The failure of one of these girders was ascribed to various causes, and much discussion on the subject took place at the tune; but to whatever the accident in this particular case might be attributed, the inherent weakness of such combina tions of wrought and cast iron became perfectly clear.
The attention of the government having been for some time past, by this and aiunilar fidlures of iron girder bridges, directed to the subject, a commission was at length appointed to inquire into the whole question of the application of iron to railway- structures, In the evidence given before this commission, will he found an immense mass of valuable information, obtained under all the advantages of a Board so appointed ; and although in their report the commissioners do not express any very decided opinion on the merits of the various forms of construction so strongly recommended to them, there is inueh practical knowledge to be gained by a careful perusal of the evidence and examination of the ingenhais plans attached to it. We shall now proceed to give some extracts from this report The sin] pl est bridge," observe the commissioners, "and that which admits of the greatest possible at a given elevation, is, undoubtedly, the straight girder bridge.
The length of a simple cast-iron girder appears to he limited only by the power of making sound castings, and the difficulty large masses. Thus the practical length has been variously stated to its as •0, 50, and 60 feet. The florin resulting from 11r. Hod.1..lkinson's former experiments on this subject is universally admitted to be that which gives the greatest strength, but the requirements of construction com pel many variations from it, especially in the ratio between the top and bottom flanges. :Moreover, the convenience mid the necessity of keeping the roadway foor 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 the roadway and of the passing loads being thus thrown wholly on one side of the central vertical web of the girder, produces torsion (wh•ch is not always taken into account in determining the proportions of the girder. The
existence of this torsion is admitted on all hands, and various schemes are employed to counteract and diminish it; but the furnt 01a girder that will effectually resist this disturbing force withwithout incurring other evils, still remains a desideratum.
The requisite length of girders is increased considerably by the excessive use of skew bridges, an 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 happens to cross them at an acute angel. Partly Limn these causes, and partly from a little indulgence in the pride of construction, skew bridges may be mound, of which from the obliquity of the bridge, the girders are moire than double the length that would be required by the direct span of the opening to be crossed.
When the span of the opening, or other circumstances, render the use of single girders unadvisable, straight girders built up of several separate castings bolted together, and sometimes trussed with wrought-iron tension rods, are largely employed, and necessarily with great varieties of con st•netion. By these means the girders may be extended to spans of upwards of 120 feet.
When wroualit-iron is combined with cast-iron in the manlier of trussing, several difficulties arise from the different expansions of the two metals. and the difference of their two masses, which causes the wrought-iron rods to be more rapidly effected by a sudden change of temperature than the cast-iron parts. The constant strain upon the wrought-iron tends to I iroduce a permanent elongation, and hence tension rods may require to be occasionally screwed up. We have sought for opinions and information upon all these questions, and these show that the greatest skill and caution is necessary 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. Neverthe less, wood, felt, or other similar substances, have occasionally been introduced between surffices, to diminish the communi cation of vibration.