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Historic American Engineering Record
Smithfield Street Bridge, Pittsburgh (PA-2)

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John Roebling

After the fire the old piers and abutments were repaired and on them the famous John A. Roebling constructed a new wire cable suspension bridge for a contract price of $55,000. Work was begun on the new structure in May, 1846, a short time after the fire. D. B. Steinman has given a full and colorful account in his biography of Roebling of the construction of this first of the famous engineer's highway bridges. (17) What was begun in Pittsburgh culminated in the 1860's in his final master work, the Brooklyn Bridge.

John A . Roebling as a bridge engineer is so well known that any biographical data would seem almost redundant, but some account of his life is necessary here because of his importance in Pittsburgh pontine history. (18) He was born in MŸhlhausen, Germany, in 1806, received his engineering education at the Royal Polytechnic Institute in Berlin, and emigrated to America in 1831, settling at Saxonburg in Butler County, some twenty-five miles north of Pittsburgh. This town, which he established, became the chief focus of his early engineering career ad here he established his wire rope manufactory which was later moved to Trenton, New Jersey. His wire cables were first used on the inclined planes of the Portage Railroad in the mountains of Western Pennsylvania; his first important bridge was a suspension aqueduct which he constructed over the Allegheny River at Pittsburgh. (19) As the aqueduct was being completed the Monongahela Bridge burned and Roebling was almost immediately given the commission to construct the new one. As a result of the fame of these two structures, Roebling was now established as America's foremost bridge engineer and went on to design such famous structures as (another Pittsburgh work) the second Sixth Street Bridge over the Allegheny River (1858-60), the Niagara Railway Suspension Bridge (1851-55), the Cincinnati Bridge over the Ohio (1856-67) and finally the Brooklyn Bridge which he was never to see finished since he died as a result of an accident in 1869 when work on the bridge piers had just begun.

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.

Table of Quantities (Suspension Bridge)

Length of bridge between abutments . . . 1500 feet

Number of spans . . . 8

Average width of spans from centre to centre . . . 188 feet

Diameter of cables . . . 4-1/2 inches

Number of wires in each . . . 750

Weight of superstructure of one span, as far as supported by cables . . . 70 tons

Tension of cables resulting from it . . . 122 tons

Weight of four six horse teams, loaded with 104 bushels of coal each . . . 28 tons

Tension resulting from it when at rest . . . 48 tons

Weight of 100 head of cattle at 800 lbs . . . 40 tons

Tension resulting from it when at rest . . . 70 tons

Aggregate weight of one span as far as supported by the cables, plus 100 cattle at rest . . . 110 tons

Tension resulting from it . . . 192 tons

Ultimate strength of two cables . . . 860 tons

Section of anchor chains . . . 26 inches

Section of pendulums . . . 63 inches

Suspension Bridge Demise

In 1859 an agreement was made with the Pittsburgh and Birmingham Railway Company, a horse car line then being constructed from Pittsburgh to the South Side across the river, to permit the line to cross the bridge at the price of fifteen dollars per car each month. In 1865 the structure was lit by gas and the foot toll was reduced to one cent per person. (21) In 1861, a wooden truss bridge was built a little further upstream at South Tenth Street; (22) Roebling's span was now not the only one crossing the river and in later years it was increasingly referred to as the Smithfield Street Bridge.

The structure during its years of service was often sorely tried -- sometimes when it was crowded with people viewing a steamboat race and sudden rushes would be made from one side of the bridge to the other. (23) Such conditions afforded a real test of the designer's foresight in providing various features that assured enduring stability. The bridge continued in service for thirty-five years, carrying the heaviest kind of street traffic, horse cars, steam rollers, eight-horse teams pulling heavy trucks loaded with iron and machinery. The multiple span arrangement though quite satisfactory for an aqueduct with its loading constant or uniform in all spans is under a disadvantage in a suspension bridge carrying variable loading. Despite the system of inclined stays which Roebling had installed, a loaded span sometimes deflected as much as two feet with a corresponding smaller rise of the adjoining spans. Not only the designer but the profession profited by this experience. (24)

Due to the enormous volume of traffic on Roebling's bridge, it began to show signs of strain and the Board of Managers of the Bridge Company decided to look into the possibility of providing a new structure. On February 1, 1871, bids were presented to the Board, but soon afterward the City of Pittsburgh tried to secure the franchise. This brought out a stockholder's meeting on May 27, 1872, contesting the city's right to such action. (25) This difficulty retarded the new improvement and the Panic of 1873 with its resultant difficulties prevented anything being done, but in the summer of 1880 the Board finally decided to demolish the Roebling bridge and construct a new one. (26)

The Board of Directors of the Bridge Company called to their aid a local engineer, Charles Davis (1837-1907) who submitted a design for another suspension bridge. Davis was one of those American engineers who seem to have learned their profession "in the field", so to speak, particularly in railway surveying. He had been consulting engineer for Pittsburgh's Point (suspension) Bridge (1875-77) and in 1881 was elected Engineer of Allegheny County, a position he held until his death. (27)

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Last modified: 30-Sep-1999

Historic American Engineeering Record (HAER) Text: James D. Van Trump, 1974