MINUTES OF PROCEEDINGS OF THE INSTITU- 
 TION OF CIVIL ENGINEERS, LONDON, 
 ENGLAND, SESSION OF 1862-63. 
 
 AMERICAN IRON BRIDGES. 
 
 ABSTRACT OF THE DISCUSSION 
 
 UPON* A PAPER SUBMITTED 
 
 By ZERAH COLBURN. 
 
 EDITED BY 
 
 CHARLES MANBY, F. R. S., M. Inst. C. E„ 
 
 HONORARY SECRETARY. 
 
 AND 
 
 JAMES FORREST, Assoc. Inst. C. E., 
 
 SECRETARY. 
 
 NEW YORK: 
 
 D. VAN NOSTRAND, 192 BROADWAY, 
 
 1 8 6 7 . 
 
^ Digitized by the Internet Archive 
 in 2017 with funding from 
 
 University of Illinois Urbana-Champaign Alternates 
 
 https://archive.org/details/americanironbridOOcolb 
 
INSTITUTION OF CIVIL ENGINEEES. 
 
 May .5, 1863. 
 
 JOHN HAWKSHAW, President, 
 in the Chair. 
 
 Mr. Zerah Colburn submitted a Paper describing the 
 several plans of Iron Bridges in use upon the Bailways of 
 the United States, illustrated by a series of diagrams. 
 
 No. 1,091. — “ American Iron Bridges.” By Zerah Colburn. 
 
 After reading the Paper, Mr. Colburn said, he had re- 
 frained from entering more into detail to avoid making it 
 tedious, but he should be happy to reply to any questions. 
 No doubt, in looking at the diagrams, it would be considered 
 that American engineers had practised great economy in de- 
 signing these bridges. The fact was, that if bridges could 
 not be constructed of iron at a moderate cost, the Bailway 
 Companies in the States would not adopt them, but would 
 continue to use timber bridges, which could be built at from 
 £5 to £7 per lineal foot. It was on the score of cost, alone, 
 he believed, that American engineers had adopted cast iron 
 for all part^ in compression ; so many square inches of sec- 
 tion could be put into the tubular form for one-third the cost 
 in cast-iron that would be incurred with wrought-iron plates. 
 In the case of the Green Biver Bridge, the top chords and 
 the vertical posts were of cast-iron pipes. Care was requi- 
 site in casting the pipes. Mr. Fink tested the iron for all the 
 pipes, and holes were drilled in the side that lay uppermost 
 in the sand, to ascertain that the cores had not floated, and 
 thus that the metal was of uniform thickness. The fact of 
 cast-iron being used in the top chords was a sufficient expla- 
 nation why bridges were not made continuous over two, or 
 more, spans in America. In nearly all these bridges almost 
 the whole of the iron was made to do work in carrying the 
 load. They, no doubt, differed from what English engineers 
 were accustomed to, and he feared the diagrams would pre- 
 sent an extraordinary appearance to English eyes ; but they 
 were faithful representations. 
 
 Mr. Colburn then called attention to the drawing of a 
 bridge with truss rods to every upright post. The span was 
 
 ^ 45939 
 
4 
 
 AMERICAN IRON BRIDGES. 
 
 125 feet, and tlie depth was 23 feet from the centre of the 
 top chord to the centre of the bottom chord ; yet all the iron 
 posts, the truss rods, the diagonal tension rods, and the lean- 
 ing end columns, were not together equal to a quarter of an 
 inch in thickness of iron, if spread out as a plate, oyer the 
 whole side, and half of that thickness was cast-iron ; still no 
 portion of the iron was subjected to a greater working strain 
 than 4 tons to the square inch. Under ordinary circum- 
 stances a working strain of 3 tons to the square inch was the 
 utmost that existed. He contrasted with that design some 
 plate girder bridges, erected three years ago, on the Boston 
 and Worcester Bailway. They were 87 feet span and 7 feet 
 6 inches deep. The plates were 6 feet 3 inches wide and 7 
 feet 6 inches high. At every vertical joint there was a pair 
 of butt straps, 8 inches wide, double riveted on each side of 
 the web, and over these a pair of angle irons, 3 inches by 6 
 inches. Midway between the vertical joints, there were two 
 angle irons, 6 inches by 3 inches, the longer sides being 
 turned over at right angles, to form a knee by which they 
 were riveted to the top and bottom chords. Mr Philbrick, 
 the engineer, adopted that form of stiffening the webs of 
 girder bridges, because, as he said, he had found that in 
 England and in France, the T irons used for stiffening had 
 begun to split in some cases. He had only called attention 
 to the butt straps and angle irons used* by Mr. Philbrick, be- 
 cause these, spread out as plates over the side of the web, 
 were alone equal to one-quarter of an inch in thickness, 
 while in the Murphy- Whipple Bridge the whole of the mem- 
 bers forming the web amounted to less than a quarter of an 
 inch. 
 
 He might add, that none of the bridges shown in the 
 drawings had broken down, and he had never heard that 
 they had exhibited any signs of weakness. They were con- 
 sidered in America, and he believed them to be, very strong, 
 and good bridges. If they had any peculiar merit, it might 
 be in the fact that they were not overloaded with ballast. 
 The trusses were deeper, and, therefore, there was not so 
 much iron put into the sides as might be expected to keep 
 the strains down. 
 
 In Bollman’s Bridge the test load was 1^ ton per lineal 
 foot, for a single line. In America the assumed load was 
 arrived at, by supposing that there was on the bridge a train 
 of loaded goods wagons, with two engines, and a snow- 
 plough weighing about 15 tons ; and, in addition, when the 
 bridge had a floor, that this was covered with snow, equal to 
 a weight of 30 lbs. per square foot of floor ; and that a side 
 
AMERICAN IRON BRIDGES. 
 
 5 
 
 wind, equal to 30 lbs. per square foot, was blowing. It was 
 further assumed that, as every wheel had a break, all the 
 breaks were put on at once, which would throw the strain 
 upon the top chords of the bridge. That was the assumed 
 load of the bridge of 200 feet span, and it was adopted by 
 Mr. Murphy, Mr. Whipple, and Mr. Bollman. 
 
 In answer to inquiries from several members, Mr. Colburn 
 said that the highest breaking strain of the cast-iron alluded 
 to in the Paper was 20J tons per square inch ; ‘but it was 
 certainly never subjected to that strain in a bridge. It was 
 boiled cast-iron, that was melted and kept in fusion for sev- 
 eral hours, and partially decarbonized. Previous to the 
 present war in America, every cast gun was made of iron 
 melted two, three, or four times. 
 
 Mr. F. J. Bramwell regretted to hear that the arrange- 
 ments for the reading of other communications were such, 
 that it was desirable to limit the present discussion ; as he 
 feared there would be great difficulty in investigating the 
 principles of construction of the Trusses described in the 
 Paper, unless some considerable time were devoted to that 
 purpose. 
 
 With regard to the principles involved in the construction 
 of the Trusses of American Iron Bridges, he referred to 
 three diagrams he had caused to be prepared, showing, re- 
 spectively, the Fink truss (Fig. 1), an ordinary diagonal truss 
 which was so well known in this country (Fig. 2), and the 
 Bollman, or Harper’s Ferry Bridge, truss (Fig. 3). In these 
 diagrams, one-half of each figure was drawn merely in lines, 
 running along the centres of the various members of the 
 truss ; while in the other half of each figure the members 
 were indicated either by dotted fines simply, or by dotted 
 fines and shading on each side, so as to show, by the width 
 of the shading or of the dotted lines, the relative amounts of 
 iron required jn each member ; and by the different kinds of 
 dotted lines and the different kinds of shading, to distinguish 
 between the parts in compression and those in tension ; dis- 
 carding, for the sake of simplicity, all consideration of rivet 
 holes, joints, and such matters, and assuming the whole of 
 the iron to be brought to an uniform thickness, and to be 
 equally valuable for compression and for extension. Thus, 
 if the shading along one member of the truss were twice as 
 wide as along another, then it indicated that there must be 
 double the sectional area of metal in the first of these two 
 members, that there was required to be in the second of such 
 members. In this way the diagrams would at once instruct 
 the eye, as to the consumption of metal in each part of each 
 
6 
 
 AMERICAN IRON BRIDGES. 
 
 truss, while the difference of the shading would show which 
 part was in compression and which in tension. Further, at 
 the end of each figure a parallelogram had been drawn, 
 which indicated the relative amount of each kind of metal 
 required in the three different trusses. Assuming that there 
 would be no special circumstances of convenience, or incon- 
 venience, attending the manufacture of any particular truss, 
 there could be no question that was the best truss which 
 consumed 'the least metal in its construction. And if the 
 decision, as to the merits of the Fink truss (Fig. 1), and of 
 the Bolhnan truss (Fig. 3), were to be arrived at by the con- 
 sideration of this test, Mr. Bramwell believed he was cor- 
 rect in saying, that they were both of them inferior to the 
 ordinary diagonal truss (Fig. 2), and in the following ratio : — 
 
 Diagonal truss required 262 parts of metal. 
 
 Fink “ “ 896 
 
 Bollruan “ “ 370 
 
 The distribution of the metal would be as follows : — 
 
 Diagonal Truss : Compression 136 
 
 Tension 126 
 
 262 
 
 Fink Truss : Compression 200 
 
 Tension 196 
 
 396 
 
 Bollman Truss : Compression 185 
 
 Tension 185 
 
 370 
 
 These results were arrived at by a calculation which he 
 believed to have been based on correct principles, and to 
 have been accurately worked out. He would, however, en- 
 deavor to explain the causes of the extra consumption of 
 iron in the Fink and the Bollman trusses. 
 
 First, as to the Fink truss : — The diagram was made on 
 the assumption, that the depth of this truss, and of course 
 of the other two trusses also, was one-eighth of the span, 
 and that the truss was loaded with fifteen units of load, one 
 over each of the vertical struts. The stmts on the left-hand 
 side were lettered from A to H, the latter letter being over 
 the central strut. The effect of these units of load would be, 
 that unit A would, by its vertical A /, be carried one-half by 
 the tie-rod / a, and one-half by / B ; and that the unit B 
 would be carried on its vertical B b } which would also have 
 
AMERICAN IRON BRIDGES. 
 
 7 
 
 to carry the half of unit A, brought on by the tie-rod f B, 
 and also one-half of the unit C, brought on, in a similar 
 manner, by the tie-rod g B. In this way the vertical B b 
 would get a load of two units, one-half of which, or one unit, 
 would have to be upheld through the tie-rod b a, but as the 
 upper part,/ a, of this tie was already strained by the weight 
 due to one-half of the unit A, and as the angle of the tie 
 was 45°, the tie b a would bring a compression of exactly a 
 unit and one-half on the top member of the girder. It was 
 clear that a downward load of one unit and a half must also 
 be brought, by the tie b g D, on to the point D, and that a 
 similar load of a unit and a half must be brought on to the 
 same point by the tie d h D, making together a load of three 
 units. This, added to the unit D itself, would cause a load 
 of four units to have to be carried by the vertical D c. Of 
 these four units two would be carried by the tie c a, and the 
 other two by the tie c H. The tie c a would bring the com- 
 pression on to the top member of the girder due to these 
 two units. If the tie c a had been at the same angle as the 
 tie bf a, this compression would only have been two units ; 
 but as the tie c a was at an angle only half as favorable as 
 the tie bf a, the compression brought on to the top member 
 of the truss by the tie c a would be double that of the verti- 
 cal load carried by it, or a compression of four. The last 
 source from which compression would come upon the top 
 member of the truss would be from the strain of the tie e a. 
 This tie, it would be seen, had to bear one-lialf the duty of 
 upholding the central strut H e. This strut H e would have 
 to support, first, two units of load brought on it by the tie 
 c H ; secondly, one unit of load brought on by the tie d i H, 
 owing to that tie upholding one-half of the load of two on 
 F d, and, thirdly, one-half of a unit of load brought on by 
 the upper part i H of the same tie d i H, which upper part 
 would have to support the half of the unit of load on G i. 
 These three sources of strain would amount together to three 
 and a half units. There would, of course, be an equal load 
 brought on by the corresponding ties to the right hand of the 
 strut H e, which strut would thus be loaded with seven units, 
 „ to which must be added the unit on H itself, making eight 
 as the total load on the strut H e. Of this eight, one-half, 
 or four, would be upheld by the tie e a, and by this tie would 
 exert a compressive force on the top member of the girder. 
 It would be seen, that the angle at which this tie e a lay was 
 twice as unfavorable as that of the tie last considered, and 
 four times as unfavorable as that of the tie bf a, the angle 
 of which was 45°. Owing to this unfavorable angle of the 
 
8 
 
 AMERICAN IRON BRIDGES. 
 
 tie e a, the load supported by it would exert a compression 
 of four times its own amount, or sixteen. The sum of the 
 compressions, therefore, on the top member of the girder 
 would be : — 
 
 For the tie/ a £ unit x by the effect of the angle 1 = \ 
 
 “ “ ha 1 “ “ “ “ 1 = 1 
 
 “ “ c a 2 “ “ “ “ 2— 4 
 
 “ « ea4 t( “ “ “ 4 = 16 
 
 / 
 
 It would be seen, that this compression extended from end 
 to end of the top member of the truss, so that the top mem- 
 ber would require to have as great a sectional area at the 
 extremities as in the middle. Assuming that one part of iron 
 was required to resist one unit of load, then the sectional 
 area would be 21J, and the length of the truss being taken 
 as 8, the amount of iron consumed in the compression bar 
 would be equal to 21J x 8, or 172 parts of iron. 
 
 Before considering the amount of iron in the other mem- 
 bers of the Fink truss, Mr. Bramwell would compare the 
 compression bar, or top member, of this truss, with that of 
 an ordinary diagonal truss (Fig 2). In the latter case it was 
 well understood that, as fifteen units of load, distributed as 
 shown, were equal to a .central load of eight, and as the 
 girder was eight times as long as it was deep, the compres- 
 sion in the centre would, of course, be sixteen units of load. 
 This, then, would represent the maximum sectional area, 
 which would exist only at the very middle of the compres- 
 sion member, as it would become less and less towards the 
 ends of the girder ; the diminution being in such a ratio, if 
 the number of points of attachment of the ties and struts 
 were infinite, as would give a parabolic curve, the area of 
 which would, <5f course, be two-thirds of the parallelogram 
 containing it. Therefore 16 x § = 10§ would represent the 
 average depth of the compression member, which multiplied 
 by the length 8, would give 85^ as the amount of iron in that 
 member, or rather less than one-half of the iron in the sijni- 
 lar member of the Fink truss. In the diagonal truss, with 
 the number of ties and struts as drawn, this theoretic amount 
 was diminished only to the extent of 1^ ; that was to say, 
 the whole amount of iron was 84, arrived at thus : — 
 
AMERICAN IRON BRIDGES. 
 
 9 
 
 Pressure 
 
 Between the points a A the compression would be 3£ units, which 
 plied by the effect of the angle , 
 
 “ * AB “ 10£ 
 
 “ B C “ 16i 
 
 “ CD “ 21i 
 
 “ DE “ 25£ 
 
 “ E F “ 283 
 
 “ PG “ 30£ 
 
 “ GH “ 31 k 
 
 , multi- 
 
 t 
 
 II 
 
 54 
 
 84 
 
 104 
 
 124 
 
 144 
 
 154 
 
 154 
 
 Giving for the sum of the compressions 84 
 
 This divided bj the number of spaces 8, made 10| as the 
 average depth. 
 
 It was, he thought, not improbable that a cursory view of 
 the drawing of the Fink truss might lead to the idea that it 
 was one that must be economical ; inasmuch as it appeared 
 to have a compression member only, and to be without any 
 corresponding formal tension member, the diagonal ties be- 
 ing made to serve the purpose of a regular tension member. 
 But even if this were so, and if the ties were made at no 
 greater consumption of metal than the ties and struts of a 
 diagonal girder, which he would prove was not the case, even 
 then the Fink truss would have no advantage ; inasmuch as 
 its one compression member contained slightly more iron 
 than the compression member and tension member together 
 of the diagonal truss. He had stated that the compression 
 member of this latter girder contained 84 parts. The ten- 
 sion member contained rather more, thus : — 
 
 Tension. 
 
 Between the points b c the tension would be 4 units, which, multiplied 
 
 by the effect of the angle = 2 
 
 a 
 
 c d 
 
 66 
 
 11 
 
 66 
 
 5 h 
 
 n 
 
 d e 
 
 66 
 
 17 
 
 66 
 
 H 
 
 a 
 
 ef 
 
 66 
 
 22 
 
 
 11 
 
 cc 
 
 f g 
 
 66 
 
 26 
 
 6 6 
 
 13 
 
 a 
 
 gh 
 
 66 
 
 29 
 
 66 
 
 14 h 
 
 a 
 
 h i 
 
 66 
 
 31 
 
 
 15 h 
 
 66 
 
 i k 
 
 66 
 
 32 
 
 
 16* 
 
 Giving for the sum of the tensions 86 
 
 This, divided by their number, 8, made 10| as the average 
 depth. The amount of metal, 86, in that member, added to 
 that in the compression member, gave 170 parts for the 
 metal in those two members, as compared with 172 parts in 
 the compression member alone of the Fink truss. 
 
 He would now revert to the consideration of the Fink 
 truss : — 
 
10 
 
 AMERICAN IRON BRIDGES. 
 
 It would be remembered that the main tie bar e a had to 
 transmit four units of load ; but these became multiplied in 
 their power of strain on the tie, by the number of times the 
 length of that tie exceeded the depth of the girder. This 
 depth being 1, and the half length of the girder being 4, the 
 length of the tie would of course be equal to Vl 2 + 4 2 ='v/17. 
 The strain, therefore, would be the load y Vl7. But as the 
 length of the tie was also Vl7, it followed that the amount 
 of metal must be Vl7 x -\/l7 x the load of 4 ; in other 
 words, the amount of metal increased as the square Parts of 
 of the length of any sloping tie. In this case the Metai. 
 
 amount was 2 ties each 17x4 = 136 
 
 Similarly, it could be shown, that the ties ca , cH, 
 and their Counterparts on the other side of the 
 truss, would require V5 x V5 x 2 load x 4 of 
 
 them = . 40 
 
 That ba, bT>, d D, (7H, and their counterparts on the 
 other side of the truss, would require V2 x V2 x 1 
 
 load x 8 of them = ..... 16 
 
 and that the upper parts of these ties, fa, gD, hD, 
 
 ^'H, and their counterparts on the other side of the 
 
 truss, with the corresponding short ties, / B, gB, 
 hF, iF, and their counterparts on the other side 
 of the truss, would require Vj x Vj x J load x 16 
 of them = 4 
 
 Making the total of iron in tension, as already stated 196 
 
 Further, the compression member, it had been P Metai! f 
 
 proved, would contain 172 
 
 The strut He would contain 1x8 units = 8 
 
 The strut D c, and the corresponding one on the 
 other side of the truss, would contain 1x4 units 
 
 x 2 of them = 8 
 
 The struts B b, F d, and their corresponding ones on 
 the other side of the truss, would contain 1x2 
 
 units x 4 of them = 8 
 
 The struts A/, Cg, E li, Gf, and their corresponding 
 ones on the other side of the truss, would contain 
 ^ long x 1 unit x 8 of them == 4 
 
 Making the total of iron in compression, as already 
 
 stated 200 
 
AMERICAN IRON BRIDGES. 
 
 11 
 
 In order to complete the comparison between the Fink 
 truss and the diagonal truss, it would be necessary to ascer- 
 tain the amount of metal in the struts and ties of this latter 
 truss. The lengths of the various struts and ties would be 
 uniformly Vl*, the multiple of strain would therefore be Vl j, 
 and the amount of iron in each of them 1J x the units of 
 load : — 
 
 On the strut A b there would be 4 units. 
 
 tt 
 
 B c 
 
 (t 
 
 3J 
 
 a 
 
 it 
 
 C d 
 
 a 
 
 3 
 
 a 
 
 it 
 
 D e 
 
 (( 
 
 2J 
 
 a 
 
 tt 
 
 E/ 
 
 tt 
 
 2 
 
 a 
 
 tt 
 
 E 9 
 
 tt 
 
 li 
 
 a 
 
 tt 
 
 Gh • 
 
 tt 
 
 l 
 
 a 
 
 tt 
 
 Hi 
 
 a 
 
 1 
 
 2 
 
 a 
 
 Making a total of 18 
 
 This sum multiplied by the 1 j, would give 22 
 
 To which must be added the end upright ah = 3 
 
 Making 26 
 
 Or, for the two sides of the truss 52 
 
 Adding the amount of metal in the compression mem- 
 ber before given 84 
 
 The total of iron in compression would then be, 
 as already stated . 136 
 
 Next, as regarded the parts in tension : — 
 
 On the tie a c there would be 34 units. 
 
 vyii me tiu 
 
 a 
 
 Uj V lilfcJI 
 
 Ad 
 
 tJ WUUJLU 
 
 ft 
 
 3 
 
 uiiil». 
 
 tt 
 
 a 
 
 B c 
 
 • tt 
 
 2i 
 
 tt 
 
 a 
 
 c/ 
 
 it 
 
 2 
 
 tt 
 
 a 
 
 Vg 
 
 tt 
 
 li 
 
 it 
 
 a 
 
 E h 
 
 tt 
 
 i 
 
 tt 
 
 a 
 
 F i 
 
 tt 
 
 1 
 
 2 
 
 tt 
 
 it 
 
 Gh 
 
 tt 
 
 0 
 
 tt 
 
 Making a total of 
 
 This, multiplied by 1|, would 
 
 give. . . , 
 
 14 
 
 
 
12 
 
 AMERICAN IRON BRIDGES. 
 
 Making for the two sides of the truss 35 
 
 Adding the amount of metal in the tension member be- 
 fore given 86 
 
 The total of metal in tension was thus found to be ... . 121 
 
 4 
 
 But to this should be added the ties G 1c, which, although 
 not called into action, when an uniform load was on the 
 girder, would have to bear a strain if the load were a 
 passing one. They might then be required to support a 
 single unit of load. Under similar circumstances two 
 ties on each side might require to be strengthened, so as 
 to bear a further half unit each ; then 4 ties 1 unit x 
 
 thelj = 5 
 
 • 
 
 Giving as the total of iron in tension, as already stated . 126 
 
 He would not occupy any further time, by entering into 
 the details of the calculation of the quantity of metal in the 
 Bollman Truss ; but would simply refer to the figures already 
 given, by which it would be seen, that while it was more 
 economical than the Fink Truss, in the proportion of 37 to 
 39.6, it was less economical than the ordinary diagonal truss, 
 in the proportion of 37 to 26.2. 
 
 The cause of the waste of metal in the Fink and in the 
 Bollman trusses appeared to him, on a little consideration, 
 to be sufficiently obvious. The strains which related to the 
 centre part only of a truss, and which might be got 
 rid of in a comparatively short distance near that centre, 
 were, in the Fink and in the Bollman trusses, carried to the 
 very ends of the compression member, so that the whole 
 length of that member sustained a pressure that need only 
 be borne near the centre. Further, to carry out this bad 
 arrangement, the tie-rods were, of necessity, placed at most 
 unfavorable angles, by which not only was an unnecessary 
 amount of metal consumed, but the already useless strain on 
 the compression member was aggravated. 
 
 It was unnecessary to investigate the construction of the 
 Murphy- Whipple truss, as the drawing showed that truss to 
 be free from the radical error, of carrying the whole of the 
 compression to the very ends of the top member. It was, 
 however, clear, that the use of struts in a vertical position 
 was not so economic as the use of struts in a diagonal posi- 
 tion, inasmuch as the struts when placed vertically did not 
 assist in the progression (if such a term might be allowed j 
 
AMERICAN IRON BRIDGES. 
 
 13 
 
 from end to end of the truss. The ties, also, were not dis- 
 posed at the most economic angle ; but with those excep- 
 tions, he had little doubt the Murphy- Whipple truss would, 
 on investigation, prove to be one of good construction, and 
 equal to such as were in ordinary use in England. 
 
 He regretted having to pronounce so unfavorable an opin- 
 ion on these trusses ; but he was glad to be enabled to do 
 so, without the fear of his criticism being displeasing to the 
 Author, who, it was understood, had merely submitted these 
 particular trusses, as matters of interest in the history of, 
 railway works in America, without expressing approval of 
 their construction, and for the very purpose of having their 
 merits, or demerits, fully discussed. 
 
 Mr. Phipps said, the Paper was for the most part limited 
 to several varieties of trussed girders. On these he would 
 not at present offer any observations, but would confine his 
 remarks to one or two iron arched constructions, also referred 
 to in the Paper, as he had paid particular attention to the 
 subject of cast-iron arches. It had been truly remarked, 
 that whenever these arched constructions were loaded irreg- 
 ularly, it became difficult to calculate the strain to which the 
 material then became subject. For instance, iron arches, 
 particularly on railways, might be loaded upon one-half of 
 the span only. In such a case the curve of equilibrium 
 would often shift so much from the middle of the arched rib, 
 as almost to touch the extrados and the intrados of the arch 
 on opposite sides of the centre. Having obtained, however, 
 the position of the curve of equilibrium, it then became a 
 question, how to estimate the effect of the pressure in its 
 detrimental action on the outer ‘fibres of the rib. This was 
 a point upon which, he agreed, that no practical, or reliable, 
 information was to be obtained from books, and, in conse- 
 quence, he had invented a method which he had found both 
 simple and accurate in its application. To illustrate the de- 
 gree to which the shifting of the line of pressure might affect 
 the strain upon an iron arch, when placed under compression, 
 he stated that in a prismatic bar of iron, in shape like a 
 3-inch plank, the pressure on the outer fibres on one side 
 would be doubled, and on the other side be reduced to noth- 
 ing, by the removal of the line of pressure only the one-sixth 
 part of the whole width away from the centre of gravity. 
 Now in this, as in every other case, whenever a piece of 
 metal, or other elastic material, was compressed by a force 
 on any other line than that of its centre of gravity, the action 
 upon the outer fibres might be obtained, by conceiving the 
 material, first of all to be compressed squarely throughout, 
 
14 
 
 AMERICAN IRON BRIDGES. 
 
 as if the strain were applied over the line of centre of grav- 
 ity, and then, finding the strain due toHhe angular motion, 
 by the same process that would be used for obtaining the 
 strain on that portion of a cantilever contiguous to where it 
 was attached to a wall ; using the total compressive force 
 applied as the weight, and the distance of that pressure from 
 the line of centre of gravity as the leverage. Then the 
 strain on the outer fibre thus obtained, added to the former 
 square-on pressure, would give the whole detrimental action 
 upon the outer fibre of the material. Thus, as he had pre- 
 viously explained in reference to the Charing Cross Bridge, 
 when a pressure of 475 tons was applied, square-on, to a 
 piece of iron of 161.25 square inches sectional area, all the 
 fibres would be strained uniformly up to 2.94 tons on the 
 square inch ; but by removing the line of pressure 3.6 inches 
 away from the centre of gravity, the pressure on the outer 
 fibres would be increased up to 6.86 tons on the square inch. 
 Mr. Phipps added that, in cases where the arched rib proper 
 was well connected with the roadway bearer above, by an 
 efficient system of diagonal bracing, the centre of gravity of 
 the whole section of the arched rib and roadway bearer to- 
 gether must be taken, as that around which the previously- 
 named angular motion must be ’computed. 
 
 Mr. F. W. Sheilds could not agree, that the object of the 
 bracing in a girder bridge was merely to keep the top and 
 bottom members asunder, and to enable them to do their 
 work. In his opinion, the chief object of bracing in all 
 parallel girders, was to carry the vertical weight of the 
 bridge, with the extraneous load upon it, to the points of 
 support at the piers ; and that duty was just as important 
 as, and quite distinct from, the work to be performed by the 
 top and bottom flanges. The bracing, ought to be specially 
 designed for that duty, and in that view he thought the 
 American bridges brought under their notice, even those of 
 the simplest form, showed a want of scientific knowledge in 
 their construction. Thus, if a girder bridg# be supposed to 
 consist of twelve bays, and the driving-wheels of a locomo- 
 tive loaded with twelve tons rested over the third bay, that 
 load would be transmitted to each pier, by the bracing, in 
 the inverse proportion of the distance of the loaded point 
 from the piers, being nine tons to the nearer, and three tons 
 to the further pier. One of the diagrams exhibited was just 
 in such a case, and he observed that at the' third bay, there 
 was no diagonal tie from the lower flange which could carry 
 the load to the further pier. Consequently the girder would 
 have to depend merely upon the rigidity of the top flange, 
 
AMERICAN IRON BRIDGES. 
 
 15 
 
 considered as an independent beam of perhaps a few inches 
 deep, for the transmission of that part of the load which 
 reached the further pier, and which should be sustained by 
 the whole girder of several feet deep. Diagonals of great 
 inclination were not advisable, independent of all increase of 
 strain arising from their inclination, as they were free to de- 
 flect on a curve struck from one of their ends as a centre and 
 the other end |ts a radius, and that curve would coincide in 
 practice, for a considerable distance, with a vertical line, so 
 that there was little or no resistance to deflection. There 
 were some bridges in the Regent’s Park upon that principle, 
 which fully bore out his foregone conclusions, for he found 
 that by jumping upon those bridges, the whole could be set 
 in motion by his own weight alone. 
 
 Mr. Zerah Colburn remarked, with reference to the com- 
 parison which had been made between the various trusses, 
 that it was well understood in the States, as applying to the 
 Fink and B oilman Bridges. The Murphy- Whipple Bridge 
 was believed to be the best class known in the States ; and 
 if there was not much difference between that and many 
 bridges in this country, it was, no doubt, owing to the fact, 
 that when the principles were fixed, there could not be 
 much room for difference between one good bridge and 
 another. 
 
 Mr. Hawkshaw, President, expressed his regret that, owing 
 to the late period of the Session, and the necessity for read- 
 ing two other Papers before its close, the discussion upon a 
 subject of so much general interest as American Iron 
 Bridges should have been necessarily restricted. The Insti- 
 tution was under great obligations to Mr. Colburn for the 
 able and elaborate manner in which the communication had 
 been made ; and he hoped that the Author would, on other 
 occasions, contribute to, and take part in, the proceedings. 
 

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