Blackwell's Island Bridge.

Progress, Volume IV, Issue 5, 1 March 1909, Page 162

Black well's Island Bridge.

Another Edition of Quebec. When the Quebec bridge fell into the water, drowning so many workmen, the ability of the American engineers was quickly challenged. Professional men on the British side of the water said that they sacrificed solidity to cheapness, taking too many risks. Comparisons of various designs for bridge construction were published in the technical papers of Great Bri-

tain and America. Several representative Americans replied with ridicule of what they chose to consider the hidebound British prejudice in favour of weightycostliness. But before the end the whole weight of expert opinion on both sides of the Atlantic was against them, and the end, which was the final report of the Commission of Enquiry into the Quebec disaster, left their prestige greatly shattered. During these discussions one particular American railway train and traffic bridge was much in evidence. This was the bridge at Blackwell's Island, over the East River, New York. In comparison with the Quebec design, this appeared to be of a

strength far greater. Men repeatedly said, ' ' If the Quebec specifications had been like those of BlackwelTs Island things would have been different." But a thoroughly alarmed profession was looking into things, and once out of the Fool's Paradise, where the spread-eaglers had lived so long, they quickly pounced on the Blackwell's Island bridge, then approaching completion. The bridge had cost between five and six millions sterling; it was the greatest cantilever structure in the world, it had the prestige of America written large all over its immense and imposing decks, traffic ways, railroad tracks, and the rest. A remorseless profession examined, checked, learned, inwardly digested, and without hesitation condemned the bridge as dangerous, insisting upon an investigation to determine the actual strength of the structure. The Bridge Department of the United States consented, and the investigation was entrusted to three men — Professor Burr, of Colombia University, and Messrs. Boiler and Hodge, bridge engineers of New York — all of them possessing the confidence of the engineering profession. While they investigated, the fears of the experts of America grew to white heat. When they reported the revelation was a catastrophe. The report meant that if the bridge, when opened, was subjected to the loads for which it was designed it must collapse, as the actual strains would in every case exceed the strains for which the bridge was designed, in one case going as high as 47 per cent, over the point of safety. In the beginning, when the bridge was designed, the determination was to have it made strong enough to carry the greatest possible congested load that could be put upon it. Tnis was found to be 16,000 pounds of moving weight for every lineal foot of the structure. Subsequently the Department determined to add to the capacity of the structure, by adding four elevated tracks. This involved a very large increase to the live loads, and ought to have meant also the thickening and strengthening of the general design proportionately. Prudence, says an expert authority, would have suggested that r,t this point, a complete recalculation of the bridge be made, and a new strain sheet drawn up. Instead of this, the first strain sheet was accepted as correct and pro rata increases were made m the sections of the various numbers. Into the new condition of the bridge the investigators went very carefully, until they were able to make, after actual weight and measurement, a strain sheet of the whole. The result is written in the pronouncement above quoted, and it has been held by good authority to spell "the most momentous case of faulty design in the whole history of the designing of long span bridges. ' ' Of course, this was bad enough. But there was worse to come. It was found that when the maximum stress was first calculated no allowance was made for secondary stresses or for snow load. The latter omission seems amazing, in view of the fact

that sometimes a blizzard might, besides loading up the tracks and frames with enormous weights of snow, block long trains adding immensely to the load. Anyhow, the strain sheets of these investigators proved that if the bridge were subjected to the ordinary strains designed for it, the structure must certainly collapse. The next thing to find out was what weight the bridge can carry at the very most with safety. After due investigation they found that the bridge designed originally — before the capacity was increased — for a load of 16,000 pounds was equal to a