56 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[Febkcart, 



highly improbable, and cites the connecting rod of an engine having vibrated 

 25,000,000 times, and yet being perfectly fibrous. In the cases of axles the 

 iron may not have been fibrous in the first instance, for though when a piece 

 of iron is rolled from 1 foot in length to 20 feet it necessarily becomes 

 fibrous, it does not necessarily become so when rolled from 1 foot in length 

 to G feet. He says that in all the cases of change of structure which he has 

 heard of there has always been some important link wanting. Mr. Locke 

 considers that concussion would alter the structure of iron, but would not 

 offer an opinion as to whether the fracture of axles arose from that cause; 

 he mentions that a great many axles broke when the crank axles were in 

 use, but tliat since straight axles have been adopted fewer breakages have 

 occured. Mr. Brunei doubts the change of internal structure, and thinks the 

 various appearances of different fractures result as much from the mode in 

 which the iron has been broken as in any change in structure, and that 

 change of temperature will also produce a variation in the fracture ; that iron 

 in a cold slate shows a more crystalline fracture than the same iron warmed 

 a little, and that wrought iron docs not actually become crystalline and 

 fibrous, but breaks either fibrous or crystalline according to the combination 

 of circumstances under which it is broken, but with the combination re- 

 quired he is not acquainted; he cites the stratification and planes of cleavage 

 of rocks, which may be broken with different fractures according to the mode 

 of applying the blow. Mr. Brunei exhibited various specimens broken, some 

 with a fibrous fracture by means of a slow heavy blow, and some with a crys- 

 talline fracture by means of a sharp short blow. Mr. Charles May cites the 

 beam of a steam engine as an instance of continued vibration not. affecting 

 iron, and mentions as an instance in favour of the change the fact that a gun 

 used in his works to break pig iron across, at last dropped in two as if it had 

 been cut. 



Greatest Weights on Railways. — Mr. Hawkshaw states that locomotive 

 engines are the greatest "weights which can come on railways, and reckon IJ 

 tons per foot linear as the greatest weight for a single line of way. Mr. Fox, 

 Mr. Fairbairn, and Mr. Brunei mention 1 J tons. Mr. W. II. Barlow states 

 that on the Midland there are engines on four wheels weighing 32 tons ex- 

 clusive of the tender, but that that weight is too great for the permanent 

 way, and that the rails are crushed and flattened by it. Mr. Stephenson 

 and Mr. Locke state, 1 ton per foot linear is the greatest weight which 

 comes on a single line of rail. 



Analysis of the Evidence given by the Witnesses examiner!. 



John Vrpeth Rastrick, Esij., Civil Engineer. — Has experimented on 

 Staffordshire aud Shropshire iron. Prefers forge iron for large castings. 

 AVith pure mine hot-blast iron is equal in strength to cold blast, but the 

 hot blast enables cinder to be used, which deteriorates the quality. The 

 temperature of the blast alters the quantity but not the quality of the metal 

 produced, about 500° or 000° is preferred. The only guarantee against bad 

 iron is to contract for a particular quality. There is no mode of detecting 

 the difference between two kinds of iron. A mixture of the I'euistone ore 

 from Shropshire with the Staffordsliire irouslones improved the quality of 

 iron. For strong castings a mixture of pig iron is preferable to mixing the 

 ores ; a good mixture is formed from Low IMoor iron, Old Park iron, and 

 Colebrooke Dale iron. Cast the bridge at Chepstow. Allows a ton as 

 the breaking weight of a bar I inch square and 1 foot between the bearings. 

 Proves a beam to l-3rd of the breaking weight, but never trusts it to carry 

 more than 1-Gth. Iron girders may be cast of almost any length provided 

 they have strength in proportion ; made beams for the British Museum 41 

 feet long in 1821 or lb2,'j, they had open v\ork in the web, and were 3 feet 

 or 3 ft. C in. deep ; they were proved by laying on l.i or 20 tons of actual 

 weight, and struck with a lieavy hammer of 14 or 20 pounds weight. In 

 simple girders, if the height is too confined, the strejigth required must be 

 given by thickness. A girder will bear the same weight on the bottom 

 flange as on the top. The torsion caused by plaring the weight on the 

 bottom flange is very trifling, and canuot take place without a greaier 

 amount of deflection in the bearer than should be allowed. Puis on 

 brackets to unite the flange to the girder. The strength of the joists sup- 

 porting the roadway should be sufficient to prevent them pushing out the 

 flanges. A flange never breaks ofl'. As long as a weight on a girder is 

 not suflicient to injure the elasticity no matter how long it remains. A 

 beam taken out of a mould while hot will break by its own weight. Cast 

 iron is more fragile in winter than in summer. In the Chepstow Bridge 

 of 112 feet span, versed sine 3 feet, the difference of temperature between 

 summer and winter altered the position of the crown of the arch by 2 

 inches. Bridges requiring a flat soflit are best supported by a bow above 

 the roadway. No combination of wrought or cast iron is equal to a solid 

 casting, the two metals hamper each other. An arch is the best form for a 

 bridge of cast iron. Vibration and impact will not injure the joints and 

 rivets of compound girders if they are strong enough. Railway girders 

 should be so strong that the deflection should be immaterial. At the 

 Pont-y-pool Iron Works a bar of wrought iron 1 inch square was hung up 

 by one end, and struck at the bottom by a small hammer continually for 

 12 months until the bar dropped in two. The vibration upon a railway 

 bridge is too small to affect it. Doubts whether the fractures of railway 

 axles can be attributed to vibration. If in a railway bridge no permanent 

 deflection has taken place after it has been in use for 12 or ItJ months. 



considers it has not been affected by the running of the train. Has not 

 observed that fish-bellied rails break from becoming crystallized. In 

 proving a girder allows a deflection of ^J^ of the length. Considers a 

 rapidly passing weight will cause less deflection than a stationary weight. 

 Prefers cast iron in all cases to wrought iron. In a span of 100 to 200 

 feet an arch is best ; if the height does not admit of it under the road- 

 way it should be placed above. The difficulty of transport is the only 

 limit to the length of castings. 



Jolm Hawkshaw, Esq., Civil Engineer. — Low Moor iron is the best for 

 girder bridges, good grey Staffordshire the next best. l-3rd of No. 1 and 

 2-3rds of No 2 of the best Staffordshire or South Wales iron is a good mix- 

 ture for large castings. Hot-blast iron is not so strong as cold-blast iron. 

 The only guarantee against the use of hot-blast iron is the character of the 

 founder. The strength of a girder should be seven times the load, and 

 would test it to at least double the load. The spans for simple cast-iron 

 girders might be increased beyond those in use. Would not hesitate to 

 make a simple cast-irun girder of 100 feet span. In designing a simple 

 cast-iron girder obtains the form for the requisite strength by Sir. Hodg- 

 kinson's formula, and trebles the area of the top flange to get lateral stabi- 

 lity, thus making the top flange half the area of the bottom. In testing 

 beams it is desirable to give vibration by blows while the pressure is on, 

 or if actual weight is applied, to throw the weights into the scale. A gir- 

 der cannot bear so much weight on one of the bottom flauges as if applied 

 at top. The weight so applied produces a torsion. By increasing the top 

 flange aud adding brackets, the torsion is diminished. It would be nearer 

 a practical residt to test a beam in the way in which the weight will be 

 applied. The objection to contrivances for throwing the weight in the cen- 

 tre plane of the girder is that by departing from the simple form the liabi- 

 lity to unsoundness from tlie casting is increased. It is possible that weak 

 girders loaded with a permanent weight might increase in deflection after 

 a length of time. The deflection of a girder should be almost impercep- 

 tible. The Knotliugly Canal Bridge of 89 feet span deflected half an inch 

 with an engine of 22 tons going at -50 miles per hour; the bridge is too 

 weak. Prefers not using compound girders, it is however possible to 

 make them strong and safe. Prefers plain girders with the top flange 

 increased to prevent lateral twisting. It would be useful to ascertain the 

 strength of beams under loads applied as in practice. For spans of 100 or 

 200 feet which must be crossed with a level soflit a truss like that for a 

 roof is preferable, or a bowstring bridge. .Joints and rivets will not 

 suffer from vibration if made originally strong. Where there is impact or 

 vibration there should be large surplus strength, a breaking strength of 

 seven times the load. Has seen nuaiberless cases of broken axles aud 

 broken rails, when frequently crystallization existed, but cannot say whe- 

 ther it is attributable to a succession of blows. Experiments on the sub- 

 ject are desirable. Mill-gearing affords examples already made ; tbe cast 

 iron is there subjected to blows and vibration, and the machinery goes on 

 running for years. The use of cast iron in mill-gearing gives confidence 

 in its application to other purposes ; by inquiring into the wear and tear 

 of mill gearing, the length of time that iron will hear shocks might be 

 ascertained. The irregularity in the surface of the rails would cause a 

 weight moving with velocity to deflect a beam more than a similar station- 

 ary weight. No practical velocity would be such as not to give time for 

 deflection. Ice does not afford a parallel case ; ice has a better surface, 

 aud time must be allowed for the displacement of the water. Is erecting 

 two bridges with wrought-iron tubular girders. Wrought iron gives more 

 warning than cast iron. The load on railway bridges may be taken at 1 J 

 tons per linear foot. The heaviest load is a locomotive engine: there is a 

 rule on all railways prohibiting trucks being loaded beyond a certain 

 poiut. Locomotives weigh about 22 tons, and the tenders 10 or 12 tons. 

 The weight on a bridge covered with locomotive engines, including tbe 

 roadway would be 2 tons per linear foot. It is desirable to ascertain the 

 real facts with regard lo the trustworthiness of cast iron. The conditions 

 under which cast iron is placed in railway structures is similar to that in 

 mill-gearing, «nd the quantity of cast iron ehafting aud length of time it 

 has been in use might be ascertained. Is making a wrought iron bridge 

 of 100 feet span ; it appears easier to construct oue of that span of malle- 

 a' le than of cast iron. The cost determined the adop'iou of wrought irou; 

 objects lo the combination of wrought and cast iron except in bowstring 

 bridges. The wrought iron girders are made double, to obtain lateral 

 stiffness. Without reference to expense, an arch is the best form for cast 

 iron. The level soflit is adopted from necessity. For the strength of 

 wrought iron girders, Mr. Hodgkinsou's formula for cast iron was used, 

 adopting 70 as a coeffioient iustead of 2S, and taking care to make the 

 upper flange strong enough ; has not had enough to do with that form of 

 girder to be certain of the precise proportions. 



Charles Fox, Esq., Civil Engineer. — Tbe mixtures to be preferred for 

 particular works depend upon the locality, as the cost must be considered ; 

 would use in the Rlidland Counties iron from Stall'ordshire and Shropshire, 

 on the sea coast \\ elch and Scotch. Tivo-thirds Blaeuavon (cold blast 

 M'elch), aud one-third of Scotch in equal proportions from the Blackbaud 

 and from the Red Hematite, is a very good mixture. Is convinced that 

 metal made by tbe hot blast would be as good as from cold blast if the 

 mine were properly treated ; but the custom in Scotland has been to care 

 for quautitj not for quality. Tbe only guarantee against inferior metal is 

 to contract that girders shall nut break with less than a specified weight, 

 and to cast one more than is required, and select any one for trial, and if 



