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Suspension Bridge, 1846

John Roebling was not only a more than competent bridge engineer, but he was also a prolific writer ad he published his achievements as they appeared. Consequently the best description of the second Monongahela Bridge remains that from his own pen. (20)

"The new Suspension Bridge over the Monongahela was commenced in June, 1845, and opened for travel in February, 1846. The piers and abutments of the old wooden structure,which was destroyed by the great fire, required extensive repairs to be fitted for the reception of the new superstructure. The whole length of the work between the abutments, is exactly 1,500 feet, and is divided into eight spans of 188 feet, average distance from centre to centre. The piers are 50 feet long at bottom, 36 feet high, and 11 feet wide on top, battering 1 inch to the foot.

"Two bodies of substantial cut stone masonry, measuring 9 feet square and 3 feet high, are erected on each pier, at a distance of 18 feet apart. On these the bed plates are laid down for the support of the cast iron towers, to which the wire cables are suspended by means of pendulums. Each span being supported by two separate cables, there are therefore, 18 cables suspended to 18 towers.

"The towers are composed of four columns moulded in the form of a two-sided or cornered pilaster; they are connected by four lattice panels, secured by screw bolts. The panels up and down stream close the whole side of a tower, but those in the direction of the bridge form an open doorway, which serves for the continuation of sidewalks from one span to the other.

"On top of the pilasters or columns, a massive casting rests, which supports the pendulum to which the cables are attached. The upper pin of the pendulum lies in a seat which is formed by the sides and ribs of a square box occupying the center of the casting. For the purpose of throwing the whole pressure upon the four columns underneath, 12 segments of arches butt against the centre box, and rest with the other end upon the four corners.

"The pendulums are composed of four solid bars of 2 feet 6 inches long, from centre to centre of pin, 4 inches by one inch -- the pins are three inches in diameter. To the lower pin the cable of one span is attached directly and the connection formed with the next cable by means of four links of 3 feet 6 inches long and 4 inches by 1-1/4 inches.

"The opposite cables, as well as the pendulums, are inclined towards each other -- the distance being 27 feet at the top of the towers, and 22 feet at the centre of a span. The pendulums on the abutments, however, occupy a vertical position.

"The two sidewalks are outside of the cables, and 5 feet wide. The roadway is contracted to 20 feet, and separated from the sidewalks by fender rails, which are raised from the floor by means of blocks of 6 inches high, 8 feet apart. The total width of the bridge between the railings is 32 feet.

"The anchor chains which hold the cables of the first and last span, are secured below ground in the same method which was applied to the aqueduct -- their oxidation is guarded against in the same manner.

"The cables are 4-1/2 inches in diameter, and protected by a solid wrapper; they are assisted by stays, made of 1-1/4 round charcoal iron; the suspenders are of the same material, 1-1/2 inch diameter, and placed 4 feet apart.

"The peculiar construction of the Monongahela Bridge was planned with the view of obtaining a high degree of stiffness, which is a great desideratum in all suspension bridges; this object has been fully attained. The wind has no effect on this structure, and the vibrations produced by two heavy coal teams, weighing seven tons each, and closely following each other, are no greater than is generally observed on wooden arch and truss bridges of the same span.This bridge is principally used for heavy hauling; a large portion of the coal consumed in the City of Pittsburgh passes over it in four and six horse teams.

"As a heavy load passes over a span, the adjoining pendulums, when closely observed, can be noticed to move correspondingly -- the extent of this motion not exceeding on half inch. By this accommodation of the pendulums, all jarring of the cast iron towers is effectively avoided. Another object of the pendulums is to direct the resultant of any forces to which the work may be subjected, through the center of the towers, as well as of the masonry below.

"Two of the piers of the old structure had once given way in consequence of the shaking and pressure of the arch timbers, when subjected to heavy loads. Such an accident can never take place on the new structure, as the piers are only subjected to the quiet and vertical pressure of the towers.

"I do not recommend the application of pendulums in all cases; but in this, it appeared to me the best plan which could be adopted.

"The two towers on each pier are connected by a wooden beam, properly encased and lined by the same mouldings which ornament the top of the castings.

"The lightness and graceful appearance of this structure is somewhat impaired by the heavy proportions of these connections, but I had to resort to it for motives of economy.

"The whole expense of this structure does not exceed $5,000 -- a very small sum indeed for such an extensive work.

"A great portion of this work had to be done during the winter, and in cold weather; it was accomplished without any accident, with the exception of one of the workmen who was seized by fits and killed by falling off a pier.

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