18 March 1905
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Scientific American New York 18 March 1905 |
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The city bridge department, knowing the bridge is loaded to near the maximum load it was designed to carry, as a cautionary measure to prevent overloading, has made rules as to loading. At least 102 feet of empty track space must exist between every two trolley cars on the bridge, and but one elevated train on each track is permitted upon the center span at the same time. These rules are necessary as the load on the bridge during periods of heaviest traffi is within 125 tons of the maximum load the bridge was intended to carry. A single elevated train or a few street cars would exceed this margin of weight, and if through carelessness they should move onto the span before the train ahead has left the span, the bridge would be overloaded. If it were thus overloaded. it would not. however, be in danger of failure, because it was built with a large factor of safety. But that was twenty-two years ago. What is its factor of safety now? The bridge department should be the best authority. During several administrations it has enforced precautionary rules. The chief engineer stated before the State Railroad Commission: "The Brooklyn Bridge is an antiquated structure, unfitted for the demands made upon it, and should be rebuilt after the completion of the Manhattan Bridge within the next five years. The problem involves a combination of difficulties. The solution is to transport the people over the bridge as fast as they arrive. The factors in the problem are: 1. Safety of passengers. 2. Weight of load on the bridge. 3. Number of persons to be transported. 4. Number of cars required. 5. Speed of cars necessary. 6. Time required for loading and unloading cars. 7. How to make the change without interrupting transportation. 8. Cost of the new system. 9. Tithe required to put a new system in operation. 10. A proper terminal station. Lack of cars moving over the bridge, and not lack of loading facilities is the cause of the congestion. More cars and lighter cars is the only remedy. This forces us to a plan for special bridge cars without heavy machinery. The system of transportation illustrated on the front page of this issue is designed to meet all the above-named conditions. An endless train of cars is operated across the bridge with a circular loop at each terminus. Cable traction is used, and the motors, brakes, third- rails, trolley-wire supporters, wires, etc., are dispensed with. Light trucks with small wheels are substituted for the heavy ones now used, and more than double the number of cars can be operated, without increasing the weight; and by operating them on one set of tracks we have an endless train of cars, and may increase the speed with no danger of collision. But this requires that the train shall not stop, and we are forced to use a slowly-rotating loading-platform with access to it by stairways located at the center, where motion is slow. Two cables would be used, driven by electric motors, the motors and cable in duplicate to be used on alternate days. This would reduce the danger of a "tie-up" to a minimum. The electric current could be independently generated, or purchased from power companies. 
This plan includes no untried feature, unless it be in the combination. Moving sidewalks were tested at the Chicago and Paris expositions, with difference in speed between adjoining platforms of three miles an hour. These cars are inclosed, and the difference in rotary speed of stairways is reduced to one-half mile an hour. Such a plan as this would accommodate all the present passenger trams on the tracks used by the elevated trains. and the space now used by trolley cars could be used for vehicle trams, and the capacity of the bridge would be doubled. As the number of cars would be more than double that now used, more than twice the number of persons could be transported without increasing the load on the bridge. A speed of ten miles an hour would do this, but the speed could be changed to meet the changing demands for transportation. All the time now wasted by stopping, starting, backing, switching, and waiting trains would be devoted to moving the people over the bridge, and the motive power required to do it would be less.At each terminal station there is a circular platform, 200 feet to 400 feet in diameter. The platforms are kept , in constant rotation. They ride on wheels, which roll on tracks laid in concentric circles, and the whole is carried on an elevated structure. Stairways at the center of the platforms give adequate and convenient means of access to the rotating platform from the street below and from passageways above (from and to the elevated stations). These stairways also serve as exits for arriving passengers. The loops in the endless train of cars are arranged so as to encircle about three-fourths of the platforms, the cars locking with the platform edge, and rotating the platforms at the same speed as the moving train. People will then be able to step from one to the other with as much ease and safety as they now step from the parlor-car to the dining-car of a moving express train. The stairways, which are attached to the platforms near the center and extend downward to near the ground, rotate with the platforms, but the motion is so slight as to be scarcely noticeable. If the platforms are made 400 feet in diameter, at twenty-miles-an-hour speed of the cars, the stairways would have a rotary speed of one mile an hour. Beneath each stairway, and leading to it, would be an intermediate circular platform, twenty feet in diameter, on a level with the ground. It would rotate in the same direction, with a speed one-half that of the stairways, or one-half mile an hour. An automatic fence prevents a person falling off the platform. Each platform would have four stairways, twenty feet in breadth, which would accommodate more than 100,000 persons an hour. Congestion on the platforms would be impossible, for if persons packed the stairways as closely as they could, each occupying two feet of breadth, forty persons would fill the eighty feet of stairways; but when they arrived at the edge of platform, beside the cars, they would be more than thirty-one feet apart, the edge being 1,250 feet long, and 940 feet of cars being always in contact with the platform. If a person failed to step off the car, moving at ten miles an hour, he would be carried over the bridge again, and back, and would lose twelve minutes; but if he did not step on the car during the first revolution of platform, he would lose but fourteen seconds; when he would begin his second revolution. He would have sixty-four seconds to step on or off the car as against twenty-eight at present. The proposed plan would reduce the load on the bridge and distribute the load more uniformly. 
Roebling's report says: "I propose a speed of 20 miles an hour, as being perhaps the one most likely preferred. But this may be increased to 30 or 40 miles per hour, with absolute safety." (Page 246, Franklin Institute Journal, 1867.) To render the above plan most effective, the Manhattan loop should extend over Park Row, where a curvature with radius of 125 feet to 200 feet could be obtained. The Brooklyn loop could be built near Tillary Street, between Fulton and Washington Streets. This woutd practically connect City Hall with Borough Hall. 
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