52 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



[February, 



upon the strength and nroperties of the material produced: and 

 we have also made carcfiil inquiries of civil enf^ineers with respect 

 to the qualities and mixtures of iron preferred by them, for the 

 large castings used in tlie construction of railway bridges, and to 

 the respective properties of hot-blast and cold-blast iron: this in- 

 vestigation has been greatly facilitated by the liberality and can- 

 dour with which tliese gentlemen have communicated to us the 

 results of their experience. 



As no ma]) of tlie kingdom had been constructed representing 

 the districts in which iron is found and worked, we applied to the 

 officers of tlic Museum of Practical Geology for their assistance, 

 and tliev caused one to be prepared expressly to accompany this 

 Kejiort, in which the principal furnaces now in blast are shown. 



Great differences of opinion exist with respect to the best qua- 

 lities and mixtures of iron; and, after all, it appears that those 

 employed for large castings depend practically so much upon the 

 commercial question of relative cost that engineers arc rarely able 

 to select the very best material. It is generally admitted that en- 

 gineers have no guarantee that the mixture for which they have 

 stipulated in a contract shall be that used by the founder, and no 

 certain test by which to determine whether a given piece of iron 

 has been manufactured by hot or cold blast. A ^ery good protec- 

 tion appears to he contained in the recommendation of JNIr. Fo.x, 

 that engineers in contracting for a number of girders, should sti- 

 pulate that they should not break with less than a certain weight 

 (leaving the mixture to the founder), and cause one more than 

 the required number to be cast. The engineer may then select one 

 to he broken, and, if it 1)reak with less weight than that agreed 

 npon, tlie whole nuiy be rejected. 



At the beginning of the railway system the bridges were natu- 

 rally constructed upon similar principles to those which had been 

 already employed for common roads or aqueducts. Some of these 

 ordinary constructions have proved inadequate to sustain the enor- 

 mous loads and vibrations of railway trains. Some have been con- 

 sidered too expensive; others, as the suspension bridges, have been 

 found wholly unfitted for railway purposes. Moreover, tlie neces- 

 sity for preserving the level of a railroad as much as pi>ssible, com- 

 bined with that of passing under or over existing canals, rivers, or 

 roads, has created a demand for those forms of bridges which ad- 

 mit of being kept as low as possible, consistently with the proper 

 headway or passage below; or, in other words, of making the least 

 possible difference of level between the road or stream which the 

 bridge has to carry and that which it has to cross. 



From these causes, combined with the innumerable opportuni- 

 ties of building new- bridges which the railways have given occa- 

 sion to, and a constant endeavour to reduce the expense of build- 

 ing them, a variety of new constructions have been projiosed and 

 essayed, most of them of great merit and value, while others 

 appear to be of very doubtful stability. 



On the whole, the art of raihi ay bridge-building cannot be said 

 to be in that settled state which would enable an engineer to apply 

 principles with confidence, ^\'e have tlierefore thought it our 

 duty to inquire into the present methods of railway bridge-build- 

 ing, to collect in evidence the opinions and practice of the leading 

 members of the profession of civil engineers upon this branch of 

 construction, and especially with respect to the form and jiropor- 

 tions of simple cast-iron girders, the practical limits to the em- 

 ployment of such girders, the methods of combining them with 

 the rest of the structure, the various forms of compound girders, 

 the expediency of several combinations of wrought-iron with cast- 

 iron: and, finally, the comparative merits of plain girders, and of 

 other forms in which the principles of the aixh, or otlier methods 

 of giving stiffness, are introduced. 



The simplest bridge, and that which admits of the greatest por- 

 sible headway at a given elevation, is, undoubtedly, the straight 

 girder bridge. 



The length of a simple cast-iron girder appears to be limited 

 only by the power of making sound castings, and the difficulty of 

 moving large masses. Thus the practical length has been variously 

 stated to us as 4U, 50, and GU feet. The form resulting from Mr. 

 Hodgkinson's former experiments on this subject is universally ad- 

 mitted to be that which gives the greatest strength; but the re- 

 quirements of construction compel many variatians from it, espe- 

 cially in the ratio between tlie top and bottom flanges. Moreover 

 the convenience and the necessity of keeping the roadway for rails 

 as low as possible has introduced a practice of supporting the 

 beams which sustain the rails upon one side of the bottom flange. 

 The pressure of tlie roadway and of the passing loads being thus 

 thrown wholly on one side of the central vertical web of the girder 

 produces torsion (which is not always taken into account in deter- 



mining the proportions of the girder). The existence of this 

 torsion is admitted on all hands, and various schemes are employed 

 to counteract and diminisli it; but the form of a girder that will 

 effectually resist this disturbing force, without incurring other 

 evils, still remains a desideratum. 



The requisite length of girders is increased considerably by the 

 excessive use of skew bridges; and it is much to be regretted that 

 difficulties should often be thrown in the way of altering the course 

 of existing roads and canals when the line of a proposed railway 

 ha])pens to cross them at an acute angle. Partly from these causes 

 and partly from a little indulgence in the priile of construction, 

 skew bridges may be found, of which, from the obliquity of the 

 bridge, tlie girders are more than double the length that would be 

 required by the direct span of the oi)ening to be crossed. 



M'lien the span of the opening or other circumstances render 

 the use of single straight girders unadvisable, straight girders 

 built up of several separate castings bolted together, and some- 

 times trussed with wrought-iron tension rods, are largely em- 

 ployed, and necessarily with great varieties of construction. By 

 tliese means the ffirders may be extended to spans of upwards of 

 120 feet. 



When wrought-iron is combined with cast-iron in the manner of 

 trussing, several difficulties arise from the different expansions of 

 the two metals and the difference of their masses, which causes the 

 wrought-iron rods to be more rapidly affected by a sudden change 

 of temperature than the cast-iron j)arts. The constant strain upon 

 the wrought-iron tends to produce a permanent elongation, and 

 hence tension-rods may require to be occasionally screwed up. 

 We have sought for ojiinions and information upon all these ques- 

 tions, and these show that the greatest skill and caution are neces- 

 sary to insure the safe employment of such combinations. It is 

 not admitted that the vibration of railway trains would loosen or 

 injure the bolts or rivets of compound girders. Nevertheless, 

 wood, felt, or other similar substances have occasionally been in- 

 ti'oduced between surfaces to diminish the communication of vi- 

 bration. 



The general opinion of engineers appears to be that the cast- 

 iron arch is the best form for an iron bridge when it can be selected 

 without regard to expense or to tlie height above the river or road 

 which is to be crossed. For low bridges the bowstring girder is 

 recommended. Lattice bridges appear to be of doubtful merit. 



The latest mode of construction that has been introduced con- 

 sists of boiler plates riveted together as in iron ship-building, and 

 combined in various ways with cast-iron. Hollow girders are thus 

 formed, which are either made so large as to admit of the road and 

 carriages passing through them, as in the Conway and Britannia 

 bridges, or else these tube girders are made on a smaller scale and 

 employed in the same manner as the ordinary cast-iron girders, to 

 sustain transverse joists which carry the road. The first kind is 

 applicable to enormous spans, those of the two bridges above men- 

 tioned being 100 and 462 feet respectively. The second kind are 

 said to be cheaper and more elastic than other forms for spans that 

 e.vceed tO feet. These methods appear to possess and to promise 

 many advantages, but they are of such recent introduction that no 

 experience has yet been acquired of their powers to resist the 

 various actions of sudden changes of temperature, vibrations, and 

 other causes of deterioration. We have thought it our duty to 

 seek for information with respect to them, and we find engineers 

 to be for the most part exceedingly favourable towards tliem ; but 

 for the reasons above stated we are unable to express any opinion 

 upon them. At the same time we desire to bear testimony to the 

 patient care and scientific manner in which the forms and propor- 

 tions of the great tubes of the Conway and Britannia bridges have 

 been elaborated; and we must beg to refer to the ^liuutes of Evi- 

 dence for the details of the information which we have collected. 



The investigation in which we have been concerned has made it 

 evident that the novelty of the railway system has introduced a 

 variety of new mechanical causes, the effects of which have not 

 yet had time fully to develope themselves, on account of the ex- 

 tent and number of new railways, and the rapidity with which they 

 were constructed, in many cases scarcely giving breathing time to 

 the engineers, by which to observe and profit by the experience of 

 each successive new construction. Thus it has happened that some 

 portions of mechanism and structure lia\e been made too weak, or 

 placed in unfavourable combinations; and hence some unavoidable 

 but most lamentable, and sometimes fatal accidents, have been 

 occasioned. It also appears that there exists a great want of uni- 

 formity in practice in many most important matters relating to 

 railway engineering, which shows how imperfect and deficient it 

 yet is in leading principles. 



