1849.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



15 



which they have heen or may be applied. No general rule, he 

 said, could be laid down, but one principle should never be lost 

 sight of — that the strongest form is the simple one of the roof : the 

 two rafters butting against eacli otlier at the top producing a 

 simple compressive strain through their length, and at the bottom 

 a horizontal thrust on the extremities of the tie or tension-rod, 

 producing simple distention. Forms might require to be modified 

 by circumstances, but to this they ought all to tend, as to a centre 

 involving the most perfect distribution of the forces. He then 

 gave several illustrations of the mode of applying these ties, 

 jjointing out where they would be of essential service, and where 

 they were objectionable and inefficient. 



In many cases of railway bridges, the space between the level 

 of the railway on tlie one hand, and sometimes a road, sometimes 

 a stream, or navigable river, on the other, is so confined, that even 

 with tension-rods well ap])lied, or massive girders, the span is so 

 great as to occasion too enormous a strain to be safe or expedient; 

 it is much better in such cases, rather than attempt to span the 

 opening with too limited a depth of beam or girder, to acquire 

 height by setting tlie girders on the outside of the railway, where 

 an unlimited height can be obtained for arching, or framework of 

 timber or iron ; and this leads to the consideration of a remark- 

 able species of bridge much used in crossing the vast openings of 

 the American rivers, both for common roads and railways. It is 

 termed the Frame Bridge or Lattice Bridge. These bridges have 

 been most extensively applied, and with complete success, and, by 

 successive impro\'ements, have now been brought to great perfec- 

 tion ; and as they possess some remarkable properties, and form 

 en e.xcellent illustration of the principle of dividing the tensile 

 and compressive strain into distinct members of the bridge, he 

 thouglit it might not be uninteresting to the Society, to give a 

 short explanation of them here. A very interesting account of 

 these bridges, one !of which, over the Susquehannah at Columbia, 

 of twenty-nine arches, each of 200 feet space, is about a mile and a 

 quarter in length, will be found in Mr. Stevenson's excellent work 

 on the "Civil Engineering of North America," and through two 

 engineering friends he had been favoured with farther information 

 and drawings, which were exhibited.* The great principle of the 

 frame bridge he then illustrated by reference to a small model. 

 It is nothing but a simple modification of the principle of the 

 roof; two rafters meeting in the centre of the bridge, and resting 

 at their extremities on a tie-beam ; from tliis centre the tie-beam 

 is extended longitudinally on each side, and running horizontally 

 to the opposite abutments; and along with it, at regular intervals, 

 a series of rafters, running parallel with each other and parallel 

 with the centre one, are extended the whole length of the bridge ; 

 the feet of these rafters rest on the tie-beam ; the tops of tliem 

 cannot meet, but are connected by an upper longitudinal beam 

 running horizontally. On this the rafters are all abutted and act 

 exactly as if each pair had met in the centre, only that the inter- 

 mediate connecting beam is subjected to a compressive strain, 

 arising from these rafters all pushing on towards the centre, in 

 the same manner as the lower beam is subjected to a tensile 

 strain from all the feet of the rafters pushing off from the centre. 

 The upper beam is termed the top chord, the lower beam the 

 bottom chord, and the rafters are called the braces ; and one mem- 

 ber more is only wanted to make the structure complete — namely, 

 a beam or tie standing vertically, to connect the top of the one 

 rafter with the foot of the next adjacent, towards the centre ; and 

 in this manner every part of the frame is supported, and there 

 being no cross strains w-hatever, it is truly astonishing how much 

 such a structure will bear. The last-mentioned beam, from being 

 subject to the tensile strain, is termed the tie, and a great im- 

 provement has been effected by the introduction of malleable iron 

 rods in place of timber ; by means of this, and of screws and nuts, 

 the whole structure can be brought to a perfect degree of tension, 

 60 that every joint and member may bear its due share of the 

 load ; and in the case of shrinkage of the timber, or other derange- 

 ment, the equilibrium and perfect form of the structure can 

 easily be restored and maintained. By screwing up the ties in 

 tliis manner, the bridge tends to assume an arched form, rising 

 with a camber in the middle ; to prevent this, another member has 

 been introduced, termed the counter-brace, which is a beam of 

 timber, extending from the top of one rafter to the bottom of the 

 next adjacent, from the centre towards the extremities of the 

 bridge. This counter-brace crosses the braces and resists any 

 change of form whicli the screwing-up of the rods would bring on; 

 and there is this remarkable advantage obtained, that the action 



* For one of these drawings he was indebted to his old friend and assistant, Mr. 

 Laurie, now engineer on some of the American railways; and for the other, along with 

 Interesting views as to the principles of construction, to Mr. Lawson, a friend and 

 engineer, lately returned to Britain. 



of these counter-braces, thus screvved-up to a certain degree of 

 tension, prevents the weight of the passing loads from having any 

 effect in straining or deflecting the bridge. Instead of any addi- 

 tional strain on any part, these loads rather relieve the counter- 

 braces from the tension to which they are subjected. No deflec- 

 tion or change of form can occur, except what may arise from the 

 mere compression or distention of the parts, and these being all 

 strained endways, and there being no cross or oblique action, this 

 effect is absolutely nothing ; and the form of the structure and 

 strains on it become in a great measure, if not entirely, indepen- 

 dent of the fluctuating traffic. The bridge is already strained to 

 the utmost extent of any passing load, and cannot be affected by 

 it ; whence arises a principle of stability and safety, w ell worthy of 

 consideration, particularly in the case of railways. The strength 

 and stiffness of the small model was then shown, and the enormous 

 load whicli it carried ; and the whole of this interesting subject 

 was concluded by the exhibition of a much larger model of one of 

 the American bridges. Bridges of so great a span as 200 feet are 

 common enougli in that country, and the model represented one of 

 these. It was exactly 1-lOth of the dimensions, being 20 feet long, 

 and the frames on each side 2 feet deep, consisting of the top and 

 bottom chord, ] inch by 2^ inches; the braces in pairs | of an 

 inch square; the tie-rods in pairs 5-inch diameter; and the 

 counter-braces each single, |-inch square. One of these frames 

 w as placed at each side of the bridge, connected at the bottom by 

 ci'oss beams on which was laid the planking of the roadway. The 

 whole weight of the bridge was only 113 lb., and although 20 feet 

 span and of such slender materials, it carried six persons, equal to 

 at least 7 or 8 cwt., standing on the centre, without deflecting 

 more than j of an inch. 



Another advantage he also mentioned of these bridges was the 

 simplicity of construction; the braces and counter-braces were all 

 cut exactly to the same length and square on the ends; no 

 morticing or jointing of any kind, but resting simply on blocks 

 attached to the top and bottom chords, through which blocks 

 were also passed the tie-rods. Nothing could be simpler, and the 

 whole britlge could be taken down, removed to another site, and 

 there put up with facility. It is easy to see also, that as the 

 malleable iron ties have been substituted for the upright timbers, 

 so may it be for the bottom chord; and the braces and counter- 

 braces could be made of cast-iron in the form of hollow square 

 tubes, which would altogether form an extremely simple and 

 strong girder or bridge. One remark, however, he must make re- 

 garding all these bridges or girders of a rectangular form — namely, 

 that though they may be practically convenient in many respects, 

 they are all attended with a sacrifice of material, in so far as they 

 deviate from the principle of the arch. Tlie top chord might be 

 in the form of an arch, starting from the lexel of the roadway at 

 the abutments, and rising to the original height at the centre. In 

 this way much material would be saved, while the strain on the 

 chord would be greatly less, owing to the curvature; and this 

 form is often used in America, but not combined with the princi- 

 ple of the braces and counter-braces as it might be. The same 

 remark applies also to the iron tubular bridges, which possess ad- 

 vantages in simplicity and great steadiness, but no doubt the 

 material at the ends is redundant, and the strain in the centre 

 beyond what would occur in the case of the arch. He now begged 

 to conclude these expositions, which had been extended to much 

 greater length than he had anticipated. 



After the reading of the paper. Professor Fobbes observed, that 

 in regard to these American bridges, which appeared to possess 

 ])eculiar projierties, he vvould put a question or two which Mr. 

 Buchanan might probably answer — namely, how far they did not 

 resemble in principle the iron tubular bridges of Mr. Stephenson. 

 The frame-work in the model of the American bridge at the sides 

 was no doubt open, but he observed from the drawing that there were 

 double the number of frames as there were in the model, and he 

 understood that the sides were often formed by numerous little 

 lozenges, or lattice-work, foi-ming nearly a continuous web, and 

 he should like to know whether the sides of the tubular bridge, 

 with the angle-irons to strengthen them, did not much resemble 

 this web of lattice-work. In regard to the top and bottom also, 

 it appeared from the model that those also were united by diagonal 

 beams or frames, forming a sort of enclosure at top and bot- 

 tom, and as the side frames were twenty feet in height, this left 

 ample head-room for passengers and carriages. 



Mr. Buchanan could not help expressing how much he felt gra- 

 tified by the countenance and approval of such high authority as 

 Professor Forbes. In regard to the American bridges, the resem- 

 blance in principle to the tubular bridges had struck liimself 

 forcibly in considering these structures. The top and bottom 



