1841.1 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



355 



8ibly differ, but between tbat point and the breaking weight, extension 

 yielded in a higher ratio than compression. 



Wrought Iron. — Similar experiments were next made on bars of wrought 

 iron, 2i in. square; the supports were 13 ft. 6 in. apart, and the strips of 

 oop iron were 12 feet long. 



Weight. Deflection. Compression. Extension, lilaslicity. 

 lbs. inches. inches. inclici. impaired. 



500 0-55 003 003 



1000 1-55 0-06 0-06 



1280 1-4.') 0-07 OO" 0-15 



15C0 1-85 008 008 



1800 2-20 009 009 



2000 2-70 Oil Oil 0-65 



2280 4-15 0-18 0-19 205 



With this weight the beam was permanently bent, and its elasticity nearly 

 cstroyed. 

 These experiments showed tbat, differing from cast iron, the amounts of 

 extension nd compression in wrought iron continue to be equal up to the 

 complete destruction of the elasticity of the beam. 



Fir battens. — The amounts of extension and compression in rectangular 

 beams of fir timber, when subjected to transverse strain, were next deter- 

 mined ; the manner of proceeding was precisely the same as in the preceding 

 experiments. 



A batten, i in. by 3 in., with the supports 8 ft. 2 in. apart, and with strips 

 7 ft. 6 in. long, when subjected to transverse strain, gave these results : — 

 Weight. Deflection. Compression. K.vtensioc. 



lb. inches. inches. inches. 



500 110 0-12 0-12 



1000 2-30 0-24 0-24 



Results. — From these experiments on the amount of extension and com- 

 pression of cast iron, measured at the under and upper surfaces of rectangular 

 beams, subjected to transverse strain, the author assumes, that within limits 

 which considerably exceed those of elasticity, and equal to at least two-thirds 

 of the breaking weight, there is no sensible difference between the amounts 

 of compression and extension, and that as the breaking point is approached, 

 extension yields in a higher ratio than compression, and gives way first. 



It would appear certain that up to the point when the elasticity of wrought 

 iron is completely destroyed, and the beam is bent, the amounts of com- 

 pression and extension continue exactly equal, and it is therefore probable 

 that this equality would continue to the last. 



It is clear that the amounts of extension and compression up to three- 

 fourths af the breaking weight do not sensibly differ in fir battens, but that 

 as the ultimate strength of the beam is approached, compression yields in a 

 much higher ratio than extension, and may be actually seen to give way first. 

 He states also, that the amounts of extension and compression are in direct 

 proportion to the strain, within the limits of elasticity, and that even after 

 those limits are greatly exceeded, and up to three-fourths of the strength of 

 a beam, they do not sensibly differ. 



The apparatus with which these experiments were made was exhibited, 

 and presented by the author to the Institution. 



Mr. Donkin eulogised the novel and ingenious manner in which Mr. Col- 

 thurst had conducted the experiments, which he considered to be highly 

 satisfactory. They not only determined the position of the neutral axis of 

 the beams experimented upon, but showed also the relative amounts of com- 

 pression and extension, so as to demonstrate that the elasticity of a body 

 was the same in compression as in extension. These experiments also con- 

 firmed the correctness of Tredgold's opinion as to the pernicious effects of 

 attempting to produce peculiar forms in beams by cambering and inserting 

 wedges into their upper sides. 



Mr. Vignoles reminded the meeting of the discussions which bad taken 

 place relative to the position of the neutral axis in the raUway bars, which 

 had the upper and under tables similar ; it was contended tbat the neutral 

 axis was situated close beneath the upper lip, or table of the rail, whereas, if 

 Mr. Colthurst's mode of experimenting had been adopted, a different and 

 more correct result would have been arrived at. 



Mr. Cubitt accorded great merit to Mr. Colthurst for the experiments, 

 which had determined the question as regarded rectangular beams. It ap- 

 peared tbat no attempt had been made to use the same mode of proceeding 

 with beams of irregular figures ; in them, therefore, it might he concluded, 

 that the neutral axis would he found in the centre of gravity of the section 

 of the beam. 



Mr. J. Home remarked, that these experiments perfectly accorded with 

 those which be laid before the Institution in 1837. His object had been to 

 show that the neutral axis was always in the centre of gravity of the section, 

 as well as to determine the figure which should resist the greatest amount of 

 pressure with a given quantity of materials ; the strongest form was shown 

 to be a prism, placed with the base upwards, and the same figure reversed 

 was the weakest ; the strength of the former figure exceeded that of the lat- 

 ter by at least one-third. 



■ipril 27. — The President in the Chair. 



Memoir of the Montrose Suspension Bridge. By J. M. Rendel, M. Inst. C.E. 



Previous to the year 1792, the passage of the River Esk at Montrose was 

 eflfected by common ferry boats j at that period an act of parliament was ob- 



tained for the construction of a wooden bridge, with numerous arches, or 

 rather openings formed by heams, supported upon piles, ivith stone abut- 

 ments at either end ; the action of the tide undermining the piles, and the 

 usual progress of decay causing great expense for repairs, it was decided in 

 the year lh2.>, to erect a suspension bridge, the iron work of which was con- 

 tracted for by Captain bamuel Urown, R.N., for the sum of £9,430, and the 

 masonry of the towers for £9,080. The total cost being £18,510, exclusive 

 of the land arches and approaches; those of the old bridge being preserved 

 for tlic new one. ° '^ 



The dimensions of the new bridge were 



Peet. 



432 



42 



412 



26 



21 



68 



40 by 20 



Distance from centre to centre of the towers 

 Deflection of the chain or versed sine of the catenary 

 Length of the suspended roadway 

 Width of ditto ......' 



Height of ditto above low water . . [ 



Ditto of tlie towers above ditto 

 Base of the towers at the level of the roadway 

 Archways through the towers . . 16 wide, 24 high 



The towers were built of red sandstone ashlar, raised on a base of the 

 same material, carried upon piles. 



Construction.—lhere were two main chains on each side, arranged above 

 each other in parallel curves, 12 inches apart. Each chain was composed of 

 four bars of iron, 5 inches wide by 1 inch thick, and 10 feet long, united by 

 short plates, and strong wrought iron pins. The roadway was suspended to 

 these chains by perpendicular rods. 1^ inch in diameter, attached at intervals 

 of 5 feet, alternately to the upper and lower lines of main chains, at the 

 joints, which were arranged so that those of the upper chain should be over 

 the long bars of the lower one ; at the lower end of each suspending rod 

 was a stuTup, which received and carried the cast-iron bearers for supportine 

 the roadway. ° 



Upon these bearers was laid and rivetted longitudinally a flooring of fir 

 planks, 3 inches thick, and well caulked ; upon this a sheathing of fir, 1 J in. 

 thick, was placed transversely, and spiked to the lower planks ; over all was 

 spread a coating of about 1 inch thick of fine gravel and sand, cemented with 

 coal tar. 



The suspending rods were without joints. The main chains rested upon 

 detached cast iron saddles, built into the masonry of the towers, and passing 

 down at either extremity, were secured behind cast iron plates in masses of 

 masonry, 10 feet under ground. 



The construction was commenced in September, 1828, and was finished in 

 December, 1829, a period of only sixteen months. 



Accident to the Bridge.— On the 19th of March, 1830, about 700 persons 

 assembled on the bridge to witness a boat race, when one of the main chains 

 gave way, and caused considerable loss of life. The injury was speedily re- 

 paired, but a careful survey of the structure was ordered, and it was dis- 

 covered that the intermediate or long Unks of the chains bore so unequally 

 upon the saddles as to he bent and partially fractured. Mr. Telford, who 

 was consulted on the subject, proposed the addition of two other main chains 

 placed above the original ones, aud having the same curve, so as to increase 

 the sectional area 40 inches— thus giving six chains of 20 inches area each, 

 instead of four chains, as originally constructed. 



Mr. Telford's decease occurring at that period, the author was instructed 

 to report upon the state of the bridge, and advise such alterations as he 

 judged to be necessary. 



After a minute personal inspection he concurred in Mr. Telford's idea of 

 the necessity of increasing the strength of the bridge, but instead of aug- 

 menting the number of the chains, he advised the addition of two bars in 

 width to each of those existing, by which means the required strength might 

 be gained. He was led to this by an opinion that, in all cases, it is'desirable 

 to have as few chains as possible. 



It appeared that there bad been hut little precision in the workmanship of 

 the chains ; for on releasing them they immediately became twisted ; thus 

 showing that all the links had not a true bearing. On taking them apart, 

 many of the traversing pins were found to be bent, and some of them were 

 cut into, evidently by the friction of the links. This was to be rectified, and 

 new saddles of a different principle and stronger form were recommended : 

 also, that those parts of the chains which rested in the saddles should be en- 

 tirely composed of short plates. Additions to the masses of masonry holding 

 the chains were likewise deemed advisable. 



Between the years 1835 and 1838, all the principal works, with many 

 minor improvements, were executed. 



In the author's report on the state of the bridge, he noticed what he 

 deemed defects in the construction of the roadway, but as there was no posi- 

 tive symptom of failure, it was allowed to remain. He conceived, that in 

 the anxiety to obtain a light roadway, mathematicians and even practical 

 engineers had overlooked the fact, that when hghtness induced flexibility, 

 and consequently motion, the force of momentum was brought into action, 

 and its amount defied calculation. 



On the 11th of October, 1838, the roadway of the bridge was destroyed 

 by a hurricane, the effect of which upon this structure is the subject of a 

 paper by Colonel Pasley, published in part 3, vol. 3, of the Transactions of 

 the Institution C. E. 'fo that account the author refers for the principal 

 details, only adding, that on inspecting the bridge, he found the chains, the 

 saddles, and the fastenings or moorings, quite sound ; the principal portion 



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