1850.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



49 



IRON FOR RAILWAY STRUCTURES. 



Report of the Commissioners appointed to Inquire into the Application 

 of Iron to Railwaij Structures. 



From the information supplied to us, it appears that the propor- 

 tions and forms at present employed for iron structures, have been 

 generally derived from numerous and careful experiments, made 

 by sulijecting bars of wrought or cast iron of different forms to 

 the action of weights, and thence determining by theory and cal- 

 culation such principles and rules as would enable these results to 

 be extended and applied to such larger structures and loads as are 

 required in practice. But the experiments were made by dead 

 pressure, and only apply therefore to the action of weights at rest: 

 — On the contrary, from tlie nature of the railway system the 

 structures employed therein are necessarily exposed to concussions, 

 vibrations, torsions, and momentary pressures of enormous magni- 

 tude, produced by the rapid and repeated passage of heavy trains. 



These disturbing causes, in smaller degree, have always occurred 

 in structures connected with mill-work or other mechanism. But 

 the effects upon their stability have not been found gi-eater than 

 could be met by increasing the dimensions of the parts without 

 especially inquiring into the exact principles upon which such in- 

 crease should be made. Thus, we are informed that the dimen- 

 sions of cast-iron girders, intended for sustaining stationary loads, 

 such as water-tanks and floors, are usually so proportioned that 

 their breaking-weight shall be three times as great as the load they 

 are expected to carry, or in some cases four or five times as great. 

 But when the girders are intended for railway bridges, and there- 

 fore subject to much concussion and vibration, greater strength is 

 given to them by altering the above proportions, and making the 

 breaking-weight from six to ten times as great as the load, accord- 

 ing to the practice of different engineers. On the other hand, 

 some consider that one-third of the Ijreaking-weight is as safe a 

 load in the latter case as in the former. 



As it soon appeared, in the coui'se of our inquiry, that the effects 

 of heavy bodies moving with great velocity upon structures had 

 never been made the subject of direct scientific investigation, and 

 as it also appeared that in tlie opinion of practical and scientific 

 engineers such an inquiry was highly desirable, our attention was 

 early directed to the devising of experiments for the purpose of 

 elucidating this matter. 



The questions to he examined may be arranged under two heads, 

 namely — 



1. Whether the substance of metal which has been exposed for 

 a long period to percussions and vibrations, undergoes any change 

 in the arrangement of its particles, by which it becomes weak- 

 ened ? 



2. What are the mechanical effects of percussions, and of the 

 passage of heavy bodies in deflecting and fracturing the bars and 

 beams upon which they are made to act ? 



A great difference of opinion exists among practical men with 

 respect to the first of these questions. Jlany curious facts have 

 been elicited by us in evidence, which show that pieces of wrought- 

 iron which have been exposed to vibration, such as the axles of 

 railway carriages, the chains of cranes, &c. employed in raising 

 heavy weights, frequently break after long use, and exhibit a pecu- 

 liar crystalline fracture and loss of tenacity, which is considered by 

 some engineers to be the result of a gradual change produced in 

 the internal structure of tlie metal by the vibrations. In confirma- 

 tion of this, various facts are adduced, as, for instance, that if a 

 piece of good fibrous iron have the thread of a screw cut upon one 

 end of it by the usual process of tapping, which is always accom- 

 panied by much vibratory action, and if the bar be then broken 

 across, it will be found that the tapped part is a good deal more 

 crystalline than the other portion of the bar. Others contend that 

 this peculiar structure is the result of an original fault in the pro- 

 cess of manufacture, and deny this effect of vibration altogether, 

 whilst some allege that the crystalline structure can be imparted to 

 fibrous iron in various ways, as by repeatedly heating a bar red- 

 hot, and plunging it into cold water, or by continually hammering 

 it, when cold, for half an hour or more. 



Mr. Brunei, however, thinks the various appearances of the frac- 

 ture depend much upon the mode in which the iron is broken. The 

 same piece of iron may be made to exhibit a fibrous fracture when 

 broken by a slow heavy blow, and a crystalline fracture when 

 broken by a sharp short blow. Temperatiire alone has also a de- 

 cided effect upon the fracture; iron broken in a cold state shows a 

 more crystalline fracture than the same iron warmed a little. 



The same effects are by some supposed to be extended to cast- 

 iron. 



^V'e have endeavoured to examine this question experimentally 

 in various ways. 



A bar of cast-iron, 3 inches square, was placed on supports about 

 It feet asunder. A heavy ball was suspended by a wire 18 feet 

 long, from the roof, so as to touch the centre of the side of the 

 bar. By drawing this ball out of the vertical position at right 

 angles to the length of the bar in the manner of a pendulum to 

 any required distance, and suddenly releasing it, it could be made 

 to strike a horizontal blow upon the bar, the magnitude of which 

 could be adjusted at pleasure either by varying the size of the ball 

 or the distance from which it was released. Various bars (some of 

 smaller size than the above) were subjected by means of this appa- 

 ratus to successions of blows, numbering in most cases as many as 

 t,000. The magnitude of the blow in each set of experiments 

 being made greater or smaller, as occasion required. The general 

 result obtained was, that when the blow was powerful enough to 

 bend the bars through one-half of their ultimate deflection (that is 

 to say, the deflection which corresponds to their fracture by dead 

 pressure), no bar was able to stand 1,(100 of such blows in succes- 

 sion; hut all the bars (when sound) resisted the effects of 4,000 

 blows, each bending them through one-third of their ultimate de- 

 flection. 



Other cast-iron bars, of similar dimensions, were subjected to 

 the action of a revolving cam, driven by a steam-engine. By this 

 they were quietly depressed in the centre, and allowed to restore 

 themselves, the process being continued to the extent even in some 

 cases of 100,000 successive periodic depressions for each bar, and 

 at a rate of about four per minute. Another contrivance was tried 

 by wliich the whole bar was also during the depression thrown into 

 a violent tremor. The results of these experiments were, that 

 when the depression was equal to one-third of the ultimate deflec- 

 tion, the bars were not weakened. This was ascertained by break- 

 ing them in the usual manner with stationary loads in the centre. 

 When, however, the depressions produced by the machine were 

 made equal to one-half of the ultimate deflection, the bars were 

 actually broken by less than 900 depressions. This result corre- 

 sponds with and confirms the forzner. 



By other machinery a weight equal to one-half of the breaking- 

 weight was slowly and continually dragged backwards and forwards 

 from one end to the other of a bar of similar dimensions to the 

 above. A sound bar was not apparently weakened by 96,000 tran- 

 sits of the weight. 



It may, on the whole, therefore be said, that as far as the effects 

 of reiterated flexure are concerned, cast-iron beams should be so 

 proportioned as scarcely to suffer a deflection of one-third of their 

 ultimate deflection. And as it will presently appear, that the de- 

 flection produced by a given load, if laid on the beam at rest, is 

 liable to be considerably increased by the effect of percussion, as 

 well as by motion imparted to the load, it follows, that to allow the 

 greatest load to be one-sixth of the breaking-weiglit is hardly a 

 sufficient limit for safety even upon the supposition that the beam 

 is perfectly sound. 



In wrought-iron bars no very perceptible effect was produced by 

 10,000 successive deflections by means of a revolving cam, each 

 deflection being due to half the weight which, when applied static- 

 ally, produced a large permanent flexure. 



Under the second head, namely, the inquiry into the mechanical 

 effects of percussions and moving weights, a great number of 

 experiments have been made to illustrate the impact of heavy 

 bodies on beams. From these it appears that bars of cast-iron of 

 the same length and weight struck horizontally by the same ball 

 (by means of the apparatus above described for long-continued 

 impact), offer the same resistance to impact whatever be the form 

 of their transverse section, provided tlie sectional area be the 

 same. Thus a bar, GXli inches in section, placed on supports 

 about 14 feet asunder, requii-ed the same magnitude of blow to 

 break it in the middle, wliether it was struck on the broad side or 

 the narrow one, and similar blows were required to break a bar of 

 the same length, the section of which was a square of 3 inches, and 

 therefore of the same sectional area and weight as the first. 



Another course of experiments tried with the same apparatus 

 showed, amongst other results, that the deflections of wrought-iron 

 bars produced by the striking ball were nearly as the velocity of 

 impact. The deflections in east-iron are greater than in propor- 

 tion to the velocity. 



A set of experiments was undertaken to obtain the effects of 

 additional loads spread uniformly over a beam, in increasing its 

 power of bearing impacts from the same ball falling perpendicu- 



8 



