Bridge Disasters in America: The Cause and the Remedy

by George L. Vose

The question is frequently asked, Does not extreme cold weaken iron bridges? To this, it may be replied, that no iron bridge, made by a reliable company, has ever shown the slightest indication of any thing of the kind, though they have been used for many years in Russia, Norway, Sweden, and Canada, and nothing that we know in regard to iron gives us any reason to suppose that any thing of the kind ever will happen. But here, again, every thing turns upon the quality of the iron. Iron containing phosphorus is "cold-short," or brittle when cold, and will break quicker under repeated and sudden shocks in cold weather than when it is warm. With good iron, properly used, we need have no fear on this point. The securing such iron is a matter to which the utmost attention is paid by our first-class bridge-building firms, but it is a matter to which no attention is paid by the builders of cheap bridges. We might suppose that a person, in putting an insufficient amount of iron into a bridge, would be careful to get the best quality; but exactly the reverse seems to be the case, on the ground, perhaps, that the less of a bad thing we have, the better.

Many persons, in building wooden bridges, take no pains to get iron rods which are suitable for such work, but purchase what is easiest to be had in the market, and in many cases never find that the iron was bad until a bridge tumbles down. There are, without the slightest question, hundreds of bridges now in use in this country, which, as far as mere proportions and dimensions go, would appear to be entirely safe, but which, on account of the quality of the iron with which they are made, are entirely unsafe; and there always will be, as long as public officials purchase iron which they know nothing about, to put into bridges. When a bridge is finished, the ordinary examinations never detect the quality of the iron; so that the wise remarks of many inspectors, or the opinions of those in charge of these structures, as to the exact condition of a bridge, are of little or no value.

We often hear iron bridges condemned, while wooden ones, so called, are supposed to be free from defects. It does not seem to occur to persons holding such ideas, that wooden bridges rely just as much upon the strength of the iron rods that tie the timbers together, as upon the timber. As a matter of fact, where one iron bridge falls, a dozen wooden ones do the same thing. One very decided advantage which an iron bridge has over a wooden one, is that we can make sure of good iron in the beginning, and that we can also be sure that it does not decay; while, however good our timber may be in the beginning, we never can be entirely sure of its condition afterwards. There are wooden bridges now standing in this country, all the way from sixty to eighty years old, which are apparently as good as ever; while there are others, not ten years old, which are so rotten as to be unfit for use. It will not do to assume, that, because no defects are very evident in a wooden bridge, therefore it has none. When a wooden bridge, originally made of only fair material, has been in use under railroad trains for twenty-five or thirty years, and in a position where timber would naturally decay, we are bound to suspect that bridge. To assume such a bridge to be all right until we can prove it to be all wrong, is not safe. To assume a bridge to be all wrong until we can prove it to be all right, is a safe method, though not a popular one. Any person who has had occasion to remove old wooden bridges, will recall how often they look very much worse than was anticipated.

There is one defect in railway bridges which has often led to the most fearful disasters, and which, without the slightest question, can be almost entirely, if not entirely, removed, and at a moderate cost. At least half the most disastrous failures of railroad bridges in the United States have been due to a defective system of flooring. With a very large proportion of our bridges, the failure of a rail, the breaking of an axle, or any thing which shall throw the train from the track, is almost sure to be followed by the breaking down of the bridge. The cross-ties are in many cases very short, and the floor is proportioned for a train on and not off the rails. When an engine on such a floor leaves the track, it plunges off the ends of the cross-ties into the open space between the stringers and the chords, and generally wrecks the bridge. To prevent this, the cross-ties should be long and well supported, and placed so close that a derailed engine cannot cut through them. The track should also be provided with guard-timbers well fastened, and the width between the trusses should be so great that the wheels of a derailed train will be stopped by the guard-rail before the side of the widest car can strike the truss.

The importance of a substantial floor system has been very fully recognized by the railroad commissioners of Massachusetts, who have recently issued a very suggestive circular, accompanied by numerous examples of track construction for railway bridges. If this circular receives proper attention, it is sure to produce good results.

Another point which has often been neglected, is making sufficient provision to resist the force of the wind. A tornado, such as is not uncommon in this country, will exert a force of 40 pounds per square foot, which upon the side of a wooden bridge, say of 200 feet span, and 25 feet high, and boarded up as many bridges are, would amount to a lateral thrust of no less than 100 tons; and this load would be applied in the worst possible manner, i.e., in a series of shocks. There have been many cases in this country where bridges have been blown down; and a case recently occurred where an iron railroad bridge of 180 feet span, and 30 feet high, and presenting apparently almost no surface to the wind, was blown so much out of line that the track had to be shifted. The recent terrible disaster at the Firth of Tay was, no doubt, due to this cause.

At the time of the Tariffville catastrophe, it was gravely stated at the coroner's inquest, and by railroad officers who claimed to know about such things, that the disaster was caused by the tremendous weight of two locomotives which were coupled together, and it was stated that one engine would have passed in safety; and directly afterwards the superintendent of a prominent railroad in New England issued an order forbidding two engines connected to pass over any iron bridges. It is all very well for a company to issue such an order, so far as it may give the public to understand that it is determined to use every precaution against disaster; but such an order may have the effect of creating a distrust which really ought not to exist. If a railway bridge is not entirely safe for two engines, it is certainly entirely unsafe for one engine and the train following; the only saving in weight by taking off one engine being the difference between the weight of that engine and the weight of the cars that would occupy the same room. For example, a bridge of 200 feet span will weigh 1,500 pounds per lineal foot. An engine and its tender will weigh 60 tons in a length of 50 feet, and a loaded freight-train may easily weigh 2/3 of a ton per lineal foot. The total weight of the span, with two engines, and the rest of the bridge covered with loaded freight-cars, would thus be 320 tons. If we take off one engine, and fill its place with cars, we take off 60 tons, and put in its place 33 tons; i.e., we remove 27 tons, or just about 1/12 of the working-load. Taking off a large part of the working-load, however, is taking off a very small part of the breaking-load; with a factor of safety of six, for example, taking off 1/12 of the working-load is taking off less than 1/70 of the breaking-load. An order, therefore, like that above, can only be of use when the working-load and the breaking-load are so nearly alike that the actual load is a dangerous one: that is when the bridge is unfit for any traffic whatever; so that, if such an order was really needed, it would, in itself, be, in the eyes of an engineer, a condemnation of the bridge.

Having seen something of the structures which require inspecting, let us now see what kind of inspection we have in this country, and the result of it; and let us also see the inspection which we might have, and the results that might be produced. Looking first at railroad bridges, it might be supposed that no one could be so much interested in keeping such structures in good order as the companies which own those bridges, and which have the bills to pay in case of disaster. This is, of course, so; but, in spite of the fact, the Ashtabula bridge broke down, on one of the best managed lines in the country, and cost the company over half a million dollars in damages. No railroad bridge ever broke down, which the owners were not interested in keeping safe; but there is always a desire to put off incurring large expenses until the last moment, and thus weak bridges are very often let go too long. A short time since, the superintendent of a large railroad stated plainly before a legislative committee, that many of the smaller roads were not safe to run over, but that such roads were having a hard time, and could not afford to keep their track and bridges in a safe condition. During the past ten years over two hundred railroad bridges in the United States have broken down. These bridges were all kept under such inspection as the railroad companies owning them considered sufficient, or such as they could afford; but either the supervision was defective, or the companies knowingly continued the use of unsafe bridges, and this fault has by no means been confined to the smaller and poorer roads. It would seem, therefore, that inspection by the companies themselves has not been sufficient. It certainly has not been enough to prevent two hundred disasters in ten years. It is the custom in several of the United States to maintain what is termed a railroad commission. The original intention seems to have been for these commissions to keep the railroads under some kind of inspection, and in some way to assist in settling any questions that might arise between different companies, and between railroad companies and the public. As far as we can judge by the results produced, in the States where these commissions have been established, we can hardly pronounce them of any very great importance. In many States, it is very certain, that, in regard to matters of inspection, the work of these boards has been simply a farce; and it could hardly be otherwise in a State which pays its commissioners only $1,000 salary, or, worse yet, as in some cases, only $500. Add to this, that in many cases the appointments have been purely political ones, and we can see the absurdity of expecting any results of value. We should hardly suppose that three men, in many cases entirely unacquainted with mechanical matters, could by riding over a railroad once or twice a year, occasionally getting out to examine the paint on the outside of the boards, which conceal a truss from view, judge very correctly of the elastic limit of the iron rods which they have never seen, and of which they do not even know the existence.

For ample proof of the utter inefficiency of the present system, we have only to compare the reports of the railroad commissioners in almost any State, with the actual condition of the structures described. In one State a late annual report covers a whole railroad with the remark, "All of the bridges on this line are in excellent order;" and yet there were at that very time, and are now, on that road, several large wooden bridges with a factor of safety referred to the breaking-weight of not over two under a fair load, assuming the iron rods to be of the very best material,—a point upon which there is no evidence whatever.

There is, in fact, no difference which any ordinary inspection would detect between these bridges as they stand to-day, and the Tariffville bridge as it stood the day before it fell. In another State, an iron bridge is in use under heavy trains, which has a factor of only 2-1/2 instead of 6, and yet the State report pronounces it an excellent structure and a credit to the railroad company, which recklessly allows its trains to pass over it. In yet another State, the commissioners in 1874 reported that a certain bridge should be removed; and this was quite correct, as it was an eminently unsafe bridge. In 1875 they suggested the same thing again. In 1876 they say, "This bridge must be rebuilt the coming spring." In 1877 they again reported, "This bridge must be rebuilt before the spring opens. It is old, and will not be safe for the passage of trains over it, if the ice or freshet should take away the temporary trestles, which now in a great measure support the truss."