1841.] 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



205 



The result of the examination was satisfactory ; the whole of the masonry, 

 the main chains, their attachments to the rock, the rollers and iron work 

 upon the pyramids, and all the principal parts of the bridge, were as perfect 

 as when first constructed ; it was, however, recommended, that " a greater 

 degree of rigidity should be given to the roadways, so that they should not 

 bend so easily under vertical pressure." 



The bridge remained in the same state until the hnrricaM of the 6th and 

 7th of January, 1839 ; during the night of the 6th, all approach to the bridge 

 was impractic'.ible ; the bridge-keeper, however, ascertained that the road- 

 ways were partially destroyed ; and he in consequence traversed the strait in 

 a boat iu time to prevent the down mail from London driving on to the 

 bridge. 



\Vlien the day broke, it was found that the centre foot-path alone remained 

 entire, while both the carriage ways were fractured in several places. The 

 suspending rods appeared to have suffered the greatest amount of injury ; out 

 of the total number of 444, rather more than one-third were torn asunder ; 

 one piece, 175 feet long, of the N.E. carriage way, was hanging down and 

 flapping in the wind ; much of the parapet railing was broken away ; the ties 

 and distance pieces between the main chains were destroyed ; the chains had 

 resisted well in spite of the violent oscillation they had been subjected to, to 

 such an extent, as to beat them together and strike the heads oiT bolts of 

 three inches diameter. 



Means were immediately adopted for restoring the roadways ; and so rapidly 

 was this effected, that in live days carriages and horses passed over, while foot 

 passengers were not at any time prevented from crossing. 



The account of the restoration of the bridge, communicated by Mr. Maude 

 to the Institutiou, is then alluded to. 



The substance of the report of the author to the Commissiouers of Her 

 Majesty's \Voods is then given, and a review of the proposals made by Mr. 

 Corams, Colonel Pasley, and others, relative to the restoration. 



The opinion of Colonel Pasley, "that all the injuries which have occurred 

 to the roadways of Suspension Bridges must have been caused by the violent 

 action of the wind from below," is then examined, and reasons given for the 

 author's dissent from that opinion. 



The action of the wind upon the Conway and Hammersmith Bridges, is 

 next examined ; and from the amount of oscillation observed in all suspen- 

 sion bridges, the conclusion is arrived at, that winds act strongly and preju- 

 dicially on the fronts as well as on the horizontal surfaces of the platforms of 

 suspension bridges, and that the effect of winds is modified and varied by the 

 nature of the country, and the local circumstances connected with each indi- 

 vidual bridge. Although differing in opinion with Colonel Pasley as to the 

 general cause of injui7 to suspension bridges, the author agrees with him in 

 the propriety of giving increased longitudinal rigidity to their platforms, to 

 prevent or to restrict undulation. He advised its adoption in 1836, and ap- 

 plied his plan of stiffening by beams, in 1839. He preferred beams to trussed 

 framing, on account of the facility with which the former could be increased 

 in number, to obtain any requisite degree of stiffness, and because he feared 

 that trussed frames could not always be kept firmly in their true vertical 

 positions. 



A drawing showing the injuries sustained by the platform during the hurri- 

 cane of 1839, accompanied the communication. 



Mr. Cowper was of opinion, that the real cause of injury to suspension 

 bridges was the vibration of the chains and roadway. The whole suspended 

 part, when acted upon by the wind, became in some measure a pendulum, 

 and if the gusts of wind were to recur at measured intervals, according either 

 with the vibration of the pendulum, or with any multiplies of it, such an 

 amount of oscillation woidd ensue as must destroy the structure. He illus- 

 trated this proposition by a model with chains of different curves, and at the 

 same time pointed out the efficiency of slight brace chains in checking the 

 vibration. 



Mr. Brunei agreed with Mr. Cowper in his opinion of the cause of injnn- 

 to bridges, and with the propriety of applying brace chains, for preventing 

 the vibration. He then alluded to the introduction of lateral braces in the 

 bridge designed by Mr. Brunei, senior, for the Isle of Bourbon. He had 

 been at the Menai 13ridge during a severe storm, and had particularly noticed 

 the vibration of the chains with the accompanying undulation of the plat- 

 foitn. The force of the wind was not apparently from beneath ; it appeared 

 to act altogether laterally. The chains were too high above the roadway ; 

 their vibration commenced before the platform moved ; the unequal lengths 

 of the suspension rods then caused the undulating motion. His attention 

 had latterly been much given to the subject on account of the CUfton Sus- 

 pension Bridge, now erecting under his direction. The span would be seven 

 hundred feet, and the height above the water about two hundred feet. He 

 intended to apply the system of brace chains at a small angle to check vibra- 

 tion. To two fixed points in the face of one pyramid would be attached two 

 chains, each describing a curve horizontally beneath the platform, touching 

 respectively the opposite sides of the centre of the bridge, and thence ex- 

 tending to similar points on the other pyramid : there they were attached to 

 two levers, the ends of which were connected with a counter balance of about 

 four tons weight appended to each ; these weights would hold the chains 

 sufficiently extended to enable them to resist the lateral action of the strongest 

 winds without their being so rigid as to endanger any part of the structure. 

 By this contrivance the platform would be kept firm, .which was the chief 

 point to be attained. 



In all suspension bridges the roadways had been made too flexible, and the 



slightest force was sufficient to cause vibration and undulation. The platform 

 of the Clifton Bridge would have beneath it a complete system of trough- 

 shaped triangular bracing, which would render it quite stiff. Ho was an ad- 

 vocate for bringing the main chains down to the platform, as at the Ham- 

 mersmith Bridge, and for attachiug the bearings to the chains at two points 

 only ; when they were suspended by four rods, it not unfrequently happened 

 that the whole weight of a passing load was thrown upon the centre suspen- 

 sion rods, and the extremities of the bearers were lifted up and relieved from 

 all pressure. The extent of the expansion and contraction of the chains was 

 a point of importance. In the Menai Bridge the main chains on a summer 

 day would be as much as sixteen inches longer than iu a winter's night. At 

 the Clifton Bridge the difference under similar circumstances would be about 

 twenty inches. The whole expansion of the back chain beyond the pyramids 

 must be thrown into the suspended part, lie would prefer having only one 

 chain on each side of the liridge, and that chain much stronger than is usually 

 adopted, but in deference to public opinion he had put two; he believed that 

 they rarely expanded equally, and hence an unequal distribution of the weight 

 of the roadways upon the suspension rods occurred. A rigid platform would 

 in some degree prevent this, but he had endeavoured to lessen the effects o£ 

 unequal expansion by arranging a stirrup at the top of each suspending rod, 

 so as to hold equally at all times upon both the chains, and thus cause each 

 to sustain its proportion of the load. 



Mr. Seaward had never seen the force of wind exerted at regular intervals, 

 as Mr. Cowper had supposed ; if the gusts were repeated at such intervals, 

 no suspension bridge, nor any elevated sliaft or chimney in masonry, could 

 resist them. 



Mr. Rendel believed that the errors committed in the construction of sus- 

 pension bridges had principally arisen fron engineers theorizing too much on 

 the properties of the catenary curve, without attencUng sufficiently to the 

 practical effects of wind in the peculiar localities in which the bridges were 

 placed. He could not agree with Mr. Cowper in his view of the intermittent 

 action of the wind, or the vibrating of the chains. Observation had led him 

 to conclude that, in the positions in which suspension bridges were usually 

 placed, the action of the wind was not uniform ; for instance, it would act at 

 the same moment on the upper side of one end of the roadway, and on the 

 lower side at the other end. In this case, unless the platform possessed a 

 certain degree gf rigidity, undulation was induced and oscillation ensued. 

 Braces and stays would not counteract this — nothing but a construction of 

 platform, which made it in itself rigid by some mode of trussing, could with- 

 stand this kind of action. He agreed with Mr. Brunei iu his idea of reducing 

 the number of the suspending chains. At the Montrose Bridge, which was 

 432 feet span, he had endeavoured to avoid all complexity of contrivances by 

 adopting a complete system of vertical diagonal trussing, which was ten feet 

 deep — five feet above, and five feet below the platform — so as to insure 

 rigidity, and to produce that solidity which was essential for preventing un- 

 dulation and oscillation. 



Mr. Cowper reverted to the motion which he had found to be so easily 

 produced by repeatedly exerting a small force at measured intervals against 

 the main chains of tlie Hammersmith Bridge. He conceived that if the chain 

 oscillated, the roadway must oscillate also. 



Mr. Rendel contended that the motion produced by the impulses communi- 

 cated by Mr. Cowper to the chain resolved itself into undulation, and not 

 oscillation. He could' not understand the advantages of the trussing adopted 

 at the Hammersmith Bridge ; it appeared to him that its tendency was, on 

 the passage of a heavy weight, to relieve four out of five of the suspending 

 rods from their due proportion of the load, and to throw it upon the fifth rod. 

 His object in the construction of the framing of such platforms had always 

 been to spread the load quite equally, and rendering it rigid by means of ver- 

 tical trussed framing, to prevent the undulation which was the primary cause 

 of oscillation. He would distinguish clearly between the two motions, and 

 say, that undulation was a motion in the direct line of the platform, and that 

 oscillation was a motion at right angles with it. Vibration was identical with 

 undulatory action. 



Mr. Donkin conceived that a good system of trussed framing could alone 

 prevent undulation or oscillation ; if the framing were placed vertically, its 

 tendency would be to prevent undulation ; if placed horizontally, to prevent 

 oscillation : now, as Mr. Rendel had given it as his opinion, that the latter 

 action resulted from the former, the system of trussing adopted by him at 

 the Montrose Bridge would appear calculated to obtain the desired end. A 

 slight exertion of force would produce a perceptible undulation, and a certain 

 degree of vibration would result from the natural elasticity of the materials, 



Mr. Seaward remarked, tliat the degree of osciUation would appear to de- 

 pend iu some measure upon the distance at which the platform was suspended 

 beneath the chains, and upon the distance between the points of suspension 

 of the main chains ; if tlie platform were rigidly held at the extremities, the 

 motion would be vibratory, and not amounting to undulation. 



The railwmi tickets on the Manchester and Leeds line, invented by Mr. 

 Edmondson,'are printed by a machine which gives each a progressive num- 

 ber, and arranges them in order. Two boys lately printed 10,000 tickets in 

 four liours. 



2 F 



