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Three Sisters (Pittsburgh)
Encyclopedia
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 seif-contained unit not dependent on the river banks for anchorage, a debate ensued among engineers whether these structures were cantilevers rather than suspension bridges. While current wisdom holds them to be the latter, the erection procedure shows how each bridge was built in halves toward the center. Temporary diagonal struts between chain and deck provided shear resistance turning each incomplete half into a trussed cantilever arm. These struts 'freed themselves' when the halves were jacked together and connected to form a suspension bridge.
Political background
A a large driving force for these bridges all being constructed at once was that the predecessor bridges were all not in compliance with War Department restrictions on bridges crossing navigable waters such as the Ohio, the Allegheny, and the Monongahela. These restrictions governed minimum span widths over channels and minimum clearances.The previous bridges were all owned by private companies established to build bridges and pay them off via tolls collected. Some controversy ensued around whether public entities could take bridges or bridge rights of way away from private companies if the companies had retired their construction loans. Several of the companies in the area were paying 15% dividends yearly to stock holders as the bridges were quite profitable, even with tolls at 1 cent per man and free for women.
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) eventually ended up as owners, buying the companies out at fair market value.
But of course, now government had to deal with the problem of non compliance when the war department raised it again at the end of the 1910 decade. Through the teens and early 20s bond issues were floated before the voters to finance replacement but were repeatedly voted down. The HAER source describes the attitude of voters as seeing 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 floated for lift bridges of various sorts, including a scheme to raise the 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 USD 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 anchorages shifting due to inability to secure them. It is not clear who exactly suggested the self anchoring suspension bridge design finally chosen, the only precedent known at that time was a 1915 bridge over the Rhine at Cologne the Deutzer Hängebrücke
Deutzer Hängebrücke
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 the second President of Germany deceased the previous year. It collapsed on 28...
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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
David B. Steinman
David Bernard Steinman was an American structural 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...
’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
Quebec Bridge
right|thumb|Lifting the centre span in place was considered to be a major engineering achievement. Photo caption from [[Popular Mechanics]] Magazine, December 1917...
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 notoriety 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.
External links
- search all 4 HAER collections using "self anchored suspension bridges" term
- Historic American Engineering Record (HAER) No. PA-490, "Three Sisters Bridges, Spanning Allegheny River at Sixth, Seventh & Ninth Streets, Pittsburgh, Allegheny County, PA," 43 pages. Some material in this article was adapted from this (public domain) work.
- Drawings From Historic American Engineering Record
- Pittsburgh West section of "Bridges and Tunnels of Allegheny County, Pennsylvania, from pghbridges.comPghbridges.comPghbridges.com is a reference site created by Bruce S. Cridlebaugh. Formally titled, Bridges and Tunnels of Allegheny County and Pittsburgh, PA, pghbridges.com has information on many of the structures of the area...
- Field Notes Details of eyebar layering, construction, dimensional notes, a very thorough survey. From pghbridges.comPghbridges.comPghbridges.com is a reference site created by Bruce S. Cridlebaugh. Formally titled, Bridges and Tunnels of Allegheny County and Pittsburgh, PA, pghbridges.com has information on many of the structures of the area...
- Three Sisters entry at BridgeMeister.com
- Google Map link showing all 3 bridges
Roberto Clemente Bridge (Sixth Street Bridge)
- entry at pghbridges.comPghbridges.comPghbridges.com is a reference site created by Bruce S. Cridlebaugh. Formally titled, Bridges and Tunnels of Allegheny County and Pittsburgh, PA, pghbridges.com has information on many of the structures of the area...
- entry at BridgeMeister.com
- August 1998 press release about renaming to Roberto Clemente Bridge
Andy Warhol Bridge (Seventh Street Bridge)
- entry at pghbridges.comPghbridges.comPghbridges.com is a reference site created by Bruce S. Cridlebaugh. Formally titled, Bridges and Tunnels of Allegheny County and Pittsburgh, PA, pghbridges.com has information on many of the structures of the area...
- entry at BridgeMeister.com
Rachel Carson Bridge (Ninth Street Bridge)
- entry at pghbridges.comPghbridges.comPghbridges.com is a reference site created by Bruce S. Cridlebaugh. Formally titled, Bridges and Tunnels of Allegheny County and Pittsburgh, PA, pghbridges.com has information on many of the structures of the area...
- entry at BridgeMeister.com