84 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[March, 



counter-hraces removed for the purpose of repairs during tlio re- 

 gular transitu of trains. 



Tlie term Lattice is therefore incorrectly applied to these 

 bridges ; and in America, where the two are quite distinct struc- 

 tures, the lattice has fallen into disrepute amonj; railway en- 

 gineers, having generally failed, and exhibited a want of rigidity — 

 a natural result where the parts are not abutted but placed side by 

 side, and fastened with pins p.issing through the very heart of the 

 material itself, on which, as so many pivot points, there is a con- 

 stant working of the whole structure, which splits the material, 

 and produces fracture and sinking, unless, as in the extensive 

 lattice bridges now in use in America, the whole fal)ric be timely 

 supported by heavy queen-post trusses inside, and bolted to tlie 

 lattice beams, or by timber arches, which really do the whole duty. 

 But the strain on a lattice bridge is felt as in all other common 

 rectangular beams ; and it fails, tlierefore, in the centre of the 

 span invariably, there being no strut or brace, with properly abut- 

 ting surfaces, to relieve the weak part, and carry back the strain 

 to the legitimate points of support — viz., the abutments. 



Now, in truss beams of the former construction the strain com- 

 municated by a load is at once taken up and distributed by the 

 forces which act only in the direction of the grain of the material 

 employed, — the weight felt by the vertical ties in a tensile 

 strain acting on the braces in compression, these communicating 

 it in tension to the bottom chord, and in compression to the top 

 chord ; thus getting rid of all direct transverse strain, and afford- 

 ing one of the greatest elements of strength, which is materially 

 assisted by the fact that the heavy weight of all surplus material 

 is dispensed with. 



Having for some years given this subject a good deal of time 

 and consideration, and had the benefit of experience in the erec- 

 tion of many of these structures, I have observed — 1st, that a 

 tensile strain is not confined to the bottom chord, but that the 

 top chords immediately over the points of support on the abut- 

 ments are subjected to a considerable degree of tension. 2ndly, 

 that in case of any tendency to failure of the bottom chords, ver- 

 tical ties, or braces, such invariably takes ])lace at about ^ of the 

 span from the abutments ; and that it is therefore advisable to 

 increase the sectional areas of all these parts for some distance 

 from each abutment. Indeed, in spans of any great length, the 

 sectional areas should be as large as possible at the points of sup- 

 port, and gradually decrease towards the centre of the bridge ; 

 and 3dly, that a due proportioned height of truss on the abut- 

 ments is requisite to ensure the stability of the structure. 



There are two chief points only to be determined by the en- 

 gineer before erecting a bridge of this kind, which, if correctly 

 obtained, will enable him to carry it to any extent known — viz., 

 the sectional areas of the parts, and the height of truss due to 

 the required span which must control the proportions adopted. 



After explaining the principle of certain timber bridges in North 

 America, Mr. Buchanan suggests the adaptation of this principle 

 to iron, for the purpose of forming extremely simple and strong 

 beams, by making the bottom chords of malleable iron and the 

 braces and counter-braces of cast-iron in the form of hollow square 

 tubes. 



The perishable character of timber, and the difficulty which has 

 been always experienced by its inability to withstand the great 

 tensile strain to which the bottom chords are necessarily sub- 

 jected, engaged the attention of the subscriber some years ago, as 

 affording an opportunity for the adaptation of the principle to iron 

 exclusively ; and it is gratifying to him to find tliat the views 

 which have controlled his efforts in the work, so closely correspond 

 with the opinions of such high authorities as Professor Forbes and 

 Mr. Buchanan. 



In the year 184i, after sundry experiments for the purpose of 

 ascertaining tlie iiroportions of the various parts, and for adajiting 

 those jiarts to iron, tlie suliscriber built the first iron bridge on this 

 principle fur railway travel in the United States of America, and 

 in 1H45 introduced it into this country; since which he has con- 

 structed al)(iut a dozen iron bridges, varying in span from 30 to 90 

 feet, in the latter of wbich, it is not a little curious, Mr. Bucha- 

 nan's suggestion of hollow, square, cast-iron braces has been actu- 

 ally anticipated, by which strength and lightness of material is 

 certainly affected. 



The advantages derived by this adaptation are — 



1st, Economy combined with rigidity. 



Sndly, Simplicity and facility of construction and erection. 



3rdly, The great ease with which any camber can be given, and 

 a certainty of its being permanently retained. 



On the first of these points, it will suffice to give a brief state- 

 ment of the cost of two of the bridges just alluded to. They 



have been in use for upwards of two years, and were built under 

 the disadvantages of having inexperienced workmen, and without 

 the aid of any machinery whatever. 



Length of beam 98 feet. 



£ t. A. 



Cast iron, 56 tons, at 12/. 672 



A\r()ught-iron, 37 tons 1 cwt. 3 qr. 3 lb. at U/. 519 4 10 



M'orkmauship and erecting 270 10 6 



Total £1461 15 4 



equal to almost 15/. per running foot of bridge, which is higher 

 than any of the others, being unnecessarily heavy, particularly in 

 the item of cast-iron, although it has the hollow traces suggested. 

 It is on a skew of 24|°. 



The next statement exhibits the cost of a beam 71 feet long, 

 across an opening of 50 ft. 10 in. in the clear. 



£ t. d. 



Cast-iron, 16 tons, at 12/. per ton 192 



Wrought-iron, 13 tons, at 14/. per ton ... 182 

 Workmanship and erecting 103 10 o 



Total £477 10 



equal to about 6/. 15*. per lineal foot of bridge, which will give a 

 better idea of economy. 



About a year ago, one of these bridges was severely tested by 

 the breaking down of a ballast-train on it ; and though several of 

 the axles were broken, and five of tho wagons were heaped in per- 

 fect wreck upon the structure, tearing up the floor, breaking out 

 four of the adjacent braces of one of the outer beams, and frac- 

 turing others, it was found after the wreck was removed, that the 

 bridge had not yielded in the least, either vertically or laterally, — 

 and was subsequently repaired, at a cost of 36/. 



Simplicity of construction, which naturally produces economy, 

 as the second point, presents itself for consideration. 



The braces on being furnished from the foundry, are put under 

 the plane or vertical chisel, to bring them to an exact uniform 

 length by taking off a mere shaving and leaving the abutting ends 

 square : the remainder of the cast-iron requires only a careful 

 cleaning. Thus, with very little workmanship, near two-thirds of 

 the material is ready for use. 



In ordinary spans, the chords may be composed of plates of 

 "Welsh iron, welded into one continuous piece (where careful 

 smiths can be procured); or they may be left in convenient lengths, 

 and fastened at the joints by suitable scarfs and bands or rivets. 

 The chords are then clamped together, placed on edge, and the re- 

 cesses for the blocks cut out in the rough, and afterwards dressed 

 up with the file ; the blocks are then fitted, and they are ready for 

 erection. In addition to the w rought-iron plates which compose 

 the top chord, there is usually a cast-iron cap which covers it, 

 formed with flanges on tlie under surface, which fill up the spaces 

 between the wrought-iron (dates. The whole being clamped to- 

 gether, forms a beam more or less solid in proportion to the com- 

 pression it is calculated tii resist ; while the wrought-iron in the 

 top chord resists any tensile strain to which it may be subjected 

 over the points of support, as before referred to. These cast-iron 

 caps are made in convenient lengths, and require no workmanship 

 but fitting on during the erection of the bridge. 



The vertical tie-rods are of cable iron if the span is large, and 

 are made without a weld, liaving the lieads and nuts uniform. 



All the respective )iarts of these structures are unifcu-m, and any 

 previous fitting together is unnecessary; indeed, they may be made 

 in different establisliments, brought to the abutments, and put 

 together at once for the first time. 



From the position of the abutting surfaces, the beam, on screw- 

 ing up the vertical ties, adjusts itself in line and camber, and no 

 force will prevent its assuming the required form with truth. The 

 sectional areas are left whole and available, there being not a pin 

 or bolt through any of the parts. 



Tlie portions of the structure liable to injury from accidents can 

 be repaired without deranging the whole, or rendering it unfit for 

 constant use. 



The third point is the facility of cambering, which is peculiar to 

 this structure. In ordinary horizontal beams, about four inches in 

 the 100 feet has been adojrted ; but less can with a great degree of 

 nicety be given, and if carried to even a semicircle involves j(o ad- 

 ditional expense wliatex-er in framing or general construction, — the 

 unifiu-mity of the parts being preserved as in the ordinary straight 

 beam. It would appear that so small a camber as four inches, in a 

 structure comjiosed of a number of parts, would soon exhibit irre- 

 gularities in its curve ; but under the heaviest traffic this camber is 



