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THE CIVIL ENGINEER AND ARCHITECrS JOURNAL. 



205 



the tension. I may here state that wrought iron, when acted upon, will 

 elongate considerably without breaking, but cast iron will not without 

 breaking. There have been a number of bridges of this description erected 

 on railways in various parts of England, both before and after I held the 

 appointment of Inspector-General of Railways, none of which, with the 

 exception of this one, ever failed. They were not quite of the same ex- 

 tent, but I will allude to a cast iron girder bridge at York, over the river 

 Ouse, of the York and Scarboro' Railway, which has two openings of 70 

 feet span. The least depth of the iron girder on that bridge is 3 feet. The 

 least depth of those on tlie Dee bridge is 3 ft. 9 in. ; and as the bridge at 

 York and other similar bridges have stood, I concluded that this one 

 would, as it had an extra depth. I may also mention another bridge over 

 the Tees, at Stockton, although I have not seen it. I may vouch for what 

 I state to be correct. It has a span of 83 ft. 4 in., and the least depth of 

 iron is 3 feet. 



Mr. R. Stephenson said it was 87 feet. 



General Pasley. — I thought it was what I slated, but yon may be cor- 

 rect. The flange on this bridge is greater than on others. I have fre- 

 quently mentioned to engineers that wrought iron tension rods would do 

 little good. In my inspection of a cast iron girder bridge, on the Syston 

 and Peterborough Railway, built by Mr. Liddie, the resident engineer, I 

 found that he had omitted tension bars, and in my report to the Earl of 

 Clarendon I approved of the omission, and considered it a preferable con- 

 struction. It appeared that Mr. Liddie could not get the tension rods in 

 time, and therefore he built the bridge without them. Having mentioned 

 this repeatedly to engineers, and having been given to understand that Mr. 

 Bidder and Mr. Gooch have made experiments with a view of testing the 

 strength of girders without rods, I am informed that the trial was in favour 

 of the latter. As to the cause of the accident ; — it has been stated that 

 some time after the Shrewsbury and Chester railway was opened, and after 

 I had inspected it, a girder was cracked, and was replaced by a new one. 

 This circumstance, which I did not know, and which was never reported 

 to government, coupled with the fracture of this one, induces me to think 

 they are not safe, and that it is the mere cast of a die between tlieir safety 

 and danger. 1 consider that the tension rods are of very little use indeed. 

 The tension rods are connected with the girder alone, as if they were part 

 of it. They have no independent support, and there is a diflerence be- 

 tween this bridge and Mr. Stephenson's former iron girder bridges. In all 

 his former girder bridges there is a connexion from girder to girder, on the 

 central pier or piers, from one end of the bridge to the other, so that when 

 the pressure is on one girder, the other girder in the same line contributes to 

 assist. That is the case with the one on the river Ouse at York. The 

 horizontal portion of these bars appears to be useless. The oblique tension 

 bars would be of use if the upper ends yvere fixed to some independent 

 support to each pier, and similar independent support on each abutment, 

 and if the standards or support over the abutments had tension bars ex- 

 tending inland to resist the heavy weight going over the extreme bays or 

 openings of the bridge. There is a swing bridge over the river Wensham, 

 at Norwich, strengthened on this principle, and which is extremely judi- 

 cious. In this case I consider that the girder broke on a train passing 

 over, added to the weight of the ballast tiiat had been thrown on it in the 

 course of the morning. The masonry gave way from the girder breaking, 

 and from that cause alone. I examined the girder ; and the castings 

 seemed very good, and I believe it is generally admitted that they are 

 good ; but the girder was too weak after the ballast that was put upon it. 

 The girder was far enough in the masonry to support it. There was quite 

 bearing enough to render it secure. I do not think the engme driver sud- 

 denly putting on the steam would cause the engine to bound with such 

 force as to break the girder. I saw nothing to throw the carriages ofl" the 

 rails on the bridge, which had strong guard rails. I should say that no 

 girder could have withstood a deflection of Sj inches. It would have 

 broken short at once. A continued deflection of 4 inches must have 

 broken it long before this occurred. 



Mr. Robert Stephenson put in a written report on the accident; from 

 which it appeared that on the day it occurred, and only a few hours pre- 

 viously, he had narrowly inspected every part of the bridge, and saw no- 

 thing to indicate weakness. He had carefully examined into every cir- 

 cumstance connected with the disaster, and for reasons which he gave, was 

 satisfied that it arose from a violent blow against the girder, near to the 

 abutment on the Salteney side, caused by the train getting otf the rails. 

 The report said: — 



" It has been suggested that the unequal expansion' and contraction of 

 the girder, during great changes of temperature, might probably interfere 

 with the uniform strength of the metal. It is impossible to deny that this 

 circumstance does sometimes interfere with the strength of cast iron beams, 

 but generally this influence may be regarded as confined to castings where 

 the thickness of the difl'erent parts vary considerably. In the present case, 

 the form of the castings was carefully studied, and with only such small 

 deviation from absolute uniformity in all thicknesses of the diti'ereut parts 

 of the section, as practice has long proved to be justifiable. 



" With regard to the competent strength of the structure, I concur gene- 

 rally in the deduction drawn by Mr. Yarrow, in which I am confirmed by 

 an extensive experience in the construction and use of similar structures, 

 tried under circumstances that demonstrate their capabilities to meet all 

 the ordinary contingencies of railway traffic." 



Mr. James Kennedy, of the firm of Kury and Kennedy, Liverpool, was 

 recalled, and confirmed the opiuiuu he had git en at the lust meeting as to 



the probable cause of the accident. The girder might have given way 

 either from a blow, or the extra weight of ballast and the train on it. Cast- 

 iron girders were capable of sustaining in the centre 70 tons; but if the 

 tension rods were not perfectly adjusted, he did not think the bridge safe 

 for ordinary trains. He did not lliiuk damp ballast placed on the bridge 

 would alfect the temperature of the girders so as to cause tlieni tti break. 



Mr. H. Robertson, the engineer of the Shrewsbury and Chester Rail- 

 way, was next called, and said — I have examined the bridge since the ac- 

 cident. My opinion is that the bridge broke under the weight of the en- 

 gine and train, increased to a large extent by the laying down of 25 tons of 

 ballast on the pla*form just previous to the accident. The witness then 

 handed in a lengthened report which he had made to the Directors, respect- 

 ing the failure of the bridge, in which he stated that the fracture spoken of 

 by Mr. R. Stephenson, as having been produced by a lateral blow, was, in 

 his opinion, caused after the girder had fallen, and that the fracture which 

 caused the bridge to give way was that in the centre. He considered that 

 the tension-rods tended more to weaken the girder than to strengthen it. 



Mr. Robertson then read the following report which he had made to the 

 Directors of the Shrewsbury and Chester Railway : — 



" I minutely examined the I>ee bridge on the Chester and Holyhead 

 Railway on the occurrence of the accident, and have since examined re- 

 peatedly the points which bear upon the accident. I have caused drawings 

 to be prepared and also a model, showing the details ot the structure and 

 the fragments of the beam, in so far as now discovered ; and to these I 

 would refer you, instead of attempting to give a written description of the 

 bridge. (These were produced in Court, for the inspection of the coroner 

 and jury.) You will perceive that there are two principal fractures in the 

 beam — one near the centre, 5^ feet from the west abutment, in the middle 

 portion of the girder; the other in the portion of the girder next to tlie 

 abutment, and 20 feet from its ' fence.' The latter fracture appears to me, 

 from its form, and especially from the position in which the fragment lay, 

 as shown in the ground plan taken the morning after accident, to have been 

 caused by the fall ; any disturbing cause previously to the fall is quite in- 

 consistent with the close proximity of the fragments. The fracture at the 

 centre, from the position of the fallen portion, and of the middle tension-rod 

 wrapped over the girder, and especially from the form of the fracture, ap- 

 pears to me to have first taken place. This fracture I consider to have 

 resulted from the weakness of the top flange, which was compressed and 

 broken by the strain arising from the rolling weight of the engine and ten- 

 der, and the vibratory motion of the structure itself, increased to a large 

 extent by the deposit of 25 tons of ballast on the roadway immediately be- 

 fore the accident. This compression is remarkably evident by the bulging 

 out of the metal at the point of the parting at the top of the web, or vertical 

 portion of the girder. 

 "In estimating the strength of the girder, I am of opinion that the tension- 

 rods, from the form of the section of the girder, weakened it, and threw an 

 undue strain, by compression, on the top flange ; but, assuming that they 

 did not weaken it, and applying the formula, as given by Eaton Hodkin- 

 son, F.R.S., to the girders — by one formula, the breaking weight is equal 

 to 61 J tons; and, by the other, the breaking weight is equal to 70 tons. 

 Now, it has been an established rule in practice, that one-third or one- 

 fourth of the breaking weight is the safe working weight to which a girder 

 should be subjected, and the larger the size, the smaller ought to he the 

 proportion ; taking, therefore, one-fourth of 56 (the breaking weight), it 

 follows that the safe weight to which one of the girders ought to be sub- 

 jected is I8J, and the two girders 37 tons. The weight of the timber, plat- 

 form, beams, rails, chairs, &;c., exclusive of the girder, according to an ap- 

 proximate calculation I made, is 10 tons 6 cwt. ; and, adopting the rule 

 that a uniform weight is difTused over the beam, is equivalent to one-half 

 that weight suspended at the centre, this becomes equal to a weight sus- 

 pended at the centre of 9 tons 13 cwt. The equivalent weight of an engine 

 and tender of 33 tons 10 cwt. 2 quarters, suspended at the centre of the 

 beam, I estimate at 32 tons — making a strain of 41 tons 10 cwt. against 37 

 tons — the safe working strain to which the bridge ought to be subjected. 

 However, on the afternoon of the accident, immediately previous to the 

 passing of the train, the bridge was subjected to an additional strain, by 

 the laying on of 5 inches of broken red sandstone ballast, amounting to a 

 weight over the bridge of 25 tons, which is equivalent to a weight suspend- 

 ed at the centre of 12 tons 10 cwt. This makes a total of 54 tons against 

 the safe strain of 37 tons formerly stated ; and the last addition appears to 

 me to be the immediate cause of the accident. In these calculations, how- 

 ever, it is assumed that everything is at rest, and that the forces applied 

 are those resulting from direct pressure, whilst the evidence shows that 

 there is a vibratory movement of the whole structure to a hirge extent ; and 

 there is, besides, a percussive movement of the engine and tender, which, 

 with a heavy long-boiler engine, with outside cylinder, is considerable. 

 "The weight of the structure, and of the train in motion, will be about 164 

 tons in all, and the strain from this cause must be added to that formerly 

 stated. This strain, although it cannot be ascertained by accuracy of cal- 

 culation founded on experiment, experience shows to be great ; and I am 

 of opinion that it formed a large element in the strain which broke the 

 bridge down. There is also the whole gross strain arising from the pres- 

 sure and the percussion of the structure and its load, with the apportion- 

 ment of that strain between the girders ; for I am of opinion that, from the 

 loose and independent connection of the girders, and the giving of the 

 structure, the strain may have been unequally divided between the girders. 

 These investigations, iudepeudeutly of the evidence of the eye-witnesses^ 



