Three Sisters (Pittsburgh)

Last updated
The Three Sisters
HAER PBG 6thStreet 361497cv.jpg
View of Sixth, Seventh and Ninth Street Bridges from downriver
Coordinates 40°26′46″N80°00′07″W / 40.44611°N 80.00194°W / 40.44611; -80.00194 Coordinates: 40°26′46″N80°00′07″W / 40.44611°N 80.00194°W / 40.44611; -80.00194
Carries2 Vehicular lanes, 2 sidewalks
Crosses Allegheny River
Locale Pittsburgh, Pennsylvania
Maintained by Allegheny County
Design Self-anchored suspension bridge
Designated1988 [1]
Sixth Street Bridge
HAER PBG 6thStreet 361482pv.jpg
From north bank of the Allegheny, downtown Pittsburgh in background
Coordinates 40°26′43″N80°00′12″W / 40.44528°N 80.00333°W / 40.44528; -80.00333
CarriesSixth Street
Official name Roberto Clemente Bridge
Total length884 feet (269 m) in main plus two 215 feet (66 m) side spans; 995 feet (303 m) with approaches
Width38 feet (12 m) roadway (formerly 2 vehicle, 2 tramway tracks, now 2 wide vehicle lanes) with 10 feet (3.0 m) sidewalks outside the plate girders
Longest span430 feet (130 m)
Clearance above 78 feet (24 m) towers
Clearance below deck is 40 feet (12 m) above Emsworth Dam normal pool level or 710 feet (220 m) above sea level
OpenedOctober 19, 1928

The Three Sisters are three very similar self-anchored suspension bridges spanning the Allegheny River in downtown Pittsburgh, Pennsylvania at 6th, 7th, and 9th streets, generally running north/south. The bridges have been given formal names to honor important Pittsburgh residents:


Designed by the Allegheny County Department of Public Works, they were all built in a four-year period, from 1924 to 1928, by the American Bridge Company, replacing earlier bridges of various designs at the same sites. Their construction was mandated by the War Department, citing navigable river clearance concerns. They are constructed of steel, and use steel eyebars in lieu of cables.

The Three Sisters are historically significant because they are the only trio of nearly identical bridges, as well as the first self-anchored suspension spans, built in the United States. They are among the only surviving examples of large eyebar chain suspension bridges in America, and furthermore, unusual for their self-anchoring designs. The bridges’ design was viewed as a creative response to the political, commercial, and aesthetic concerns of Pittsburgh in the 1920s.

The bridges were designed under the auspices of the Allegheny County Department of Public Works, by T. J. Wilkerson, consulting engineer; Vernon R. Covell, chief engineer; A. D. Nutter, design engineer; and Stanley L. Roush, architect. The American Bridge Company built the superstructure; while the Foundation Company built the substructure. All three bridges are owned by Allegheny County.


From 1926 through 1931, four suspension bridges were constructed across the Allegheny and Monongahela Rivers. The fourth bridge (not covered in this article) is a wire cable bridge carrying South Tenth Street across the Monongahela.

The "Three Sisters Bridges" represent an adaptive engineering design response to political and technical concerns. County engineers successfully overcame challenges presented by federally mandated clearances, aesthetic and financial considerations raised by local agencies, and the lack of adequate anchorage points along the river banks. They were the first self-anchored suspension bridges built in the United States. The design’s solid plate deck-stiffening girder provided compressive support while lowering visual barriers between Pittsburgh and the historically distinct North side (formerly Allegheny City), annexed in 1907 in a contentious fight.

The American Bridge Company, the builder and steel supplier for these three bridges, was headquartered nearby, as was the Foundation Company which had the pier/abutment masonry contract. Local election campaigns during the period highlighted the intent to use local suppliers and labor.

Engineering background

A portion of the Erection Sequence and Force Diagram for the bridges, produced by the Historic American Engineering Record in the 1990s. The full diagram includes the following text: Haer PBG erection force Diagram part.png
A portion of the Erection Sequence and Force Diagram for the bridges, produced by the Historic American Engineering Record in the 1990s. The full diagram includes the following text:
Seventh Street Bridge
HAER PBG 7thStreet 361498pv.jpg
From north bank of the Allegheny, looking SW, downtown Pittsburgh in background, Sixth Street Bridge at right. Shows main plate girder (bearing compressive forces) and sidewalk support
Coordinates 40°26′45″N80°00′05″W / 40.44583°N 80.00139°W / 40.44583; -80.00139
CarriesSeventh Street
Official name Andy Warhol Bridge
Total length884 feet (269 m) in main plus two 221 feet (67 m) side spans; 1,061 feet (323 m) with approaches
Width38 ft roadway (formerly 2 vehicle, 2 tramway tracks, now 2 wide vehicle lanes) with 10 feet (3.0 m) sidewalks outside the plate girders; total 62 feet (19 m)
Longest span442 feet (135 m)
Clearance above 83.5 feet (25.5 m) towers
Clearance below deck is 40.1 feet (12.2 m) above Emsworth Dam normal pool level or 710 feet (220 m) above sea level
OpenedJune 17, 1926

A suspension bridge works by hanging a roadway from cables or chains under tension. Though a few unstiffened suspension bridges exist, a longitudinal stiffening truss or girder is usually added to prevent excessive movement of the deck. The cables pass over towers and are anchored at both ends. Conventional suspension bridges use massive concrete or rock anchorages to resist the cable’s tension. In self-anchored suspension bridges, however, the cables are fastened to both ends of the longitudinal girders. These girders are therefore compression struts in addition to stiffening the roadway.

Because each of Pittsburgh's Three Sisters appears to be a self-contained unit not dependent on the river banks for anchorage, a debate ensued among engineers whether these structures were cantilevers rather than suspension bridges. Current wisdom holds them to be the latter. Each bridge was built in halves toward the center, with temporary diagonal struts placed between chain and deck to provide shear resistance. These struts temporarily turned each incomplete half into a trussed cantilever arm. The struts 'freed themselves' when the tension on them was removed by jacking each halve and connecting them together to form a suspension bridge.

Ninth Street Bridge
HAER PBG 9thStreet 361504pv.jpg
From south bank of the Allegheny, looking NE, oblique view of roadway and south tower, showing eyebar links for main suspenders and roadway suspenders, as well as solid plate main compressive stiffening girders dividing roadway from sidewalks
Coordinates 40°26′47″N79°59′59″W / 40.44639°N 79.99972°W / 40.44639; -79.99972
CarriesNinth Street
Official name Rachel Carson Bridge
Total length840 feet (260 m) in main plus two 215 feet (66 m) side spans; 995 feet (303 m) with approaches
Width38 feet (12 m) roadway (formerly 2 vehicle, 2 tramway tracks, now 2 wide vehicle lanes) with 10 feet (3.0 m) sidewalks (outside the plate girders; total 62 feet (19 m)
Longest span410 feet (120 m)
Clearance above 78 feet (24 m) towers
Clearance below deck is 40.3 feet (12.3 m) above Emsworth Dam normal pool level or 710 feet (220 m) above sea level
Opened26 November 1926

Political background

A driving force for the three bridges all being constructed at once was that the predecessor bridges were all out of compliance with War Department standards for minimum width and clearance over channels for bridges crossing navigable waters such as the Ohio, the Allegheny, and the Monongahela. [2]

The previous bridges were all owned by private companies established to build bridges and pay them off through toll collection. The bridges were quite profitable, with several of the companies in the area paying 15% dividends yearly to stock holders, even with tolls at 1 cent per man and free for women.

Some controversy ensued over whether public entities could take bridges or bridge rights of way away from private companies if the companies had already retired their construction loans.

At some point, the public supported a "free bridges" movement that supported government buying out the bridge companies and abolishing tolls. Between that and the War department ruling that government-owned bridges would not need to be corrected as quickly, the bridge companies were willing to sell and did so before 1910. The County (rather than City or State) bought the companies out at fair market value.

Thus the county had to deal with the problem of non compliance when the war department raised it again at the end of the 1910s. Through the teens and early 20s bond issues were floated before the voters to finance replacement but repeatedly failed, in part because voters of the time saw public works as a source of corruption, especially at the county level.

In 1924, voters finally approved a $29.2 million bond issue at the county level to improve bridges and municipal structures. Several proposals were made for lift bridges of various sorts, including a scheme to raise an existing bridge on mechanical jacks, but the War Department did not approve, insisting on a failure proof means of clearance. The Seventh Street Bridge was razed in 1924, and the War Department forced the razing of the 9th as well, despite the inconvenience to the city.

After much design work, two truss bridges at 6th and 9th and a cantilever bridge at 7th were approved, and submitted to the Metropolitan Art Commission, a forgotten body that had approval rights for any bridge over $25,000 in the City of Pittsburgh. Expecting a rubber stamp, contracts were let, but to everyone's shock the commission vetoed the designs as unaesthetic, preferring suspension bridges. But shoreline clearances were tight and two of the older suspension bridges had experienced problems with insecure anchorages. It is not clear who suggested a self-anchoring suspension bridge design, as the only precedent known at that time was a 1915 bridge over the Rhine at Cologne the Deutzer Hängebrücke.

However, by 1922 one of the better known advisors on cantilever and suspension bridge structures noted the peculiar strengths represented in the Cologne bridge’s design. David B. Steinman’s running dispute with J. A. L. Waddell about the relative costs of suspension and cantilever bridges continued a debate with roots in the Quebec Bridge collapse. The 1907 construction disaster, accompanied by the 1916 accident during a second attempt, heightened a growing preference for suspension bridges. In light of the popular and engineering support for suspension structures, Steinman’s first edition of A Practical Treatise on Suspension Bridges fed a growing demand for technical information about components such as eye-bars and stiffening systems. Steinman’s reputation and the fact that his book was reviewed in the engineering press make it likely that Allegheny County’s engineers could have examined the first edition of this book.[ citation needed ]

See also

Related Research Articles

Suspension bridge type of bridge

A suspension bridge is a type of bridge in which the deck is hung below suspension cables on vertical suspenders. The first modern examples of this type of bridge were built in the early 1800s. Simple suspension bridges, which lack vertical suspenders, have a long history in many mountainous parts of the world.

John A. Roebling German-American engineer

John Augustus Roebling was a German-born American civil engineer. He designed and built wire rope suspension bridges, in particular the Brooklyn Bridge, which has been designated as a National Historic Landmark and a National Historic Civil Engineering Landmark.

Self-anchored suspension bridge suspension bridge in which the main cables attach to the ends of the deck

A self-anchored suspension bridge is a suspension bridge in which the main cables attach to the ends of the deck, rather than to the ground via large anchorages. The design is well-suited for construction atop elevated piers, or in areas of unstable soils where anchorages would be difficult to construct.

David B. Steinman American civil engineer

David Barnard Steinman was an American civil engineer. He was the designer of the Mackinac Bridge and many other notable bridges, and a published author. He grew up in New York City's lower Manhattan, and lived with the ambition of making his mark on the Brooklyn Bridge that he lived under. In 1906 he earned a bachelor's degree from City College and in 1909, a Master of Arts from Columbia University and a Doctorate in 1911. He also received an honorary Doctor of Science in Engineering on 15 April 1952 from degree mill Sequoia University, but would distance himself from it after the 1957 inquiry and not include it in his biographies. He was awarded the Franklin Institute's Louis E. Levy Medal in 1957.

Eyebar straight bar, usually of metal, with a hole ("eye") at each end for fixing to other components

In structural engineering and construction, an eyebar is a straight bar, usually of metal, with a hole ("eye") at each end for fixing to other components. Eyebars are used in structures such as bridges, in settings in which only tension, and never compression, is applied. Also referred to as "pin - and eyebar construction" in instances where pins are being used.

Roberto Clemente Bridge Bridge in Pittsburgh, Pennsylvania, United States

The Roberto Clemente Bridge, also known as the Sixth Street Bridge, spans the Allegheny River in downtown Pittsburgh, Pennsylvania, United States.

Waldo–Hancock Bridge bridge in Bucksport, Maine

The Waldo–Hancock Bridge was the first long-span suspension bridge erected in Maine, as well as the first permanent bridge across the Penobscot River below Bangor. The name comes from connecting Waldo and Hancock counties. The bridge was built in 1931 and retired in 2006, when the new Penobscot Narrows Bridge was opened just a few yards away, and it was demolished in 2013.

Hutsonville Bridge

The Hutsonville Bridge or Sullivan-Hutsonville Bridge connecting Crawford County, Illinois, and Sullivan County, Indiana, over the Wabash River, built 1939 and replaced in 1988, was an example of the relatively rare self-anchored suspension bridge type. It was designed by Robinson & Steinman, with R. V. Milbank as the resident chief engineer, and constructed by Wisconsin Bridge and Iron Company as general contractor and Vincennes Steel Corporation as steel fabricators and Charles J. Glasgow as a subcontractor.

Deutz Suspension Bridge

The Deutz Suspension Bridge was a self-anchored suspension bridge using eyebar chains, across the Rhine at Deutz in Cologne, Germany. It was built from 1913 to 1915. In 1935, it was named Hindenburg Bridge after Germany's second President deceased the previous year. It collapsed on 28 February 1945 during repair works and was replaced in 1948 by the world's first steel box girder bridge designed by Fritz Leonhardt and Gerd Lohmer. H. D. Robinson, who later worked with David B. Steinman on the Florianopolis Bridge, another eyebar chain bridge, consulted on the towers for the design of this Cologne bridge. It reportedly later served as inspiration for American bridge engineers and was specifically cited as a design influence on the Three Sisters bridges in Pittsburgh, Pennsylvania as well as for the Kiyosu Bridge on the Sumida River in Tokyo.

Coraopolis Bridge bridge over Ohio River back channel at Neville Island, Pennsylvania

The Coraopolis Bridge is a girder bridge over the back channel of the Ohio River connecting Grand Avenue on Neville Island to Ferree Street in Coraopolis, Pennsylvania. It opened in 1995 to replace a structure of historic significance. The original Pratt/Bowstring/Pennsylvania through truss spans, designed by Theodore Cooper, were formerly the (third) Sixth Street Bridge, spanning the Allegheny River, in downtown Pittsburgh, and were built in 1892 by the Union Bridge Company. They were floated downstream by the Foundation Company in 1927 rather than being demolished when the bridge was removed to enable construction of the present (fourth) Three Sisters (Pittsburgh) Sixth Street Self-anchored suspension bridge. However, by the late 1980s, the old bridge could no longer support traffic volumes and was replaced by a newer structure.

Liberty Bridge (Pittsburgh) bridge over the Monongahela River in Pittsburgh, Pennsylvania

The Liberty Bridge, completed in 1928, connects downtown Pittsburgh, Pennsylvania, to the Liberty Tunnels and the South Hills neighborhoods beyond. It crosses the Monongahela River and intersects Interstate 579 at its northern terminus.

South Tenth Street Bridge bridge in Pittsburgh, Pennsylvania

South Tenth Street Bridge, most often called the Tenth Street Bridge, but officially dubbed the Philip Murray Bridge, is a suspension bridge spanning the Monongahela River in Downtown Pittsburgh, Pennsylvania. It is the longest bridge spanning the Monongahela River and serves as the only cable suspension bridge across any of Pittsburgh’s three rivers. The bridge was renamed on Labor Day 2007 for Philip Murray, the first president of the United Steelworkers of America.

Tied-arch bridge type of bridge

A tied-arch bridge is an arch bridge in which the outward-directed horizontal forces of the arch(es) are borne as tension by a chord tying the arch ends, rather than by the ground or the bridge foundations. This strengthened chord may be the deck structure itself or consist of separate, deck-independent tie-rods.

Rachel Carson Bridge bridge over the Allegheny River in Pittsburgh

Rachel Carson Bridge, also known as the Ninth Street Bridge, spans the Allegheny River in Downtown Pittsburgh, Pennsylvania in the United States. The total length is 840 feet (260 m) including the 410-foot (120 m) main span and two 215-foot (66 m) side spans, or 995 feet (303 m) including the approaches. The total width of the deck is 62 feet (19 m), including the 38-foot (12 m) roadway plus two 10-foot (3.0 m) sidewalks outside the compressive plate girder. Whereas the roadway formerly carried two vehicle lanes and two streetcar tracks, it now carries four wide vehicle lanes.

Andy Warhol Bridge bridge over the Allegheny River in Pittsburgh

Andy Warhol Bridge, also known as the Seventh Street Bridge, spans the Allegheny River in Downtown Pittsburgh, Pennsylvania and is the only bridge in the United States named for a visual artist. It was opened at a cost of $1.5 million on June 17, 1926 in a ceremony attended by 2,000.

Tacoma Narrows Bridge (1950)

The 1950 Tacoma Narrows Bridge is a suspension bridge in the U.S. state of Washington that carries the westbound lanes of Washington State Route 16 across the Tacoma Narrows strait, between the city of Tacoma and the Kitsap Peninsula. Opened on October 14, 1950, it was built in the same location as the original Tacoma Narrows Bridge, which collapsed due to a windstorm on November 7, 1940. It is the older of the twin bridges that make up the Tacoma Narrows Bridge crossing of the Tacoma Narrows, and carried both directions of traffic across the strait until 2007. At the time of its construction, the bridge was, like its predecessor, the third-longest suspension bridge in the world in terms of main span length, behind the Golden Gate Bridge and George Washington Bridge; it is now the 46th longest suspension bridge in the world.

Vernon R. Covell was an American engineer. He was chief engineer of the Allegheny County Public Works Department.

Market Street Bridge (Ohio River)

The Market Street Bridge is a suspension bridge connecting Market Street in Steubenville, Ohio and West Virginia Route 2 in Follansbee, West Virginia over the Ohio River. As a project of the Steubenville Bridge Company, it was constructed in 1905 by the Ohio Steel Erection Company, the framework was created by the Penn Bridge Company, and the original steel was done by Jones and Laughlin Steel Company and Bethlehem Steel. The bridge spans a length of 1,794 feet (547 m) with a width of 20.7 feet (6.3 m). As of 2002, the average daily traffic was estimated around 15,000 vehicles. It was listed on the National Register of Historic Places in 2019.

Sepung Bridge bridge in South Korea

The Sepung Bridge is the first curved, cable-stayed bridge in South Korea. It crosses the Gwangyang western river, a railway and a roadway in South Korea. It was built by Iksan Regional Construction & Management Administration.


  1. Historic Landmark Plaques 1968-2009 (PDF). Pittsburgh, PA: Pittsburgh History & Landmarks Foundation. 2010. Retrieved 2010-07-02.
  2. Haer record, page 19
Roberto Clemente Bridge (Sixth Street Bridge)
Andy Warhol Bridge (Seventh Street Bridge)
Rachel Carson Bridge (Ninth Street Bridge)