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Historic American Engineering Record
"Three Sisters" Bridges
HAER No. PA-490

(Trinity of Bridges)
Pennsylvania Historic Bridges Recording Project - II
Spanning Allegheny River at Sixth, Seventh, and Ninth streets
Allegheny County

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HAER No. PA-490
(Page 6)

abutment, and the bridge's roadway was reinforced with buckle plates, iron beams, and iron plates for smoke protection. [16]

At the time of its construction, the Hand Street Bridge was the third-largest Burr arch truss in the United States, according to Kaufman. The combination of two structural systems used in a Burr arch-truss presented problems in determining which component carried load at a given time. Despite these calculation difficulties, the Burr arch-truss remained a popular design during the second and third quarters of the nineteenth century. Only Wernwag's "Colossus" across the Schuylkill River and another structure on the Merrimac River surpassed the Hand Street Bridge in span lengths." [17]

The Hand Street Bridge combined a single span of 190'-0" with four spans each 200'-0" long. Three lines of arch-trusses divided the roadway into two sections each 11'-0" wide. Each arch contained two layers of timber 2'-4" deep and 7" thick, with timbers in the middle arch twice the thickness of those in the outer arches. Cantilever sidewalks added 7'-0" more to each side of the bridge, which measured about 28'-6" across. Howe trusses, consisting of white pine diagonals and transverse iron rods, provided lateral bracing. When workers removed the exterior sheathing to build the replacement bridge, some pedestrians were scared of crossing because of visible strains in the arches and floor beams. Nonetheless, the high-quality pine allowed the bridge to remain sound and mostly free of rot during its entire use. [18]

With a clear interior height of 11'-0", the wooden structure later faced the clearance problems of Burr arch-trusses adapted for street railways. An additional dilemma of navigational clearance arose when constructing a replacement in 1890. Engineers considered the vertical tension members extending 18" below the roadway surface to be the limit for the height of craft moving under the bridge. They found that the protuberances were a steering challenge rather than a lower limit for expert boatmen on the river below. "Such precision in steering was almost an incredible revelation" to Kaufman. [19]

Ice presented a constant danger to structures across Pittsburgh's rivers. Ice damaged a pier on the Hand Street Bridge in 1857 so severely that traffic had to be shut off while repairs could be made. Despite having insurance against ice damage, the company could not collect on its policy -- a difficulty which may have encouraged other bridge companies to forego spending money on policies thereafter.


[16] Kaufman, "Reconstruction of Ninth Street Bridge," 192-93.

[17] Kaufman, "Reconstruction of Ninth Street Bridge," 192-94.

[18] Kaufman, "Reconstruction of Ninth Street Bridge," 192-95.

[19] Kaufman, "Reconstruction of Ninth Street Bridge," 192-94.

HAER No. PA-490
(Page 7)

The Second Sixth Street (St. Clair) Bridge, 1859

Roebling's introduction to Pittsburgh gained him other jobs in the city. He designed a replacement for the Smithfield Street Bridge, destroyed by fire in 1845, and then a new structure for the profitable St. Clair Bridge, which was completely reconstructed from 1857 to 1859. [20] Although covered bridges were constructed as late as the 1876 Union Street Bridge over the Allegheny River at the Point, the suspension type became more popular after Roebling's structures were completed. [21]

The Allegheny Bridge Company, owner of the St. Clair Bridge, showed consistent profits from 1819, the first year in which it collected tolls, through 1860, when the new suspension bridge was finished. The rate of dividends increased steadily from 3 percent in 1823, when first paid, to a steady 15-percent rate in the mid-1840s. [22] A charter supplement in 1857 allowed the bridge to be replaced. John Harper, director of the bridge company, advocated a suspension bridge over the objections that such an expensive venture would not be profitable. The entire structure, from piers and abutments to superstructure, was razed prior to starting work on Roebling's design, although the stone was saved for reuse. [23]

Subsequent to reconstruction, the company became known as the Suspension Bridge Company. Harper aggressively marketed investments in the new structure, assuring prospective buyers of the value of their stock purchases -- which indeed came to pay solid dividends. Harper also reduced the toll to a penny for male pedestrians and gave women free passage, while also publicizing incentives rewarding companies and local organizations whose members committed to use the crossing. Fascination with the suspension form encouraged more intense use of the crossing, proving Harper's investment and strategies to have been a savvy business decision. [24]

Roebling's ornate design for the replacement nearly doubled the largest span lengths of the previous structure and used the elegant suspension form for which he was so well known, at a cost of just less than $300,000. One local history described Roebling's St. Clair Bridge as "the first great bridge to span a navigable stream in the United States." [25]


[20] Warner, History of Allegheny County, 570-71; Plowden, Bridges, 119; and Wilkins, "Reconstruction of the Sixth Street Bridge," 145.

[21] Warner, History of Allegheny County, 571.

[22] Pennsylvania State Archives, "State of the Allegheny Bridge Co."

[23] Everts, History of Allegheny County, 135-36; Pennsylvania State Archives, "State of the Allegheny Bridge Co." Harper also served as president of the Bank of Pittsburgh.

[24] Everts, History of Allegheny County, 136.

[25] Everts, History of Allegheny County, 135-36.

HAER No. PA-490
(Page 8)

The design called for two main spans of 344'-0" between the shore spans of 177'-0" and 171'-0". [25] The bridge was 40'-0" wide, with a 20'-0" roadway and two 8'-0" sidewalks, which decreased to 7'-0" at towers. Four cables ran over the cast-iron towers. Two large main cables attached to the structure between the roadway and sidewalk, and two smaller cables were affixed to the bridge outside of the sidewalks and handrails. [27]

The suspension system required 7-1/2"-diameter main cables, each containing seven bundles of 100 wires, spaced 22'-0" apart the crown and flaring out 5'-0" wider at the towers. The 4-1/2"-diameter, 300-wire sidewalk cables, hung between 35'-0" and 42'-0" apart at the same points. Four cast-iron column towers, approximately 27'-0" high and 22" in diameter, supported saddles for the cables. Wire-rope stays crossed from the deck beams at one side of the structure, over the saddles, and back to the opposite beams. The iron floor beams, spaced 5'-0" on center, hung from 1"-diameter wire rope suspenders affixed to the cables with wrought-iron collars. A system of timber stringers, with iron rods providing lateral bracing, carried a wooden deck. [28]

The St. Clair Bridge became more popular after Roebling's design was constructed, and rail traffic soon joined those on foot. By the 1870s, two street railway tracks used the structure for taking passengers across the river.[29] With the advent of electric streetcars in 1890, the structure could not bear the greater load and width required by the newer vehicles. Taken down later for replacement, the once-repaired and once-oiled wire cables were found nearly as strong as when installed, excepting the exterior strands. The change in traffic technologies and loads, however, made the narrower cable-supported structure obsolete. Tests on the iron components also showed corrosion from steamboat smoke. [30]

The First Seventh Street Bridge, 1884

In 1868, Pittsburgh annexed the East End and renamed city streets. Names recalling founders or landmarks disappeared into numbered sequences. The St. Clair Bridge took on a new identity as the Sixth Street Bridge; Irwin Street became Seventh Street; and Hand Street two blocks away became Ninth Street. Historic monikers and denotations such as Pitt, Hancock,


[26] Wilkins, "Reconstruction of the Sixth Street Bridge," 145.

[27] "The New Sixth Street Bridge, Pittsburgh, Pa.," Railroad Gazette (28 July 1893): 560.

[28] Wilkins, "Reconstruction of the Sixth Street Bridge," 145-47.

[29] Everts, History of Allegheny County, 136.

[30] Wilkins, "Reconstruction of the Sixth Street Bridge," 144, 147-49.

HAER No. PA-490
(Page 9)

Canal, Factory, and Mechanic streets were lost along the way of rationalizing the municipal layout. [31]

Commodore William J. Kountz led the group operating the Pittsburgh, Allegheny & Manchester Passenger Railway over the St. Clair Bridge. The route joined downtown Pittsburgh to Allegheny City, running from Market and Liberty streets in the former to Federal Street and on to Woods Run in the latter. Rail passengers endured sharp turns on the mule- or horse-pulled vehicles, as well as exposure to winter air. Rail companies provided straw during cold weather to help keep customers' feet warm. Cars traveled at most three to five miles per hour, and passengers paid six cents for a ride. [32]

In a bid to compete with the popular crossings at Sixth and Ninth streets, the North Side Bridge Company hired famous bridge builder Gustav Lindenthal to design the Seventh Street Bridge. Lindenthal's 1884 structure used a stiffened-chain suspension design. Three towers carried chains supporting two main spans of 320'-0" and two side spans of 165'-0". Rather than calling his design a suspension bridge, Lindenthal referred to it as a "suspended arch bridge" because of the unusual web bracing he specified for placement between the top and bottom chains.[33]

Moreover, eye-bars and not wire comprised the chains, which hung as pairs with the bracing connecting the two cables. Eye-bars provided a stiffness lacking in wire suspension bridges. Lindenthal also designed the Smithfield Bridge which stands today in Pittsburgh, albeit with a more modest portal than first constructed. Historian Henry Petroski notes that the pair were among "the major structures erected in the 1880s in America." [34] When discussions about tearing down the trio of downtown bridges proceeded in the 1920s, journalists and residents of Pittsburgh noted the Seventh Street Bridge as a historic site because of Lindenthal's contribution. Despite being an older crossing, the much less attractive Ninth Street Bridge received much less attention.

Lindenthal's Seventh Street Bridge also gave warning about the anchoring problems that structures at the downtown sites faced. After construction had been completed, the north anchorage proceeded to slip. A repair shoring up the anchorage with additional masonry joined to the structure's first anchor chain resolved the problem, however. Hermann Laub, who had experience working with Lindenthal, completed the repairs. [35]


[31] "Life in Pittsburgh Over Fifty Years Ago," Pittsburgh First, 6 Dec. 1924, 56, in Pennsylvania Room, Carnegie Library, Pittsburgh, Pa. Everts' map in History of Allegheny County, 1876, uses "Irvine," instead of "Irwin."

[32] Pittsburgh First, "Life in Pittsburgh," 57.

[33] Plowden, Bridges, 238.

[34] Henry Petroski, Engineers of Dreams: Great Bridge Builders and the Spanning of America (New York: Alfred A. Knopf,1995),129.

[35] Plowden, Bridges, 238.

HAER No. PA-490
(Page 10)

In the mid-1890s, a city directory listed railways crossing the Sixth and Ninth street bridges but none over Lindenthal's structure. Railway consolidations brought the Sixth Street line under control of the Pittsburg, Allegheny & Manchester Traction Company. The Federal Street & Pleasant Valley Passenger Railway Company controlled the Ninth Street Bridge. Each firm ran two streetcar tracks on each of the structures, carrying passengers in both directions between Allegheny and Pittsburgh. [36] The added weight of the electric cars made alterations of the Sixth and Ninth Street bridges necessary.

The Second Ninth Street Bridge, 1890

The Pleasant Valley Electric Street Railway Company purchased controlling stock in the Pittsburg & Allegheny Bridge Company in 1889, allowing the railway to dictate a new Ninth Street Bridge design to meet its specifications. The profitability of electric railways encouraged the Pleasant Valley company to undertake the venture.[37] Engineers Ferris and Kaufman directed construction of a second bridge out of steel to make the crossing sufficiently strong for rapid transit vehicles. [38]

The railway provided funds to construct a four-track roadway with 5'-2-1/2" gauge rails. Three river spans, measuring 205'-0" each, were flanked by side spans of 152'-6", and an 80'-0" viaduct on the Allegheny approach. The work was started in 1889 and completed by the fall of 1890. Specifications called for a Pratt truss design built of rivet steel and wrought iron. The bridge company also specified stringers of either iron or steel, and the widest roadway possible on the existing piers, showing a concern with economizing wherever possible. [39] The railway company selected the Iron City Bridge Company's proposal as best meeting the specifications. The Iron City Bridge Company built bridges in the Pittsburgh area from 1876 to 1896, according to a directory of nineteenth-century bridge-building companies. The same directory indicated that engineers Ferris and Kaufman ran a joint operation named Ferris, Kaufman & Company in 1896; their association likely extended earlier to the Ninth Street Bridge replacement. [40]

The contractor immediately took off the roof and wall coverings of the bridge so that the railway's cars could pass. New wooden towers provided lateral support after overhead bracing was torn down. Even with a mild winter, the engineers noted, the Allegheny River's level fluctuated so much that other work was difficult. Contractors removed the sidewalks and erected the new bridge around the wooden one. The poor condition of the masonry made the work


[36] J. M. Kelly, Handbook of Greater Pittsburgh (Pittsburgh: J .M. Kelly,1895), 58.

[37] Kaufman, "Reconstruction of Ninth Street Bridge,"195.

[38] Wilkins, "Reconstruction of the Sixth Street Bridge," 144.

[39] Kaufman, "Reconstruction of Ninth Street Bridge," 200-01,215,196.

[40] Victor C Darnell, A Directory of American Bridge-Building Companies, 1840-1900, Occasional Publication No.4 (Washington, D.C.: Society for Industrial Archaeology,1984), 65,67.

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Last modified: 05-May-2003

HAER Text: Haven Hawley, August 1998; Pennsylvania Historic Bridges Recording Project - II
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