Bridge Disasters in America: The Cause and the Remedy

by George L. Vose

The load which any bridge will be required to carry being determined, and the general plan and dimensions fixed, the several strains upon the different members follow by a simple process of arithmetic, leaving to be determined the actual dimensions of the various parts, a matter which depends upon the power of different kinds of material to resist different strains. This brings us to the exceedingly important subject of the nature and strength of materials.

It has been said that we know what one square inch of iron will hold. Like the question of loads above examined, this is a matter which has been settled, at any rate within very narrow limits, by the experience of many years of both European and American engineers. A bar of the best wrought-iron an inch square will not break under a tensile strain of less than sixty thousand pounds. Only a small part of this, however, is to be used in practice. A bar or beam may be loaded with a greater weight applied as a permanent or dead-load than would be safe as a rolling or moving weight. A load may be brought upon any material in an easy and gradual manner, so as not to damage it; while the same load could not be suddenly and violently applied without injury. The margin for safety should be greater with a material liable to contain hidden defects, than with one which is not so; and it should be greater with any member of a bridge which is subjected to several different kinds of strain, than for one which has to resist only a single form of strain. Respect, also, should be had to the frequency with which any part is subjected to strain from a moving load, as this will influence its power of endurance. The rule in structures having so important an office to perform as railroad or highway bridges, should be, in all cases, absolute safety under all conditions.

The British Board of Trade fixes the greatest strain that shall come upon the material in a wrought-iron bridge, from the combined weight of the bridge and load, at 5 tons per square inch of the net section of the metal. The French practice allows 3-8/10 tons per square inch of the cross section of the metal, which, considering the amount taken out by rivet-holes, is substantially the same as the English allowance. The report of the American Society of Civil Engineers, above referred to, recommends 10,000 pounds per inch as the maximum for wrought-iron in tension in railroad bridges. For highway bridges a unit strain of 15,000 pounds per square inch is often allowed. A very common clause in a specification is that the factor of safety shall be four, five, or six, as the case may be, meaning by this that the actual load shall not exceed one-fourth, one-fifth, or one-sixth part of the breaking-load. It is a little unfortunate that this term, factor of safety, has found its way into use just as it has; for it by no means indicates what is intended, or what it is supposed to. The true margin for safety is not the difference between the working-strain and the breaking-strain, but between the working-strain and that strain which will in any way unfit the material for use. Now, any material is unfitted for use when it is so far distorted by overstraining that it cannot recover, or, technically speaking, when its elastic limit has been exceeded. The elastic limit of the best grades of iron is just about half the breaking-weight, or from 25,000 to 30,000 pounds per inch. A poor iron will often show a very fair breaking-strength, but, at the same time, will have a very low elastic limit, and be entirely unfit for use in a bridge. A piece of iron of very inferior quality will often sustain a greater load before breaking than a piece of the best and toughest material, for the reason that a tough but ductile iron will stretch before giving way, thus reducing the area of section, while a hard but poor iron will keep nearly its full size until it breaks. A tough and ductile iron should bend double, when cold, without showing any signs of fracture, and should stretch fifteen per cent of its length before breaking; but much of the iron used in bridges, although it may hold 40,000 or 50,000 pounds per inch before failing, will not bend over 90 degrees without cracking, and has an elastic limit as low as 18,000 pounds. It is thus full as important to specify that an iron should have a high elastic limit as that it should have a high breaking-weight. A specification which allowed no material to be strained by more than 10,000 pounds per inch, and no iron to be used with a less elastic limit than 25,000 pounds, would, at the same time, agree with the standard requirement, both in England and in the United States, and would also secure a good quality of iron.

Two documents published some time since illustrate the preceding remarks. The first is the account of the tests of the iron taken from the Tariffville bridge after its failure, and the second is the specification for bridges on the Cincinnati Southern Railroad. The Tariffville bridge, though nominally a wooden one, like most structures of the kind relied entirely upon iron rods to keep the wood-work together. Although the rods were too small, and seriously defective in manufacture, the bridge ought not to have fallen from that cause. The ultimate strength of the iron was not what it should have been, but yet it was not low enough to explain the disaster; but when we look at the quality of the iron, we have the cause of the fall. The rods taken from the bridge show an ultimate tensile strength of 47,560 pounds per inch, but an elastic limit of only 19,000 pounds; while the strain which was at any time liable to come on them was 22,000 pounds per inch, or 3,000 pounds more than the elastic limit. The fracture of the tested rods, which, it is stated, broke with a single blow of the hammer very much in the manner of cast-iron, showed a very inferior quality of metal. The rods broke in the bridge exactly where we should look for the failure; viz., in the screw at the end. No ordinary inspection would have detected this weakness. No inspection did detect it, but a proper specification faithfully carried out would have prevented the disaster.

Look now at an extract from the specification for bridges upon the Cincinnati Southern Railway:—

"All parts of the bridges and trestleworks must be proportioned to sustain the passage of the following rolling-load at a speed of not less than 30 miles an hour: viz., two locomotives coupled, each weighing 36 tons on the drivers in a space of 12 feet, the total weight of each engine and tender loaded being 66 tons in a space of 50 feet, and followed by loaded cars weighing 20 tons each in a space of 22 feet. An addition of 25 per cent will be made to the strains produced by the rolling-load considered as static in all parts which are liable to be thrown suddenly under strain by the passage of a rapidly moving load. A similar addition of 50 per cent will be made to the strain on suspension links and riveted connections of stringers with floor-beams, and floor-beams with trusses. The iron-work shall be so proportioned that the weight of the structure, together with the above specified rolling-load, shall in no part cause a tensile strain of more than 10,000 pounds per square inch of sectional area. Iron used under tensile strain shall be tough, ductile, of uniform quality, and capable of sustaining not less than 50,000 pounds per square inch of sectional area without fracture, and 25,000 pounds per square inch without taking a permanent set. The reduction of area at the breaking-point shall average 25 per cent, and the elongation 15 per cent. When cold, the iron must bend, without sign of fracture, from 90 to 180 degrees."

A specification like this, properly carried out, would put an absolute stop to the building of such structures as the Tariffville Bridge, and would prevent a very large part of the catastrophes which so often shock the community, and shake the public faith in iron bridges. Reference has been made above to the proper loads to be placed upon wrought-iron when under a tensile strain. Similar loads have been determined for other materials, and for other kinds of strain.

The preceding remarks in regard to the loads for which bridges should be designed, and the safe weight to be put upon the material, are given to show how far the safety of a bridge is a matter of fact, and how far a matter of opinion. It will be seen that the limits within which we are at liberty to vary, are quite narrow, so that bridge-building may correctly be called a science; and there is no excuse for the person who guesses, either at the load which a bridge should be designed to bear, or at the size of the different members of the structure. Still less can we excuse the man who not only guesses, but who, in order to build cheaply, persistently guesses on the wrong side.

It will, of course, be understood, when it is said that bridge-building may be called a science, that it can only be so when in the hands of an engineer whose judgment has been matured by wide experience, and who understands that no mechanical philosophy can be applied to practice which is not subject to the contingencies of workmanship. There are many bridges which will stand the test of figures very well, and which are nevertheless very poor structures. The general plan of a bridge may be good, the computations all right, and yet it may break down under the first train that passes over it. There are many practical considerations that cannot be, at any rate have not yet been, reduced to figures. It is not enough that the strains upon each member of a bridge should be correctly estimated, and fall within the safe limits: the different members of the bridge must be so connected, and the mechanical details such, as to insure, under all conditions, the assumed action of the several parts. In fine, while we can say that a bridge that does not stand the test of arithmetic is a bad bridge, we cannot always say that a structure which does stand such a test is a good one.

We often hear it argued that a bridge must be safe, since it has been submitted to a heavy load, and did not break down. Such a test means absolutely nothing. It does not even show that the bridge will bear the same load again, much less does it show that it has the proper margin for safety. It simply shows that it did not break down at that time. Every rotten, worn-out, and defective bridge that ever fell has been submitted to exactly that test. More than this, it has repeatedly happened that a heavy train has passed over a bridge in apparent safety, while a much lighter one passing directly afterwards has gone through. In almost all such cases, the structure has been weak and defective; and finally some heavy load passes over, and cripples the bridge, so that the next load produces a disaster. For the test of a bridge to be in any way satisfactory, we must know just what effect such test has had upon the structure. We do not find this out by simply standing near, and noting that the bridge did not break down. We must satisfy ourselves beyond all question that no part has been overstrained.