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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 31)

They are also the first cantilever-erected suspension bridges in the world. [129] The bridges vary only slightly from one another in measurements. The Sixth Street Bridge, more famous because of its historic site and selection by the American Institute of Steel Construction as "The Most Beautiful Steel Bridge of 1928," typifies the set of three-span steel eye-bar suspension structures. [130]

The bridges were built with a basically north-south orientation over the Allegheny River. From the intersection of Duquesne Way and the respective streets for which the bridges are named, they traverse the river to the north shore, where the street names change. Upon reaching the North Side, Sixth Street becomes Federal Street; Seventh, Sandusky Street; and Ninth, Anderson Street. [131] The Sixth Street Bridge has a total span of 995.1' from back wall to back wall, which is identical to its complement at Ninth Street and slightly shorter spans than the Seventh Street structure. From south to north, the Sixth Street Bridge includes an approach span 75.1' long; a side span, 215.0'; the main span, 430.0'; another side span, 215.0'; and an approach span, 60.0'. The Seventh Street Bridge, with its slightly longer total length of 1061.0', includes (again from south to north) an approach span of 72.80'; a side span, 221. 12'; the main span, 442.08'; another side span, 221.12'; and two approach spans, 41.95' and 61.45'. [132]

The south and north approaches rise at a 4.175 percent grade on all three bridges. The relatively steep grade reflects not only the shift from animal-powered to motorized vehicles but also the short approaches available for each structure. The camber of just more than 15'-0" forces the plate girders to perform their stiffening duties along a noticeable curve, serving as a 885'-0" long double struts that bear about 10 million pounds of compression. The structure itself acts as a weight to counter the potential for buckling. [133]

The stiffening plate girder rises approximately 3'-0" above the roadway. The girder serves not only as an unobtrusive compressive member but also as a safety barrier separating pedestrian and vehicular traffic. The roadways measure 38'-0" wide on the Sixth and Ninth Street bridges and 37'-6" on the Seventh Street Bridge. When designed, they carried four lanes of traffic, with two lanes for vehicles and two for street railways, with a track gauge of 5'-2-1/2" (and center-to-center measurement of 9'-6-1/2"). The clear sidewalk width at the time of construction was a constant 10'-3-1/2". [134]


[129] Plowden, Bridges, 238.

[130] American Institute of Steel Construction, award citation in File AL 02, ACDPW.

[131] Engineering News-Record, "Three New Bridges," 995.

[132] "6th St. - 7th St. - 9th St, Allegheny, Bridges No. 2, 3 & 4," n.d., in File AL 02, ACDPW; and "Sixth Street Bridge, No. 2 Allegheny River, Pittsburgh, Pa., Plan & Elevation," Drawing No. 5857 (11 Feb. 1927), in file AL 02, ACDPW.

[133] Roush, "Sixth, Seventh and Ninth Street," 196.

[134] R. C. Chaney, Engineer, Columbus, Ohio, City Planning Commission, to V. R. Covell, Chief Engineer, Allegheny County Department of Public Works, 12 Nov. 1929, in File AL 02, ACDPW.

HAER No. PA-490
(Page 32)

The towers measure 77-11-3/8" in height. [135] At the towers, the eye-bar chains bear up to six million pounds of vertical load. The Seventh Street Bridge has a chain sag of approximately 54'-4", with towers 83'-5 ' over the level of piers. The plate girders vertical stiffeners are riveted to stirrups, which connect to suspenders. Floor beams and stringers carry the concrete slab deck, and the floor beams and brackets for the cantilevered sidewalk are riveted to the stiffening girder. Engineers based live-load calculations on two 18-ton trucks of a pair of 60-ton streetcars for the roadway, and a 66-pound live load for the sidewalks, making a live load total of 6,590 pounds per lineal foot. In figuring the ratios for cable and stiffening girders, the live load results were calculated using a 6.9-percent impact factor. The unit stress of the eye-bars was specified as 27,000 pounds per square inch. [136]

The Pittsburgh Railways Company, which controlled most of the rail transportation throughout the city after a period of railway consolidation in the first decade of the twentieth century, faced the problem of rerouting a dozen railway lines while work on the bridges proceeded. [137] The county allocated $85,000 to pay for a portion of the $200,000 cost of realigning tracks and posting detours, which included making the Manchester Bridge and other local roads serve as alternate routes. The process delayed the Ninth Street Bridge work from February 1926 into the next month. [138]


The Sixth Street Bridge served as a model for design of the three structures, but the Foundation Company and the American Bridge Company first constructed a replacement for the Seventh Street Bridge, which was razed beginning in September 1924. Covell explained the first project at Seventh Street:

The structure may be briefly described as a self-anchored suspension bridge. The suspension system consists of 14-in. eyebars extending from anchorage to anchorage, having two pins on the top of each tower, and carrying the roadway by 4-in. eyebar suspenders at the panel points. The stiffening system consists of triple-web plate girders placed parallel to the grade. The horizontal component of the stress in the eyebar chain is taken by the stiffening girders, while the reactions


[135] V. R. Covell, "Erecting a Self-Anchored Suspension Bridge -- Seventh Street Bridge at Pittsburgh," Engineering News-Record 97 (1926): 502.

[136] Roush, "Sixth, Seventh and Ninth Street," 196.

[137] Bion J. Arnold, Report on the Pittsburgh Transportation Problem (Pittsburgh: City of Pittsburgh, 1910), 43

[138] "County Signs Pact To Pay $85,000 For Re-Routing Changes," Pittsburgh Post, 24 Jan. 1925, in Clippings File, Pittsburgh Bridges -- Ninth Street, Pennsylvania Room, Carnegie Library.

HAER No. PA-490
(Page 33)

at the ends are vertical. The girders are thus subjected to stresses due to bending combined with direct compression. [139]

Engineers rejected using a temporary anchorage for the chain during erection because the site lacked adequate anchorages, both in clearance and in access to rock. Falsework was considered but the War Department's order to keep the river navigable precluded that method. One of the several unique aspects of the Three Sisters Bridges became, out of necessity, the erection of a suspension bridge with cantilever methods. The American Bridge Company suggested the cantilever technique, keeping the main span over the river channel navigable at all times. [140]

Erection began in July 1925 with the driving of wood piles to place metal bents at points 1 through 9 on both sides of the main span (with twenty panel points denoting each half of the structure). The piles at points 4 and 7 were reinforced to protect against possible flooding during construction. Barges delivered eye-bars and other equipment when directed. A 100-ton crane traveled finished portions of the floor to aid in erecting the lower chord between points L0 and L10. Workers put the stiffening girders and chains into place, using jacks to produce the correct camber for the girders and bolting the splices. Using bases already constructed, workers erected the towers and affixed adjustable struts to the stiffening girders and tower bottoms. [141]

Eye-bars and hangers were set in permanent positions at points U0 to U3, but intermediate pins at U4 and U6 and the cradled sections around them (U3 to U5 and U5 to U7) were connected to a supporting I-beam, with struts attached at U5 and U7. The next section, between U7 and U10, received a similar treatment, with I-beam cradles at U7 and U10 and with slotted-hole plates attached to the I-beams cradling U8 and U9. Point U10 reached the tower. [142]

Stretching the massive chain to the connection point at U20 required setting the towers 12" closer than their final position. To allow this movement, the south pier contained a roller bearing and the north pier a rocker bearing. Workers left unattached the bottom hanger pins points L4 to L6, and L17 to L17*. (* indicating the north span). Point L0 had to be shifted 12" toward the south tower and segmental rollers at the tower adjusted to anticipate the movement, then locked. The northern tower and points did not have to be altered. Cantilever deflection and camber made the task of joining the chain at U20 more exacting. Five hundred-ton jacks connected to a strut at U5 alleviated secondary stresses caused by deflection, raised point L19 for the closure process, and allowed pins from L4 to L6 to be fastened. [143]


[139] Covell, "Erecting a Self-Anchored Suspension Bridge," 502.

[140] Covell, "Erecting a Self-Anchored Suspension Bridge," 502.

[141] Covell, "Erecting a Self-Anchored Suspension Bridge," 502-503.

[142] Covell, "Erecting a Self-Anchored Suspension Bridge," 503-504.

[143] Covell, "Erecting a Self-Anchored Suspension Bridge," 504.

HAER No. PA-490
(Page 34)

Commencing with the cantilever erection, workers installed diagonal struts in panels 10-11 and 15-16, continuing at panel 17 with a truss unattached to final bridge components to correct the chain's curve. Cantilevering the independent truss brought the splices past points L19 and L19* by 3'-8". The gap of 44'-2-1/2" between splices, less the 3'-8" measurement doubled from each span, then appeared 1'-0" away from the center girder's last placement, leaving only 35'-8-1/2" for the girder. Workers prepared a girder 2" shorter than that measurement (requiring a 14" addition later), allowing the girder to be attached to cantilever truss supports and the eye-bar chains to be joined. A quartet of 500-ton jacks per chord aided in controlling the member stress during the operation. The rollers shoes were then unlocked and pins at the exact mid-point driven, relying on the center girder and jacks to make the stresses on the chain negligible. Workers then drove the remaining unattached hanger pins near the center of the bridge and pins in lower chord points on either span. [144]

As workers modified the cantilever construction into a suspension system by jacking and shifting the south span into final position, the diagonal trusses fell away, leaving a strictly suspension form. With the insertion of the last 14" member and cover splices, workers completed riveting every point along the structure and took away jacks, leaving the bridge with virtually the same stresses anticipated by engineers before erection.

Workers closed the girder in February 1926 and prepared the bridge for roadway construction. The next month, the War Department gave the county an ultimatum to begin work on the Sixth Street Bridge by the end of the year, forcing the county to rush work in progress on the Seventh and Ninth Street bridges. The Foundation Company eased the situation by providing pedestrian access across the old Sixth Street Bridge while it was prepared for removal. Combined with the opening of the first bridge in July and the second in November 1926, this allowed commissioners to avoid having all three bridges closed at the same time. Business interests had opposed working on the Seventh and Ninth Street bridges at the same time because of the disruption it would cause to commercial interests -- a concern that certainly would have been expressed even more strongly if all structures were down at once. [145]

Coraopolis and the Old Sixth Street Bridge

With the construction of the Seventh Street Bridge finished in mid-1926 and the Ninth Street Bridge nearing completion, Allegheny County's attention turned toward the main commercial thoroughfare between the two halves of Pittsburgh: the Sixth Street Bridge. The Cooper spans remained in good shape, and concerns for economizing in public works projects led to recycling of the 1892 bridge.

Commissioner Armstrong took credit for proposing to reuse the Cooper bridge instead of building an entirely new structure in nearby Coraopolis, saving Allegheny County $350,000.


[144] Covell, "Erecting a Self-Anchored Suspension Bridge," 504-05.

[145] "6th Street Bridge Must be Razed by December 31," Pittsburgh Gazette-Times, 10 Mar.1926, in Clippings Files, Pittsburgh Bridges -- Sixth Street, Pennsylvania Room, Carnegie Library, Pittsburgh' Pa.

HAER No. PA-490
(Page 35)

The Foundation Company bid on a contract to move the Sixth Street Bridge from its site in downtown Pittsburgh to Coraopolis, twelve miles away. Winning the contract at a bid of $316,200, the company also assumed any risk that the spans would sink or be damaged during the project. [146]

The company allowed pedestrians to cross the structure while roadway removal proceeded in the fall of 1926 in order to reduce the inconvenience for residents. Workers took off half of the roadway at a time, leaving pedestrian areas accessible until final segments were taken away. [147]

The bowstring trusses, in addition to weighing 1,600 tons each, presented the difficult problem of being slightly too tall to fit under a bridge along the journey. Because of the stresses inherent in a bowstring truss, the Foundation Company could not merely disassemble them; instead, the firm had to transport the entire 450'-0" spans, which were 44'-0" wide and 80'-0" high. [148] The Cooper bridge consisted of sixteen eye-bar panels pinned together. Removing one part would break the structure's rigidity and make moving it very difficult. [149]

Instead of shifting the spans off piers for lowering or pivoting them from their present support, the Foundation Company lowered the structure in position, taking off the masonry and using substitute supports for resting the structure without getting in the way of the process. The contractor attached a frame to each of the piers and abutments, used straps to bind the trusses to each frame, and lowered them using the straps. With twenty-six 7" holes punched in the strap, the company used a matching chain to counter the eight straps. Using pins to move the strap by hole sets, the company brought the spans downward 15" at a time with jacks. The pins attached to the plungers of eight 500-ton jacks. The jacks remained in place while the pins moved 15". The water-cylinder jacks were also 15" high, capable of exerting 3200 psi after pumping. By bleeding water out of the cylinders of the jacks all at once, workers used the four jacks on each side of the bridge to lower the spans on alternate sides to the full depth of 16'-0". [150]

Workers made a pontoon out of two pairs of barges, spaced to create a platform 400'-0" long and 52'-0" wide, which carried the bridge with 20'-0" of overhang. Stringers supported the bridge in forty-two places, with a 40-ton screw jack at each stringer for easier loading and unloading. When workers reached the Manchester Bridge, they had to adjust the bridge to fit the


[146] "Continuous Service of Structure, Since 1892 Ended; Will Be Used Down River," Pittsburgh Sun, 3 Jan. 1927, in clippings File, Pittsburgh Bridges -- Sixth Street, Pennsylvania Room, Carnegie Library, Pittsburgh' Pa.

[147] "Ninth St. Bridge Formally Opened," Chronicle Telegraph, 26 Nov. 1926, in Clippings File, Pittsburgh Bridges -- Ninth Street, Pennsylvania Room, Carnegie Library, Pittsburgh, Pa.

[148] "Floating Intact to Coraopolis Pittsburgh's Old Sixth Street Bridge Spans," press release from Norman F. Brown's office, n.d., in File AL 02, ACDPW For a published account and published photos of the process and the jacks used, see D. T. Jerman, "Moving the 440-Ft. Truss Spans of Sixth St. Bridge, Pittsburgh," Engineering News-Record 98 (1927): 850-51. Jerman notes that the transported spans were too high for the Ohio Connecting Railroad bridge over the Ohio River as well as the Manchester Bridge over the Allegheny.

[149] ACDPW, "Floating Intact."

[150] ACDPW, "Floating Intact."

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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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