Bridge Disasters in America: The Cause and the Remedy

by George L. Vose

A short time ago the builders of a wretchedly cheap and unsafe highway bridge, in order to quiet a fear which had arisen that the structure was not altogether sound, tested a span 122 feet long with a load of 58,000 pounds; and inasmuch as the bridge did not break down under this load, which was less than a quarter part of what it was warranted to carry safely, the county commissioners considered the result eminently satisfactory, and remarked that the test was made merely to satisfy the public that the bridge was abundantly safe for all practical uses. The public would, no doubt, have been satisfied that the Ashtabula bridge was abundantly safe for all practical uses had it stood on that bridge in the morning and seen a heavy freight-train go over it, and yet that very bridge broke down directly afterwards under a passenger-train.

Now, according to the common notion, that was a good bridge in the morning, and a very bad bridge, or rather, no bridge at all, in the evening. The question for the public is, When did it cease to be a good bridge, and begin to be a bad one? A test like the one referred to above can do no more than illustrate the ignorance or lack of honesty of those who make it, or those who are satisfied with it. Such a test might come within a dozen pounds of breaking the bridge down, and no one be the wiser. The entire absurdity of such testing has recently been illustrated in the most decided manner. The very same company that built the bridge above referred to, made also another one on exactly the same plan, and of almost precisely the same size, and tested it when done by placing almost exactly the same load upon it. The bridge did not break down; and the county commissioners, for whom the work was done, were satisfied that it was "abundantly safe for all practical uses," accepted it, paid for it; and in less than ten years it broke down under a single team and a little snow, weighing in all not over one-tenth part of the load the bridge was warranted to carry, and not over one-half the load with which it had been previously tested. If this bridge had been "tested" by five minutes of honest arithmetic, it would have been promptly condemned the very day it was finished.

In view of the preceding, what shall we say of a bridge company that deliberately builds a bridge in the middle of a large town, where it will be subjected to heavy teaming, and, owing to its peculiar location, to heavy crowds, and warrants to the town that it shall safely hold a ton per running-foot, when the very simplest computation shows beyond chance of dispute that such a load will strain the iron to 40,000 pounds per square inch? We are to say, either that such a company is so ignorant that it does not know the difference between a good bridge and a bad one, or else so wicked as to knowingly subject the public to a wretchedly unsafe bridge. The case referred to is not an imaginary one, but existed recently in the main street of a large New-England town. The joints in that bridge, which could safely hold but 20,000 pounds, were required to hold 60,000 pounds under the load which the builders had warranted the bridge to carry safely. The case was so bad, that, after a lengthy controversy, the town officers had a thorough expert examination of the bridge, which promptly condemned it as in imminent danger of falling, and as having a factor of safety of only 1-15/100, which is practically no factor at all. Notwithstanding all this, and in the face of the report, the president of the bridge company came out with a letter in the papers, in which he pronounced the bridge "perfectly safe." Thus we actually have the president of a bridge company in this country stating openly that a factor of safety of 1-15/100 makes a bridge perfectly safe, or, in other words, that a bridge can safely bear the load that will break it down, for he very wisely made not the slightest attempt to disprove any of the conclusions of the commission; and this company has built hundreds of highway bridges all over the United States, and is building them to-day wherever it can find town or county officers ignorant enough to buy them.

It might be supposed, that, under the above condemnation, the authorities controlling the bridge would have taken some steps to prevent the coming disaster. They did, however, nothing of the kind, but allowed the public to travel over it for more than a year, at the most fearful risk, until public indignation became so strong that a special town-meeting was called, and a committee appointed to remove the old bridge, and to build a new one.

One of the worst cases of utterly dishonest bridge-building that we have had of late years in Massachusetts, was that of the iron highway bridge across the Merrimac River at Groveland, a few miles below Haverhill, one span of which broke down in January, 1881. This bridge was built in 1871-1872, and consisted of 6 spans, each about 125 feet long. The whole cost of the structure was $80,000, and the contract price for the iron-work was $28,000. The company which made that bridge, agreed in their contract to give the county a structure that should carry safely 3,000 pounds per running-foot besides its own weight; but they built a bridge, which, if they knew enough to compute its strength at all, they knew perfectly well could not safely carry over one-quarter part of that load. In fact, the weight of the bridge alone is more than it ever ought to have borne. The company warranted each span of that bridge to carry safely a net or moving load of 165 tons, and it broke down under a single team and a small amount of snow. The company warranted that bridge to carry safely a load which would strain the iron to 50,000 pounds per inch, when it knew perfectly well that 15,000 pounds per inch was the most that could safely be borne.

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.