 



The platfoims of large bridges, in exposed situations, are acted upon in so 

 many different ways by the wind, that he had an objection to the use of 

 stays or braces to counteract movements which ought rather to be resisted 

 by the form of the structure. 



Holding such opinions, he determined to adopt a framing which, although 

 connectedly rigid in every direction, should nevertheless be simple, composed 

 of few parts, cajialile of l)eing easily renewed; should distribute its weight 

 uniformly over the chains, not be subject to change from variation of tempe- 

 rature, and not augment the usual weight of suspended platforms. 



The details of the alterations, and general repair of the bridge, arc then 

 given ; a few may be mentioned. 



An entirely new set of stronger suspending rods was introduced; they 

 were IJ of an inch in diameter down to the flexible joint at the level of the 

 platform ; below that point the diameter was increased to IJ of an inch, and 

 a strong thread was cut on to the lower end, so as to adjust them to the re- 

 quisite lengths. 



In the place of the cast iron bearers cross beams were substituted, com- 

 posed of two Memel planks, 13 inches deep, 3.V inches thick, bolted together, 

 and trussed with a round bar 1 -^ inch diameter ; every sixth beam had a deep 

 trussed frame on the under side, so as to give great stiffness. Above and 

 beneath the cross beams, on each side of the carriage way, were bolted two 

 sets of longitudinal timbers, four in each set ; they were further united by 

 cast iron boxes, at intervals of 10 feet ; and the ends were secured to beams 

 of English oak, built into the masonry of the towers. A curb of Memel 

 timber, 11 inches by 6 inches, was attached to the ends of the cross bearers, 

 and extended the whole length of the platform. 



The planking of the footways was composed of narrow battens, 2 inches 

 thick, laid transversely from the inner longitudinal beam to the outer curb 

 piece, with an inclination or drip of l\ incli in 5 feet. 



The carriage way was formed of four thicknesses of Memel plank ; the 

 two lower layers, each 2 inches thick, were placed diagonally with flic trans- 

 verse beams, crossing each other so as to form a reticulated floor, abutted 

 against the longitudinal beams; they were firmly spiked to tlie beams, and 

 to each other, at all the intersections, and upon them was laid and spiked a 

 longitudinal layer of Memel planking, 2 inches thick. Over the whole was 

 fixed, transversely, a layer of slit battens, \] inch thick. Each hayer was 

 close jointed and caulked, and the upper one was laid in a mixture of pitch 

 and tar. A composition of fine gravel and sand, cemented with boiled gas 

 tar, was laid over the whole, to the thickness of 1 inch, forming the road 

 track. 



To add to the stiffness afforded by tliis construction, the author caused to 

 be passed through the spaces between the pairs of longitudinal beams, a 

 series of diiigonal truss pieces of Memel timber, G inches square, with their 

 ends stepped into the cast-iron boxes, which, at every 10 feet, grasp the 

 beams. On the other ends of tliese diagonal truss pieces, cast iron boxes 

 were fixed, which received the straining pieces, placed 3 ft. fi in. above, and 

 the same depth below, the roadway: an iron screw bolt, 1] inch diameter, 

 at every 10 feet, and a contrivance of wedges in the cast iron boxes, enabled 

 any degree of tension to be given to the framing. 



The roadway was thus stift'ened by two of the strongest kinds of framing, 

 in parallel bncs, dividing the carriage way from the foot-paths ; it was deemed 

 preferable to disconnect them from the suspending rods, and, by bringing 

 them nearer together, to .avoid a twisting or unequal strain. The whole 

 formed a compact mass of braced wood work, the diagonal planking giving 

 the horizontal stifl"ness, and the two trussed frames insuring the vertical 

 rigidity. 



The weight of the new roadway was — Tons. Cwt. 



Wood work 130 19 



Cast and wrought iron about ditto . . 36 G 



AVrought iron in the suspending rods . . 20 14 



Ditto in the fencing 8 18 



Gravel concrete 30 



Total 226 1 7 



The weight of the original roadway was — 

 Wood work .... 



Cast iron about ditto 

 Wrought iron in the suspending rod 

 Gravel concrete 



Tons. Cwt. 



69 



92 



12 9 



30 



Total 203 9 



Or 47.5 lb. per square foot, superficial, for the entire roadway 



' Minutes of proceedings, pages 167, and 204. 



Or 23 tons less than the new roadway. 



Cos/. — The platform described is 412 feet long, and 27 feet wide; it cost 

 .-£4026 or about 7s. 3t/. per superficial foot. 



The works were completed in the summer of 1840; the bridge has borne 

 without injury the gales of the last winter; and the stiffness of the platform 

 has given confidence in its strength to all who have examined it. 



Five elaborate drawings of the bridge, giving all the details of its construc- 

 tion on a large scale, accompanied this communication ; they were presented 

 by Mr. Page on his election as an Associate of the Institution. 



Mr. Seaward agreed with Mr. Rcndel in the advantage of reducing the 

 number of suspension chains, aud thus rendering the whole construction as 

 simple as possible. The trussed framing, which appeared to be the main 

 feature of this bridge, was particularly deserving of commendation, as it im- 

 parted a degree of stiffness to the platform which had not hitherto been at- 

 tained in other cases, although it was demonstrated to be the best method of 

 preventing the undulation which was so prejudicial to the suspension bridges. 



Mr. Itendel had, on a previous occasion,* explained his view of the action 

 of wind upon the platforms of suspension bridges, and of the necessity of a 

 certain degree of stiffness in the construction ; tliis he conceived would 

 always be better attained by having a simple well-trussed framing to prevent 

 undulation, than by the application of braces or stays to check either undu- 

 lation or oscillation— the latter being in his opinion only the result of the 

 former. 



He would now only insist more forcibly upon those points. The roadway 

 should be so stift" as to prevent as much as possible all tendency to motion, 

 because it added to the nafiiral decay of every part of the structures; for 

 instance, he found on taking down the chain of the Montrose Bridge, after 

 seven or eight years' wear, that the pins of the links were cut some depth 

 into; demonstrating how great had been the amount of motion among the 

 links. In constructing suspension chains, after this experience, he should be 

 inclined to abandon the circular form for the pins, and forge them of a long 

 oval shape in their transverse section ; making the apertures in the links by 

 drilling two holes, and cutting out the metal between them with a machine ; 

 this form of pin would allow sufficient play for the necessary curve of the 

 chain, while the pin itself would be stronger, would weaken the link less 

 than the large circular hole, and would be less expensive to manufacture. 

 He disapproved of all the complicated contrivances for allowing expansion of 

 the main chains ; he had found that plain saddles of proper form were quite 

 sufficient to permit the expansion of the back chains, which was all that 

 required attention. 



Mr. Palmer mentioned, on the authority of Mr. Chapman, the destruction 

 of a suspension bridge in America, caused by the sudden passing of a drove 

 of cattle when frightened. This was peculiar, as it always had been con- 

 sidered that an irregular motion was innocuous, hut that when any regular 

 impulses were communicated, there was danger of fracture of the bars. 



ilr. Vignoles eulogized this excellent communication for the practical con- 

 clusions which it contained. Mr. lieudel had materially assisted in affording 

 facility of communication by the introduction of the floating bridges, in com- 

 munication with railways, and it was not difficult to foresee that, by carrying 

 out the system of adapting well-trussed framings to the platforms of suspen- 

 sion liridgcs, sufficient rigidity would be attained for locomotive engines and 

 carriages on railways, to traverse rivers or ravines by means of these bridges, 

 instead of by costly viaducts or heavy embankments. 



Mr. Kendel saw no difficulty in giving any required amount of rigidity to 

 the platfonns ; it was only necessary to increase the strength of the framing, 

 to enable tlie roadway to bear with perfect safety the passage of an engine 

 and a train of carriages. 



The President directed the attention of the members to what he considered 

 the most valuable part of this interesting communication — the detection of 

 the errors in the original construction of the bridge. This was the most 

 useful class of papers which members could present to the Institution, and 

 they were particularly valuable when they were illustrated by such complete 

 drawings as those now communicated by Mr. Page on his election. He 

 hoped this example would be extensively followed. He mentioned that an 

 attempt had been made to carry a railway across the Tees by a suspension 

 bridge, but it had been abandoned. 



Mr. Rendel understood that the weight of the trains had so stretched the 

 chains, or rather forced the moorings of the back chains of the bridge over 

 the Tees, that the platform sunk in the centre so as to prevent the passage 

 of the carriages ; piles had therefore been driven beneath each bearer of the 

 roadway, and the chains now remained merely to show that it had formerly 

 been a suspension bridge. 



May 4. — The Pbesidk.vt in the Chair. 



" Suppleinentary Account of the I'ne of aiLriliary Steam Power, on board 

 the ' Earl of Ilardxp'cke' and the ' Vernon ' Indiamen." By Samuel Seaward, 

 M. Inst. C.E. 



' Minutes of Proceedings, page 205 



