There are several concerns in the United States which make a specialty of highway bridges, and which, taking advantage of the ignorance of public officials, are flooding the country with bridges no better than that at Groveland. On an average, at least twenty of these miserable traps tumble down every year, and nothing is done to bring the guilty parties to punishment. Dishonest builders cheat ignorant officials, and the public suffers the damage and pays the bills. Is human life worth enough to pay for having these structures inspected, and, if found unsafe, strengthened or removed? Can we do any thing to prevent towns and counties from being imposed upon by dishonest builders? We certainly can, if those who control these matters care enough about it to do it. There are two ways of buying a bridge,—a good way and a bad one; and these two ways are so plain that no one can misunderstand. To buy a bad bridge, just as soon as your town or county votes money for a new bridge, certain agents—and they are as numerous as the agents for sewing-machines or lightning-rods—will call on, or write to, the town or county officers, and will offer to build any thing under heavens you want of any size, shape, or material, and for almost any price. They will produce testimonials from all the town and county officers in the country for the excellence of their bridges, and would not hesitate to give reference, even, for their moral character, if you should ask it. If they find that you don't know any thing about bridges, they will, to save you the trouble, furnish a printed specification; which document will commit you to pay the money, but will not commit the bridge company to any thing at all. When the bridge is put up, you never will know whether the iron is good or bad, nor whether the dimensions and proportions are such as to be safe or not. You will know that you have paid your money away, but you never will know what you have got for it until some day when your bridge gets a crowd upon it, and breaks down, and you have the damage to pay. This mode of buying a bridge is very common. To buy a good bridge, first determine precisely what you want; and if you don't know any thing in regard to bridge-building yourself, employ an engineer who does, to make a specification stating exactly what you want, and what you mean to have. Then advertise for bridge-builders to send in plans and proposals. Let the contractors understand that all plans and computations are to be submitted to your engineer, that all materials and workmanship will be submitted to your inspectors, and that the whole structure is to be made subject to the supervision of a competent engineer, and accepted by him for you. You will find at once, that, under such conditions, all travelling agents and builders of cheap bridges will avoid you as a thief does the light of day. You will have genuine proposals from responsible companies, and their bids should be submitted to your engineer. When you have made your choice, let the contract be written by your lawyer, and have the plans and specifications attached. Employ a competent engineer to inspect the work as it goes on; and when it is done, you will have a bridge which will be warranted absolutely sound by the best authority. This mode of buying a bridge is very uncommon.
The Ashtabula bridge, it is stated in the report of the committee of the Ohio Legislature appointed to investigate that disaster, had factors,—we can hardly call them factors of safety,—in some parts as low as 1-6/10 and 1-2/10, such factors referring to the breaking-weight; and even these factors were obtained by assuming the load as at rest, and making no allowance for the jar and shock from a railroad train in motion. Well may the commissioners say, as they do at the end of their report, "The bridge was liable to go down at any time during the last ten years under the loads that might at any time be brought upon it in the ordinary course of the company's business, and it is most remarkable that it did not sooner occur."
One point always brought forward when an iron bridge breaks down, is the supposed deterioration of iron under repeated straining; and we are gravely told that after a while all iron loses its fibre, and becomes crystalline. This is one of the "mysteries" which some persons conjure up at tolerably regular intervals to cover their ignorance. It is perfectly well known by engineers the world over, that with good iron properly used, nothing of the kind ever takes place. This matter used to be a favorite bone of contention among engineers, but it has long since been laid upon the shelf. No engineer at the present day ever thinks of it. We have only to allow the proper margin for safety, as our first-class builders all do, and this antiquated objection at once vanishes. The examples of the long duration of iron in large bridges are numerous and conclusive. The Niagara-Falls railroad suspension bridge was carefully inspected after twenty-five years of continued use under frequent and heavy trains, and not only was it impossible to detect by the severest tests any defect in the wire of the cables, but a piece of it, being thrown upon the floor, curled up, showing the old "kink" which the iron had when it was first made, and wound on the reel. The Menai suspension bridge, in which 1,000 tons of iron have hung suspended across an opening of 600 feet for sixty years, shows no depreciation that the most rigid inspection could detect. Iron rods, recently taken from an old bridge in this country, have been carefully tested after sixty years of use, and found to have lost nothing, either of the original breaking-strength, or of the original elasticity.
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.