Iftw.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL 



365 



Referring to the Journal for June, 1945 (vol. viii., p. 165), there 

 would appear to be an accidental error, in callinpr the weight of 

 the chains 715 tons: the links of those suspended hetween the 

 piers, which alone enter into the calculation, are stated by Mr. 

 Cooper to weigh only 352 tons. It is to be regretted, that in the 

 extract given from that gentleman's paper, read before the Royal 

 Institution, all mention of the remaining weights which constitute 

 the permanent load, as well as the particulars which would facili- 

 tate an approximation, are omitted. 



Mr. Cowper appears to exhibit the figures, 296 X S = 1480 tons; 

 and these are cited by Mr. Cox, in round numbers, as "about 1,500 

 tons." But there is this very serious difference between what it 

 seems probable the former meant to convey by them, and the in- 

 terpretation given to them by tlie latter — that Mr. Cowper would 

 appear to have calculated upon a tension of 5 tons per square inch 

 as the greatest that could be thrown upon the chains by the heavi- 

 est possible accidental load of 100 lb. per square foot of platform, 

 added to the permanent load of chains, rods, &c., which tension 

 would amount in all to 1,480 tons; — whereas Mr. Cox has called 

 this the value of the load itself from which the tension arises. 



If Mr. Cox has misinterpreted Mr. Cowper, his conclusion as to 

 the present critical state of Hungerford Bridge fails instantane- 

 ously. If Mr. Cowper's meaning has been mistaken here, what 

 remains to be said will be less forcibly applicable to that great 

 structure as maintaining its sufficiency of strength, but will remain 

 to invite some notice as a general question. 



No explanation is given in the Journal to show why Mr. Cowper 

 adopted a tension of 5 tons per square inch, as the greatest that 

 would probably arise ; and it is desirable to analyse his process of 

 calculation, as far as the imperfect data which are on the instant 

 accessiljle will permit. 



Such an amount of tension would be the effect of a gross load of 

 875 tons, uniformly distributed, and supported by the chains 

 between the piers. It would be made up of 



Weight of chains 352 tons 



„ load 398 „ 



Estimated weight of platform ... ... 100 „ 



„ „ suspension rods ... 25 „ 



Total ... 875 tons. 

 There may be inaccuracy in the estimate of the two last items: it 

 cannot, however, be very material, and the meagre means at com- 

 mand admit of no more certain result. 



The principal question now for investigation is, whether it be 

 possible that the bridge is liable to a load of 100 lb. per square 

 foot from the assembly of a crowd of persons on tlie platform. 



This will be answered when we ascertain how many persons can 

 be crowded into a given space, and what the aggregate weight ot 

 that number of persons may amount to. 



It is well known to military men, that, taking the average of 

 large bodies of infantry when close packed, each man covers with 

 his own person a space of 20 X 15 = 300 square inches. We 

 should therefore find 0-48 men in a square foot. 



Mr. James Walker, who, by direction of government, investi- 

 gated the circumstances connected with the fall of tlie suspension 

 bridge at Yarmouth, in May, 1845, stated in evidence before tlio 

 coroner, that he calculated the weight of peoj)le, " ])acked en maxse 

 upon the bridge," at six persons per square yard, consisting chiefly 

 of women, and children under 14 years of age, each person being 

 of tlie fair average weight of 7 stone; which, he adds, might be 

 a large average, but one adopted by him, partly because it has been 

 frequently employed before. This would give | of an individual 

 belonging to such a description of persons, as chiefly women, and 

 children under 14 years of age, for each square foot; and, follow- 

 ing Mr. AValker's average of weight, it would amount to 65 lb. per 

 square foot. 



Herr Von Mitis, who constructed the steel suspension bridge 

 across the Danube at Vienna, computed its probaiile load as arising 

 from the occupation of a square fathom (of Vienna) by 15 men, 

 each weighing 115 Vienna pounds. Hence, per unit of one square 

 foot English, we should have 0'39 men, and 54"9 lb. 



Drury, in his work on Suspension Bridges, lays down an arbi- 

 trary standard of 2 square feet per man weighing 10 stone. This, 

 per square foot, is equivalent to 0'5 men, and a weight of 70 lb. 



It is familiar information, that in France the conditions imposed 

 by government on the constructors of suspension bridges, require 

 that, before the public is admitted to the use of any such bridge, 

 the chains shall undergo the proof of carrying for 24 hours, an 

 imposed load of 200 kihigrammes per square metre of )datforni in 

 addition to the weight of chains, rods, platform, &c. This is equal 



40 

 50 



63-67 

 63-46 



to 41 lb. per square foot. The rigour of this condition is modified, 

 too, in jiractice, by permitting the use of the bridge, subject to 

 special police regulations, for six months after its completion, if 

 proof to the extent of one-half this weight has been satisfactorily 

 made ; but at the end of that time, proof to the full weight of 200 

 kilogrammes per square metre must take place. 



The enncessionnaire is required also to maintain the bridge in 

 good order, which shall be done by the authorities, at his expense, 

 in case of neglect. Annual surveys of the works take place ; and 

 the Prefect may order a fresh proof to be made whenever any 

 ground for fear arises, as to the stability of the bridge, or as to the 

 safety of using it. 



With respect to the average weight of a number of persons 

 assembled accidentally, we may form some precise judgment, with 

 assistance from the researches of Quetelet, published in his "Trea- 

 tise on Man," in which he gives a table of the average weights and 

 sizes of men and women at diiferent periods of life, — sufficient for 

 our purjjose being found in the following extract. 



Age. Males. Females. 

 Years. Kilogrammes 



5 15-77 14-36") ,.., , ^, „ ,- • , 



10 24-52 23-52 V means ('^/''S ^1-53 lb. avoird. 



15 43-63 40-37 j I female, 57-50 



20 60-06 52-28 \ /Male, 135-59 „ 



25 62-93 53-28/ " \ Female, 116-33 „ 



30 63-65 54-33| .^,^, ^^^.^^ 



56-16) " Uema'le, 121-8 :,' 



To apply this table, — 



From military experience, and assuming the age of soldiers to 

 range between 20 and 50, we should get a weight per square foot, 

 arising from a packed crowd, thus — 



0-48 X 137-9 = 66-19 lb. 

 According to Mr. AValker's estimate of the number of peisons 

 on the Yarmouth Bridge, there would be a weight per square foot 

 of I X 89-29 = 59-53 lb. 



Taking the estimate of Herr Von Mitis, as to numbers, the 

 weight per square foot would be 0-39 X 137-9 = 53-78 lb. 



And, upon the arbitrary standard of Mr. Drury, that weight 

 would be 0-5 X 137-9 = 68-9 lb. 



By bringing all our results together, the conclusion to be derived 

 from them will be more obvious :^ 



Mr. Walker's estimate of weight per square foot is 65 lb. 

 Herr Von Mitis „ „ 55 



Drury „ „ 70 



Proof load by French Government „ 41 



A packed body of Infantry „ 66 



From the numbers on Yarmouth Bridge 60 



,, „ as per Von Mitis „ 51 



„ „ as per Drury „ 69 



llie average of the whole would be „ 60 



But suppose we adopt that number which is derived from facts 

 apparently the best ascertained — viz., 66 lb. per square foot — does 

 it not seem to be inadmissible that an increase of 50 per cent, 

 should be made to it when we are about to calculate what margin 

 may be left for contingencies, in the strength of a material mem- 

 ber of a suspension bi-idge ? If this be so, let us revise the esti- 

 mate of strain when shorn of so considerable an excess. 



The distance between centre and centre of jiiers being taken at 

 676^ feet, tlie length of platform supported by the cliains would 

 appear (from the apjilication of a scale to the plates given in the 

 Journal for June, 1845,) to be 



676i — 29i — 13 = 634 feet. 

 Hence, straining-weight of platform would be 99 tons 

 „ „ load „ 261-5 „ 



„ „ suspension rods 25 „ 



„ „ chains would be 352 „ 



Total ... 737 tons. 

 Equivalent to a tension of 4-2 tons per square inch on the lowest 

 part of the suspension eliains. 



Before pronouncing any judgment on the sufficiency of the 

 Hungerford Bridge, it would be requisite to know what weights 

 the suspension rods, the timbers, and the planking of the platform 

 are capable of sustaining. Indeed, it it not impossible that in the 

 latter we might find a limit which would act as a safety-valve for 

 other parts of the structure. At all events, we should not forget 

 that dense crowds of persons generate a lateral pressure, such as 

 the parapet railing of a suspension bridge is usually unequal to 



