1850.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



55 



no hesitation in making one of 200 feet span ; but the engineers generally 

 seemed to consider that other modes of construction disposed the material 

 more advantageously. Mr. P. M'. Barlow exhibited a new form of girder in 

 separate castings, for moderate spans. 



The Arched Girder. — The cast-iron arch is a mode of construction which 

 all engineers concur in approving of, when not limited by considerations of 

 levels or of abutments. Mr. Locke states he would never willingly use cast- 

 iron in any other shape than that of an arch. Mr. \V. H. Barlow has also 

 adopted it where practicable. 



The Trussed Girder is straight and of separate castings bolted together, 

 assisted by wrought-iron tension rods. The Dee Bridge girder was on this 

 principle. Mr. Stephenson caused an experimental girder to be made, to 

 exhibit the effect produced by the tension rods, adjusted as they were in the 

 Dee Bridge girders, as well as the effect when adjusted to lie parallel with 

 the bottom flange and adjoining it; these experiments, in conjunction with 

 some made by Mr. T. L. Goocb, show that tlie tension-rods, though th eydo 

 not, when acting at the angle, as they did in the Dee Bridge girders, produce 

 the full effect ; yet, that they add considerably to the strength of the girder. 

 Mr. Rastrick and Mr. Fairhairn object to the trussed girder on account of the 

 different rates of expansion in cast and wrought-iron. Mr. Stephenson and 

 Mr. Wild propose to obviate this objection by putting the tension-rod along 

 the bottom flange, and applying to it an initial strain of five or six tons per 

 square inch, so as to cause the wrought-iron to come into play as soon as 

 any weight is applied to the girder. Mr. Fox approves of this arrangement, 

 bnt he considers that a strain upon wrought-iron tends to stretch the metal 

 permanently, and that the tension-rods would require to be tightened perio- 

 dically, whilst Mr. Stephenson and Mr. Wild have concluded from their 

 experiments, that with a less weight than ten tons per square inch, the elas- 

 ticity of the metal is not affected. The measure of the strain upon the tension 

 rods is the amount they are actually elongated by screwing up. As a com- 

 bination of wrought and cast-iron, Mr. P. \V. Barlow has proposed to cast 

 a bar of wrought-iron in the bottom flange of a girder, and not to make the 

 bottom flange so large. Mr. Locke, Mr. Stephenson, and Mr. C. May, con- 

 sidered that the different rates of expansion of the two metals would be au 

 objection to it. Jlr. Brunei objects to the use of cast-iron in long spans, 

 and its combination with wrought-iron, and prefers a framing of wrought- 

 iron and wood. 



Bowstring Girder. — Mr. Hawksbaw, Mr. Glynn, Mr. W. H. Barlow, Mr. 

 Locke, Mr. Fox, and Mr. Joseph Cuhitt, are agreed in considering the how- 

 string form of girder, with a bow either of cast-iron or wrought-iron cells 

 and the tension-rods of wrought-iron, as f °e from any objections urged 

 against other modes of combining wrought and cast-iron. It is considered 

 applicable, under almost all circumstances, as the roadway can be suspended 

 from the bow. 



Box or Tubular Girders. — Mr. Fairbairn considers these girders the best 

 for large spans, and from some experiments be made, considers them capable 

 of resisting not only dead weight but also impact. Mr. Stephenson states 

 that they are cheaper and more elastic than other forms for spans of more 

 than 40 feet, and he recommends that the top should be made of cast- 

 iron to resist compression. Mr. Glynn and Mr. Locke mention that they 

 have been used for steam-engines for some time, and consider the plan 

 sound. Mr. Brunei looks upon the introduction of wrought-iron into the 

 construction of girders as the most important step that has been taken for 

 some time in engineering; and he considers that, with ordinary care, and 

 with the improvements which have been introduced in the mode of riveting, 

 the joints made by riveting may be as permanent, and in every respect equal 

 to the other parts of the structure, and he does not consider oxidation or 

 vibration can affect them. With respect to riveting, Mr. Brunei considers 

 that two plates could be riveted together so as to ensure their not breaking 

 in any part contiguous to the rivets or joints, because the rivets should not 

 act as pins or bolts, but as clamps, whicli by pressing the plates together, 

 produce an enormous friction. Mr. Clarke, however, who has made a good 

 many experiments on the subject, does not appear to have obtained so close 

 an union of the plate, as he states that they generally broke at the riveting. 

 Mr. Hawksbaw has adopted wrought-iron girders for large spans, because he 

 considers the use of wrought-iron more advisable than cast-iron for large 

 spans : the box form is adopted to produce lateral stiffness. Mr. Fox and 

 Jlr. Rastrick consider that a large structure, like the Menai Bridge, must be 

 subject to sudden compression and extension from the changes of tem ■ 

 perature. 



Suspension Bridges. — Mr. Stephenson does not consider suspension bridges 

 applicable to railways except to very small extent; and he states that he has 

 been informed that an engine and train passing over one at Stockton (which 

 has since been replaced by a girder bridge), pushed the bridge hke a wave in 

 front of it. Mr. Brunei states that, under very peculiar circumstances, he 

 once proposed a suspension bridge himself. Mr. Brunei considers that the 

 lattice bridge is advantageous only under circumstances which would prevent 

 materials of more than a certain length being procured. Mr. Stephenson 

 objects that the compression cannot be carried through them, and that the 

 base through which the strain has to be carried is not sufliciently broad. 

 It is stated, however, that Sir J. M'Neill has remedied the want of power to 

 resist compressiou by introducing a cast-iron top. 



Best form for Bridges independently of Expense. — Mr, Rastrick, Mr. 

 Hawksbaw, Mr. Fox, Mr. P. W. Barlow, Mr. Glynn, Mr. Locke, -Mr. Brunei, 

 and Mr. Cubitt, agree in considering that the best form for ii-on bridges of 

 large span is that of a cast-iron arch. Mr. Grissell states that he considers 

 a well-made straight girder equally to be depended upon, hut admits that 

 the arch is the strongest form ; and Mr. Fairbairn says that for spans beyond 

 70 or 80 feet he would prefer wrought-iron tubular girders. Mr. Stephenson 

 would use narrow wrought-iron girders. 



Action on Skew Bridges. — It does not appear that the deflection of gird- 

 ers is sufficient to cause oscillation in engines passing over skew bridges, by 

 causing one side to be deflected to the full amount before the other. But Mr. 

 Stephenson mentions that when the road has been in had order, one wheel 

 being on the solid angle of the brickwork, while the other was on the soft 

 ballast, has caused considerable oscillation. 



Effect of Impact and Vilration. — It is not admitted that the vibration 

 caused by a railway train on bridges would injure the bolts or rivets of 

 compound girders, if well-made and strong in the first instance. Mr. Grissell 

 gives them a large amount of surplus strength, as he thinks that when no 

 greater strength of iron is put than is absolutely necessary, every jar must 

 tend to loosen the joints, and he considers that vibration has much 

 more effect on wrought iron than on cast iron. Mr. Fox states that he 

 would not depend on a cast-iron girder of separate pieces bolted together 

 without strengthening it with a wrought-iron tie-bar, but the use of wooden 

 sleepers interposes a cushion which does away with the vibration. Mr. W. 

 II. Barlow mentions that with light engines he found felt very useful in 

 diminishing vibration, but that with the heavy weights now in use on the 

 Midland line any interposing medium is crushed out. Mr. Stephenson 

 attaches no great importance to vibration, and has laid iron girders on brick 

 without interposing medium; and the fact of old caSt-iron mill-work having 

 run for so long a time without breaking is cited by Mr. Hawksbaw, as an 

 instance of the apparently small efl'ect of vibration. Mr. W. H. Barlow con- 

 siders that the irregularities wiiich exist on the road from uneven joints, &c. 

 in the rails is a greater cause of danger than vibration, and he mentions that 

 to experiment on the impact he caused the rails to be whitewashed for a 

 mile before the passage of a fast train of 12 carriages, and that the small 

 imperfections in the joints caused spaces adjoining them of 5 inches in 

 length to be left untouched by any of the wheels in the train. 



Change of Internal Structure in Iron. — Mr. Rastrick mentions that at the 

 Pont-y.Pool Iron Works a bar of wrought iron suspended, and continually 

 struck by a hammer at the bottom, dropped in two after a length of time, 

 but he knows of no instance of a change of structure on railways. Mr. Hawk- 

 sbaw, though he has observed crystallization in broken rails and axles, has 

 not traced it directly to vibration : be thinks mill-gearing and shafts would 

 furnish good examples, though when they break the various circumstances 

 under which the fractures have taken place should be observed. Mr. Grissell 

 has observed that the vibration to which crane chains are exposed changes 

 the iron from very beautiful malleable iron to the crystalline appearance of 

 cast iron. He does not consider that east iron is subject to the alteration 

 of structure. Mr. Fox considers that vibration does produce a change in 

 the internal structure of wrought iron, and instances that if the thread of a 

 screw be cut in a wrought-iron bar, and that the bar be broken across the 

 tapped part, the fracture there will be found more crystalline than at the 

 other part : he mentions the frequency with which shafts and mill-gearing 

 break, and states that cold-hammering the axles to give them a high polish 

 changes their internal structure; bnt instead of remedying the injury by an- 

 nealing, he recommends that they should be finished at a high temperature. 

 Mr. Grissell mentions tliat chains of cranes often break with a crystalline 

 fracture, which he attributes to a change in the internal structure, but he 

 does not consider the same effect is produced in cast iron. Mr. Fairbairn 

 states, that repeatedly making a wrought-iron bar red-hot, and plunging it 

 into cold water, renders it crystalUne, and that the fibrous texture may be 

 restored by annealing ; he considers that percussion renders the fibres more 

 liable to break off short, but that without it is sufiicient to cause a consider- 

 able increase of temperature, it does not produce any real internal change. 

 Mr. Glynn considers that the structure both of wrought and cast iron is 

 altered by a succession of blows, the wrought to a crystalline structure, the 

 cast to larger crystals ; he has observed this appearance particularly in axles, 

 mill-shafts, toothed wheels, crowbars, and crane chains, the latter even 

 when specially made of strong fibrous iron require to be annealed after about 

 three years ; the axles of tenders to which breaks have been applied he men- 

 tions as particularly subject to this change. He attributes the alteration to 

 galvanic action, induced by the alloys from which iron is never entirely free, 

 and considers that the action is increased by blows. He also mentions that 

 brass wire, of copper and zinc, originally tough and fibrous, continually 

 breaks off' short with a crystalline fracture radiating in the form of a star, 

 showing a change in the structure such as would have taken place if the 

 metal had been melted and had crystallized in cooling ; this effect is more 

 rapidly produced in an atmosphere containing sulphuric acid. Mr. W. H. 

 Barlow mentions having caused a piece of fibrous iron to be hammered for a 

 long time by a blacksmith, and that he found the iron changed from a 

 fibrous to a crystalline structure ; but as axles do not undergo the same sort 

 of hammering, he does not know whether the same efl'ect takes place in 

 them. Mr. Stephenson considers the fact of an alteration of structure as 



