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It has been for some time a matter of common knowledge that the Brooklyn Bridge is unequal to the present heavy duty that is laid upon it. Although there is no actual danger of collapse, there are certain elements in the design and construction which are continually giving evidence of weakness. This is particularly true of the floor system, in which is included the stiffening trusses. The latter are giving continual trouble by buckling in the lower chords under heavy concentrations of traffic. In saying this we cast no reflection upon the designers and builders of the original bridge. Considering its unprecedented size, the Brooklyn Bridge must ever be regarded as the pioneer structure in the class of extremely long-span bridges. The theory of bridge construction was not so well understood than as it is now. The structure has done noble work in the twenty years of its life, and in the latter half of that period it has been carrying far greater loads than it was originally designed for. The late Commissioner of Bridges, Mr. G. Linden-thai, has left on record in his office a most valuable preliminary study for the reconstruction of the bridge with a view to eliminating its faulty features, enlarging its capacity, and dividing up the duty of the bridge, so that all parts shall take their proper share of the: load, and the stress be so distributed that there sSall be no uncertainty whatever as to the work that each part has, to do. The chief faults of the original design are, first, the presence of diagonal wire rope stays running from the stiffening truss back to the top of the towers, to which they are rigidly attached. The sagging of the main cables, under a combination of high temperature and congested traffic, throws the greater part of the load on these diagonal stays, with the result that the trusses are strained beyond their strength and are continually buckling out of shape. Secondly, the stiffening trusses are altogether too shallow to be of any great service, some of them having a depth of only 12 feet on a span of 1,600 feet. The main cables have always been the most satisfactory element in the whole structure, for they have a margin of strength beyond the maximum load, that is much greater than the margin in other parts of the structure. In the proposed reconstruction it is sought not only to bring the stresses in each part of the structure within known limits and keep them there, but also to increase the carrying capacity of the whole structure. Briefly stated, the desired end is accomplished by the removal of the present shallow trusses and the substitution of a pair of great continuous stiffening trusses, 75 feet deep at the towers and 19 feet deep at their shallowest portion in the center of the main span. These will extend across the main span and half way across the two shore spans, terminating at each end at a steel holding-down pier, by which the inshore, overhanging portions will be tied down to a mass of masonry at the ground level, of sufficient weight to counterbalance the excess weight of the main span portion of the trusses. The total length of this great truss will be 2,628 feet, the length of the portion spanning the Bast River being 1,595 feet 6 inches and that of each of the overhanging shore arms 516 feet 6 inches. It is designed that these stiffening trusses shall carry their own weight, neither more nor less the live load, that is the elevated cars, trolley cars, etc., and the load of the floor system, being carried by the main cables. In view of the fact, however, that the stiffening trusses will be built as part of the floor system, and will be rigidly attached to the same, it would seem at first thought as though this division of the load would be impossible. It will be accomplished, however, by a very ingenious method, which consists of cutting out a section Of the bottom chord at the towers and placing between the abutting ends a hydraulic plunger which is maintained under a pressure exactly equal to that due to the load of the truss itself. Consequently, when the cables begin to bend under an accumulation of live load, there is no additional load thrown upon the trusses, for the reason that the hydraulic plunger begins to yield, thus maintaining the predetermined stresses throughout the trusses. The relieving of the cables of the great weight of the present six stiffening trusses and making the two substituted trusses carry their own load, renders it possible to admit a much larger live load upon the bridge, and this is done by providing two decks, on the upper of which will be four elevated railway tracks, and on the lower deck two trolley tracks, two 17-foot roadways and two passenger footways. The roadways will thus be restored to the full width of the roadways on the present bridge before the trolleys monopolized a third of the space. The trolleys, being separated from the roadways, can run at twice the average speed that obtains at present, while there will be a clear gain of two new elevated tracks. The diagonal suspenders, which have been the cause of so much trouble in the past few years, will be entirely removed, and the load upon the towers lightened by the amount of their aggregate weight. The weight of the big stiffening trusses, moreover, will not rest upon the towers, but upon steel piers which will be built inside the hollow spaces of the towers and will rest directly upon the masonry foundation below. The floor system will be entirely reconstructed, the present narrow latticed floor beams being replaced by plate-steel floor beams and stringers of approved modern construction. The wire-cable wind bracing will be removed, the new trusses and their lateral bracing being designed effectively to sustain any possible wind pressure. In order to take care of the increased pull on the anchorages due to the larger live loads that will be carried, the anchorages will be increased in size, a considerable addition of solid masonry being made at the shore ends. Moreover, to prevent any settlement or sliding, steel sheet piling is to be driven entirely around the base of the anchorages. It will be very gratifying to the citizens of Greater New York to learn that by the proposed scheme of reconstruction not only is the life of the great Brooklyn structure indefinitely prolonged, but its usefulness and capacity are increased over fifty per cent. Unfortunately the reconstruction cannot be taken in hand until the new Manhattan Bridge is completed. The plans for this structure, which call for eye-bar cables, were drawn particularly with a view to expeditious erection; and if the work is put in hand at once, the bridge should be ready for use in three and a half years from the present date. There will be some slight interference with traffic during the reconstruction of the present Brooklyn Bridge, but the plans have been so drawn that the bridge will be in practical service during the whole period of reconstruction. The numerous plans accompanying the report on this work will be found in the current issue of the Supplement.
