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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 11)

difficult, because old grout had to be removed and new mortar injected to keep piers sturdy. Workers took apart the skewbacks, constructed new tops on the piers, and used falsework to construct the new trusses outside of the existing arches, span by span. [41]

Kaufman reported that navigation concerns arose during the bridge's construction. The engineer was aware of federal jurisdiction over navigable waterways such as the Allegheny. With increasing discussion about a future height requirement on American rivers, Kaufman's firm tried to anticipate higher clearances that might affect the bridge owners. Midway through construction, the company decided to place 12"-high granite blocks under the span shoes in the channel and 7"-high blocks on adjacent piers, providing additional clearance. [42].

The new design included a number of improvements for rail traffic. On the Pittsburgh approach, the contractor moved the abutment 40'-0", decreasing the length of the span adjacent to Duquesne Way (present-day Fort Duquesne Boulevard). This modification made turning from the bridge onto the street easier. On the bridge itself the designers increased the vertical clearance to 20'-0" and boosted the number of stringers from eight to twelve. The roadway was designed to carry an engine somewhat heavier than that used by the Pleasant Valley Railway. Kaufman's firm used an Aveling & Porter engine weighing 15 tons for calculations because the railway's cars weighed close to 13 tons when full. Changes to the Allegheny City abutment benefitted the Pittsburg & Western Railroad operating on the river bank below. The rebuilt approach included five 16'-0" stone arch spans each accommodating one track, with an additional 1'-0" of vertical clearance. The passage was made expandable in case Anderson Street was moved and more space for railroad operations became available. [43]

The Third Sixth Street Bridge, 1892

Steel trusses provided the strength necessary for the greater load of electric streetcars at the Sixth Street crossing as well. The main route between the two cities, the Sixth Street Bridge constructed by Roebling, proved unequal to the task of carrying greater passenger and vehicular loads. Streetcars' speeds were restricted on the two tracks of the roadway, and the number of light vehicles continued to increase. Pittsburgh needed "a structure capable of carrying the heavy loads customary in an iron city," reported the Railroad Gazette. [44] The design of those trusses became a matter of public interest because of the longevity of the two previous bridges at the site.


[41] Kaufman, "Reconstruction of Ninth Street Bridge," 202-03, 205-11

[42] Kaufman, "Reconstruction of Ninth Street Bridge," 213.

[43] Kaufman, "Reconstruction of Ninth Street Bridge," 196-201 According to a story related by an engineer who helped take down the wooden bridge, a violin maker asked to procure wood from the first Hand Street Bridge, speculating that the continual vibration would condition the wood as if it had been played already. He formed three violins from a wood chosen from the disassembled wooden structure and was very satisfied with the outcome of his handiwork (ibid., 218).

[44] Railroad Gazette, "New Sixth Street Bridge," 560.

HAER No. PA-490
(Page 12)

The Allegheny Bridge Company deemed the project important enough to announce a design competition, selecting aesthetics and strength as two critical features.[45]

The Lohse system of inverted arches, which had been featured recently in a German bridge at Hamburg, served as the model for the specifications. C. L. Strobel submitted three designs closely following the announced guidelines, and the company selected one that featured a three-hinged inverted arch in the lower chords. The project proceeded far enough for Strobel to submit and have published in a major engineering journal the designs he had prepared for the structure. Before work began, however, the bridge company changed its mind about the design to be used. [46]

An entirely different design by Theodore Cooper, who had been named the bridge company's engineer, received final acceptance. His simpler design featured a pair of pin connected 440'-0"-long bowstring trusses erected around the old bridge, which was then demolished upon completion in 1892. The Drake & Stratton Company, Limited, received a contract for foundations and masonry; the Union Bridge Company for the superstructure; and Baird Brothers of Pittsburgh for erection. All work cost $560,000. The contract included a 105'-0" approach on the Pittsburgh side with a pair of 28'-0" archways; the two main spans; a 48'-6" approach span on the northern side traversing the Pittsburgh & Western Railroad right-of way; and new coping for the old Allegheny approach. The trusses were spaced 44'-6" apart, accommodating a roadway 40'-0" wide from curb to curb, and stood 79'-0" high at mid-span. Sidewalks measured 10'-0" wide on the approaches and 9'-0" between posts. The design moved the center line on the Pittsburgh shore downstream approximately 6'-0" for better alignment with Duquesne Way. [47]

Engineer W. G. Wilkins discussed foundation work for the third Sixth Street Bridge at a meeting of the Engineers' Society of Western Pennsylvania. Abutments were completed in halves in order to maintain cable strain in the old bridge. The new piers incorporated rock-faced Beaver County sandstone, with coping and cornice stone from Berea, Ohio. Wilkins noted one of the original piers was found to have been widened and reinforced with two rows of piles extending the pier 3'-0" over the piles while the original remained on timber footings. Also, the third pier had a location identical to that of the first St. Clair Bridge abutment. Contractors discovered "two courses of twelve inch timbers with the sap sides riveted together with oak pins" from the 1819 bridge, which provided material for blocking up the old floor. [48]

With bedrock about 60'-0" under river bottom in the area where the Monongahela and Allegheny rivers meet, only one bridge in the vicinity had foundations on rock, most opting


[45] C L. Strobel, "A New Truss Design And Its Analysis," Engineering News 38 (1897): 51

[46] Wilkins, "Reconstruction of the Sixth Street Bridge," 149.

[47] Wilkins, "Reconstruction of the Sixth Street Bridge," 150-51, 161-62; Railroad Gazette, "New Sixth Street Bridge," 560.

[48] Wilkins, "Reconstruction of the Sixth Street Bridge," 152-57.

HAER No. PA-490
(Page 13)

instead for pier foundations at whatever depths were deemed appropriate to the bridge structures above. All of the piers for the new Sixth Street Bridge, except for the second one, were constructed in open cuts protected by sheet piling. For the second pier, the contractor first set a breakwater in place 80'-0" upstream to shield the new pier site. The next step was to build an open coffer-dam on a wooden grillage, sinking the grillage to gravel without piling. This avoided the former bridge's problem of having timber footings out of water during low river flow and allowed the footings to be set on gravel with enough bearing capacity to avoid unnecessary excavation. With the new pier 7'-7" lower and offset 10'-6", the old pier could be maintained in place for temporary support. During dredging, additional sheet piling had to be driven to stop erosion under the old pier. As pier masonry was added, the cofferdam settled out of plumb, requiring that each successive course be adjusted to provide a level coping. The entire process, from breakwater to coping, took four months and finished in November. As a final step, workers dredged the river after the old piers had been removed. [49]

Workers began constructing the new bridge's floor after raising the old floor to accommodate the procedure. Temporary piles were topped with timber bents to allow the old floor to be jacked, blocked, and supported by the stringers. Other bents carried traveling crane tracks on either side of the structure. Bridge company directors decided to halt horse traffic across the bridge during construction because of liability for accidents if the animals became frightened, but pedestrian and non-animal vehicles were able to use the structure. To allow continued streetcar traffic, the tracks were repositioned on three occasions by installing buckle plates and stringers. [50] The Union Bridge Company prepared a cable conduit for streetcars as requested by the transit company, but the firm ultimately decided to continue using overhead trolley wires. [51]

Greater Pittsburgh, Navigation, and Free Bridges

Pittsburgh continued to build bridges to the North Side in the 1880s and 1890s, and the physical connections paralleled administrative efforts to create a Greater Pittsburgh. The vision of a metropolitan area united under one city government reflected consolidation and annexation movements beginning in the mid-nineteenth century, but the Pittsburgh Chamber of Commerce dated Greater Pittsburgh to 1894, when state legislators approved Pittsburgh's proposed annexation of Allegheny City. Allegheny City fought the proposal, forcing Greater Pittsburgh proponents to endure numerous state-level committee meetings, to power three bills through the

[49] Wilkins, "Reconstruction of the Sixth Street Bridge," 152-57.

[50] Wilkins, "Reconstruction of the Sixth Street Bridge," 151-52.

[51] Railroad Gazette, "New Sixth Street Bridge," 560.

HAER No. PA-490
(Page 14)

legislature, and to procure judicial approval for the annexation procedures. Pittsburgh's Chamber of Commerce claimed responsibility for the proposal's success. [52]

A committee of the Chamber of Commerce requested select councils of each city to meet to discuss the issue, seding the issue in motion. After numerous delays, including revising state law to lower the number of petitioners required to initiate annexation of an adjacent municipality from one-fifth to one-twentieth of the population, the state legislature passed a bill allowing the process to begin. After the last hurdle of approval by the State Supreme Court allowed the vote to proceed, citizens in the two cities at last voted in 1906 for consolidation. [53] Pittsburgh had maneuvered for legislative approval that required a majority of the combined vote of the municipalities instead of a clear majority of the area to be annexed, and the measure passed easily. A pithy history of Allegheny City noted the "end of the wrangling that started back in 1846. Pittsburgh, the larger city, annexed itself to Allegheny by outvoting the smaller city, and retained its own name." [54]

The Chamber of Commerce also promoted federal improvements to waterways in the area. Just ten days after its chartering in 1876, the Chamber lobbied Allegheny County to encourage congressional action on a bill to dam the Ohio River and improve harborage for the city. By 1880 the organization was calling for Congress to implement "radical improvements" along the entire Allegheny River. Members consistently opposed giving bridge companies rights-of-way or concessions to make profits from control over riverfront access. The Chamber sent representatives to congressional committee meetings that focused on navigation issues in the mid-1890s, and in the following decade began actively participating in regional and national river and harbor conferences. [55] Congress authorized new surveys of waterways across the nation in the 1913 River and Harbor Act, and the Chamber continued to press for more attention to area navigation, hoping to link Pittsburgh to a national waterway integrating the Ohio River and the Lake Erie Canal into an system encompassing the Mississippi Valley and eastern United States. [56]

An earlier River and Harbor Act, passed by Congress on 3 March 1899, required the Secretary of War to declare highway or railroad bridges over navigable bodies of water in violation if they posed "an unreasonable obstruction to the free navigation of such waters on account of insufficient height, width of span, or otherwise." In the course of investigations to


[52] "The Chamber and Greater Pittsburgh," Pittsburgh First, 6 Dec. 1924, 49.

[53] Pittsburgh First, "The Chamber and Greater Pittsburgh,''49.

[54] Briggs, Chronological History, entry under 1907.

[55] "The Chamber and Ohio, Monongahela and Allegheny River Improvements," Pittsburgh First, 6 Dec. 1924, 38-39.

[56] Pittsburgh First, "The Chamber and ... River Improvements," 39.

HAER No. PA-490
(Page 15)

determine whether a structure posed such a blockage, the secretary's task entailed setting deadlines and clearances for modifications. [57}

Although river interests had agitated for higher clearances long before the Secretary of War became involved, the War Department first held hearings about Allegheny obstructions in 1903 after a local district engineer, Capt. William L. Sibert, reported that Pittsburgh bridges were not in compliance with the 1899 act. Secretary of War Elihu Root ruled in 1904 after lengthy investigations that the structures posed no unreasonable problem for shipping as it existed at that time. Four years later his successor, William Howard Taft, upheld the decision because he was unwilling to overturn the decision without new evidence, although he conceded that he favored a 47'-0" clearance for all new cases he would be called upon to judge. [58]

The matter resurfaced in 1909, resulting in a recommendation by the Bixby Board, a War Department committee, generally to follow Sibert's advice. Timing proved crucial in the outcome. When a new Secretary of War, J. M. Dickinson, finally acted on the recommendation, he rejected the petition instead of approving it. The ruling that Allegheny bridges would not have to be razed to meet height requirements came on 6 March 1911, and influenced Allegheny County's decision to purchase the bridges from their private owners. Dickinson, like Taft, refused to overrule a predecessor without material alteration in the circumstances of the case. [59]

Yet again proponents of increased clearances sought a positive ruling on their petition in 1915 with acting secretary Lindley M. Garrison. Garrison agreed to review the case for new information, and Lt. Col. Shunk, serving in the same capacity as Capt. Sibert as district engineer, provided a detailed report. Like Sibert, Shunk advised that the bridges obstructed river traffic After holding hearings, visiting the site, and digesting immense amounts of information in the case, Secretary of War Newton D. Baker agreed with the district engineer. [60]

Shunk reported that the existing clearances of the Sixth, Seventh, and Ninth Street bridges were respectively 33.4', 35.0', and 33.6' above the Davis Island pool (elevation 703.0'). He recommended additional clearances of 8.1' to 9.7' in 1916. Baker revised the order for even greater heights and set firm deadlines, requiring 47.0' in the clear over the Davis Island pool for the Sixth Street Bridge, 47.1' for the Seventh Street Bridge and47.3' for Ninth Street Bridge. Work on the latter two bridges was to begin by 28 September 1918, and on the Sixth Street Bridge in 1919. The deadline for compliance was two years after commencement of work. [61]


[57] U.S. War Department, Opinion of the Secretary of War in re Elevation of Bridges over the Allegheny River at Pittsburgh (Washington, D.C.: U.S. Government Printing Office,1917),3.

[58] War Department, Opinion, 3-4; and V. R Covell,"The Bridge-Raising Program on the Allegheny River in Allegheny County," in Proceedings of the Engineers' Society of Western Pennsylvania 41 (1925): 83.

[59] War Department, Opinion, 4.

[60] War Department, Opinion, 5.

[61] Covell, "Bridge-Raising Program," 84.

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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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