Talk:Three Sisters (Pittsburgh)

Comments

that's on the talk page because the article is "in use" but I have to go to bed... More tomorrow unless the article fairy does it for me!

I plan to return to this article in the morning... I want to try to add 3 {{Infobox Bridge}} invocations and use up all the images I uploaded (these are all from HAER) but the problem is that 3 boxes take up a lot of vertical... Not sure I can get it to work. Maybe I'll have to do one box with 3 columns of stats in it??? have to see.

Below is stuff dumped to preserve it, please comment up here... All the dumped stuff will be moved to the article or condensed and moved, or discarded, as needed. I ran out of weekend so pulled the inuse tag from the main article but I am by nomeans done, it needs a LOT OF WORK... the HAER text is too detailed, and the predecessor bridges need organization... maybe each bridge needs its own article after all, comments? ++Lar: t/c 16:50, 23 January 2006 (UTC)

The images:

Sixth Street

Seventh Street

Ninth Street

Maybe the way to do it is a gallery? I figure one image each for the 3 infoboxes? These may not be the absolute best images possible but I selected them because between the set you get a LOT of detail about the different construction details, how the eyebolts run, what an anchorage looks like, what the deck underside looks like, the way the suspender eyebolts work, etc... and that first 6th street pic, showing all three bridges is just cool. That's the article lead image I think. There were only 2 HAER color photos, I took both of them and a smattering of the rest.

The data for the boxes can be gleaned from the structurae and bridgemeister links already in the article.

Question (for self? for whoever reads this?) do we mention that they used to be green towers with silver spans and eyebars and that all Pittsburgh bridges are now golden? Or not? Also, how did the diagram turn out? It's hard to reduce a TIF that much but I thought it came out OK? -- ++Lar: t/c 06:59, 21 January 2006 (UTC) --

Some OCR'ed text I need to find a home for

This text is from drawing page one (left side)

The “Three Sisters Bridges” represent an adaptive engineering design response to political and technical concerns. County engineers successfully maneuvered around federally-mandated clearances, aesthetic and financial considerations raised hylocal aqencies and the lack ofadequate anchorage points along the river banks. The structures are the only trio of neariy identical bridges and among the few surviving examples of large eyebar suspension bridges in the United States.They were the first self-anchored suspension bridges built in this county.

The Sixth Street Bridge is at the site of Pittsburgh‘s first Allegheny River bridge, Lothrops 1819-20 wooden covered St. Clair Bridge. Replaced by a John A. Roebling suspension bridge in 1860, the bridge was rebuilt again in 1892 using a Theodore Cooper bowstring truss design to support growing railway traffic between Pittsbxurgh and adjacent competitor, Allegheny City, north of the river. The current three-span structure measures 995 feet with a main span of 430 feet. The design’s deck- stiffening girder provided compressive support while lowering visual barriers between Pittsburgh and the historically distinct North side, annexed in 1907.

The American Institute of Steel Construction named the Sixth Street Bridge “The Most Beautiful Steel Bridge of 1928”.

(the right side text gives HAER bridge project history and is not germane to the article)

the following is analysis of the HAER document (43 pages) on these bridges and their predecessors. I am not going into as much detail on the predecessors as on the motivations. (all three spans had multipel predecessors, but that could be left for another author

Note that a large driving force for these bridges all being done at once was that the predecessor bridges were all not in compliance with War department restrictions on bridges crossing navigable waters (of which the Allegheny and Monongahela, not to mention the Ohio, included)... these restrictions governed minimum span widths over channels and minimum clearances (page 19 of the Haer record [1]

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 hte 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 29.2M bond issue at hte county level to improve bridges and munincipal 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 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 (look that one up!)

Cited from the text as now it is getting good! (note there is also stuff to enhance the American Bridge Company article as well as the David B. Steinman article... I knew of his rivalry with Amman but not with Quaddell...:

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.105 David B. Steinman’s nmning dispute with J. A. L. Waddell about the relative costs of suspension and cantilever follHs 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.106 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 infoirnation 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.107

skipped some, then more cites

Once the design for the self-anchored structure had been identified, the question of whether to vary bridge design solved itself: no other solution to the technical, political, and aesthetic dilemma could be found. A consensus formed among members of involved agencies that not only was the eye-bar suspension design for a self-anchored structure the only and best solution, but also that constructing a trio of bridges in such a close area would create a scenic attraction near downtown. Not to be forgotten was the potentially lower unit cost for each to be gained by awarding a single contract for all three projects)’7 Design Issues The crucial features of the Three Sisters’ final design were the eye-bar chains, the deck- stiffening girders, and vertical hangers. An eye-bar chain, draped over steel towers on the river piers, carried the floor system’s weight through vertical hangers. The deck contained a stiffening girder running the bridge’s entire length. Engineers designed the components to work as a system, anchoring both ends of the eye-bar chain to the stiffening girder, which resisted vertical forces through horizontal thrust. This stood in contrast to conventional suspension bridges, which used heavy shore masonry to maintain tension in the catenary.118 The Three Sisters caused some debate within the engineering community because they confounded this aspect of the archetypical suspension form. Nonetheless, the design met the surest test of a suspension bridge: if the chains were cut, the entire structure would collapse. 119 In a suspension bridge, catenary elements carry loads in tension rather than compression. Cables or chains are draped over towers and anchored at bridge ends. They function like inverted arches, with the roadway hung from the catenaries.120 New materials like heat-treated eye-bars and drawn steel wire offered greater strength in suspension bridges. With the greater span lengths and elegant catenary lines, a suspension bridge would satisfSi both navigational and aesthetic considerations. Calculating stress required in stiffening trusses was difficult, however. In 1930 this remained a point of contention among engineers who could not explain why some

stiffening trusses used in suspension bridges worked in practice while failing to meet theoretical stress thresholds.121 Although the Three Sisters essentially behaved as suspension bridges when complete, they were erected by cantilever methods. The 1889 Fifth of Forth Bridge inspired renewed interest in cantilever erection, which had great application on long-span work. Generally, cantilever erection does not save money when used on spans less than 700’-O” in length if falsework can be contracted for without exceptional cost. When deciding whether to use cantilever erection, engineers had to consider the difficulty of using falsework, the length of the span, and whether navigation must be maintained under the structure. Cantilever methods made erection possible without falsework, for longer spans, and with free navigation.122 Contractors Erecting suspension bridges using cantilever methods was a complex job demanding only the most experienced of contractors. After advertising the bids, the county awarded the contract for the three bridges’ superstructure to the American Bridge Company and the substructure to the Foundation Company of New York. County commissioners, anxious to assure the public that contracts were in no way marred by corrupt deals, publicized the savings provided by awarding the contracts to the same company. By building all three bridges at a single price, the county shaved about $0.5 million off of the project budget.123 The American Bridge Company, perhaps more than any other company in the world, was best qualified to work on a bridge with a continuous stiffening component. The company was then constructing the Florianopolis Bridge in Brazil, working from 1922 to 1926 on the longest eye-bar suspension span in existence at that time. The 11 13’-O”-long design featured towers with rocker bearings. The company brought to that project its own experimental heat-treated eye-bars, and as project contractor pioneered stiffening techniques that saved materials and money while providing greater rigidity.’24 H. D. Robinson, who had worked with Steinman on the Florianopolis Bridge, had also consulted on towers for the Cologne bridge’s design. Robinson used similar towers on the Rondout Bridge at Kingston, New York, which was constructed in 1922 and used a stiffening truss that functioned apart from the towers. By 1924, the American Bridge Company had many opportunities to acquire a base of knowledge that specifically would have helped in building a bridge such as that specified by Allegheny County engineers. Whether the firm advertised its services to the county before specifications were devised is not known. The American Bridge Company used its knowledge from the Florianopolis Bridge to prepare alternative designs for bridges over the Ohio River in Point Pleasant and in St. Marys, West Virginia, however, securing both projects.’25 The American Bridge Company also accumulated vast experience by company acquisition. In 1900, J. P. Morgan consolidated two dozen bridge-building companies under the umbrella of the American Bridge Company, which a year later became a subsidiary of Morgan’s U.S. Steel. Thereby he created an industrial giant that controlled half of the country’s bridge- building capacity. The acquisition united large Pin sburgh competitors, like Carnegie’s Keystone Bridge Works, and smaller area concerns, such as Schultz Bridge & Iron Works of nearby McKee’s Rocks. Morgan’s reach also encompassed New York’s Union Bridge Company, Gillette-Hezon Manufacturing Company of Minneapolis, and Berlin Iron Bridge Company of 126 Connecticut in that first year, as well as other operations in coming years. The Foundation Company of New York brought its own wide experience in to Pittsburgh. Incorporated as the Foundation & Contracting Company in 1902 and specializing in “design and construction of difficult foundations,” the firm shortened its name to the Foundation Company to reflect its specialized focus. Known for constructing foundations for skyscrapers at Manhattan Island’s southern tip where bedrock was exceptionally deep, the company also expanded its operations to Canada and the Midwest. The Foundation Company’s projects in the first decades of the twentieth century included bridges, mine shafts, tunnels, dams, and sea-walls. The firm used a variety of techniques, such as pneumatic caisson work, pile driving, and coffer-dam construction. The most prominent of its projects included sinking foundations for the Woolworth Building in New York City, at the time the tallest building in the world, 750’-O” above street level, and weighing 136,000 tons. The company also constructed foundations for the Miramachi and Pitt River bridges in Canada and the Penn Bridge in Coshocton, Ohio.’27 With rock lying 50-0” to 60’-O’ under the Allegheny River’s vigorous current, an experienced firm was needed for the Three Sisters Bridges.’28 The Three Sisters, 1924-28 The Sixth, Seventh, and Ninth Street bridges foi iii a striking trio of structures crossing the Allegheny River in downtown Pittsburgh. These bridges are unique in the United States for the clustering of three similar bridges in such close proximity, their place as the first American self- anchored suspension bridges, and as significant examples of eye-bar suspension bridges. They are also the first cantilever-erected suspension bridges in the world.’29 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.13° 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.’3’ 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 ofjust more than 1 5’-O” forces the plate girders to perform their stiffening duties along a noticeable curve, serving as a 885’-O”- 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’-O” 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’-O’ 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 l0’-3-l/2”) The towers measure 77’-l 1-3/8” 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 16.9-percent impact factor. The unit stress of the eye-bars was specified as 136 27,000 pounds per square inch. 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.’37 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.’38 Construction 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 at the ends are vertical. The girders are thus subjected to stresses due to bending combined with direct compression.’39 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.’4° 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 LO and Li 0. 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 inteiuiediate pins at U4 and U6 and the cradled sections around them (U3 to US and U5 to U7) were connected to a supporting I-beam, with struts attached at U5 and U7. The next section, between U7 and UlO, received a similar treatment, with I-beam cradles at U7 and UlO and with slotted-hole plates attached to the I-beams cradling U8 and U9. Point UlO reached the tower.’42 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 Li7 to L17* (S indicating the north span). Point LO 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 ofjoining 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)43 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 L19t 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.’” 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.’45 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. 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)47 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’-O” spans, which were 44’-O” wide and 80’-O” high.’48 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.’49 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 tl5O side of the bndge to lower the spans on alternate sides to the full depth of 16-0 Workers made a pontoon out of two pairs of barges, spaced to create a platform 400’-O” long and 52’-O” wide, which carried the bridge with 20’-O” 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 346 “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. ‘47 “Ninth St. Bridge Formally Opened,” Chronicle Telegraph, 26 Nov. 1926, in Clippings File, Pittsburgh Bridges — Ninth Street, Pennsylvania Room, Carnegie Library, Pittsburgh, Pa. 48 “Floating Intact to Coraopolis Pittsburgh’s Old Sixth Street Bridge Spans,” press release from Norman F. Brown’s office, nd., 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.

clearance, which was 14’-O” and 12’-O” less than the bridge’s height. After supporting the bridge under the floor beams, they disassembled the top chord and stabilized each panel point on the trusses.’51 Steering the bridge under the Manchester Bridge and the railroad bridge at Brunot’s Island, the tugboat captains went through the Ohio River lock and up the back channel toward Coraopolis, where piers and abutments had been constructed. Reversing the process and using the same jacks and steel frames to raise the structure 32’-O”, the company erected the Cooper bridge 30 days after the project began.’52

Completing the Sixth Street Bridge

Work proceeded on the new Sixth Street Bridge, with the Foundation Company called to accept last-minute revisions of piers 1 and 4 to accommodate subway construction at a later date. Bion J. Arnold’s influential transportation report called for converting street-level areas downtown to pedestrian and automobile use. He recommended building tunnels for subways to connect Pittsburgh’s central business district with selected areas, including the North Side. Although business owners and city officials hotly contested the exact route for such a system, the North Side access required by any future subway system was not in doubt.153 The revisions proposed constructing tunnel shields that could be pierced when the city decided on a final route. The tubes were 68’-O’ on center, with suggested radii ranging from 7-9” to 9’-O’, and steel reinforcement overhead. One proposal for the construction shows plans using three separate open box caissons to foiiii the base of each pier)54 The tube openings provided slightly different angles for each direction to adapt to the grades and length of approaches on north and south shores, anticipating lines that bent outward to avoid the two main piers. The city of Pittsburgh absorbed the cost of the alterations to the original plan, paying the Foundation Company the same contract costs negotiated for the bridge work of up to $40,000 for the south pier and $80,000 for the north pier.’55 (The subway openings have yet to be used, however. Pittsburgh’s underground light rail system, constructed between 1980 and 1985, follows a Sixth Street alignment for several blocks but does not cross the river.)

Even with the subway alterations, the Foundation Company completed its work four months ahead of schedule, leaving to the American Bridge Company a working schedule during Pittsburgh’s bitter winter weather. The company continued to fabricate components at its Ambridge shop, completing two-fifths of the 6,000 tons needed while it waited for better river conditions. In the meantime, workers pre-assembled smaller components off site. Brown’s office justified the apparent delay in a press release, responding to business owners who insisted that work continue amidst the winter by noting no contractor could reasonably be expected to erect the side-span falsework necessary for the project with the dangerous currents and ice faced by the American Bridge Company. The uniqueness of a suspension bridge erected by cantilever 156 methods required exceptional caution. Conclusion Its predecessor razed in September 1924, the Seventh Street Bridge was the first completed. The county opened the Seventh Street Bridge on 17 June 1926, and the Ninth Street Bridge on 26 November of the same year. County commissioners soothed area business owners anxious for a return to normal commerce by allowing pedestrians to use the Sixth Street Bridge’s downstream sidewalk on 14 September 1928, in advance of the bridge’s official opening. In a move that symbolized commitment to keeping its public works promises, the capstone of Allegheny County’s 1924 construction program was brought to a close almost exactly four years after workers began tearing down the first of the three bridges. The Sixth Street Bridge was opened to all traffic on 19 October 1928.157 The county’s public works projects in the 1920s employed local labor instead of contracting with outside consultants or companies, and materials were procured from area manufacturers. “Every pound of the thousands of tons of steel used in our bridges was manufactured in the mills of our district,” Armstrong wrote in his re-election campaign literature in 1931. He reminded voters that he had helped oversee the construction of ninety-nine bridge projects at a cost of $47.2 million Included in the list were the Coraopolis Bridge, at Neville Island in Coraopolis Borough in 1928, costing $0.8 million; the Sixth Street Bridge in 1928, $1.5 million; the Seventh Street Bridge in 1926, $1.4 million; and the Ninth Street Bridge in 1926, also $1.4 million. Significant in themselves, the bridges were only part of a massive $31 million construction plan beginning in 1924 and ending in 1931. Armstrong boasted, “More county

156 Carbon copy of memo,”New 6th Street Bridge,” n.d., in File AL 02, ACDPW. ‘“Plowden, Bridges, 239; “Ninth St. Bridge Formally Opened,” Pittsburgh Chronicle Telegraph, 26 Nov. 1926, in Clippings File, Pittsburgh Bridges — Ninth Street, Pennsylvania Room, Carnegie Library, Pittsburgh, Pa.; “Parade Marks Sixth Street Span Opening,” Pittsburgh Post-Gazette, 20 Oct. 1928, in Clippings File, Pittsburgh Bridges — Sixth Street, Pennsylvania Room, Carnegie Library, Pittsburgh, Pa.; and “Opening of the Sixth Street Bridge Sidewalk, Friday, September 14, 1928,4:00 P.M. Daylight Saving Time,” press release from Norman F. Brown’s office (12 Sep. 1928), in File AL 02, ACDPW.

improvements were made during this 8 year period than during the previous 136 years of the county’s existence.”58 One of Aiiiistrong’s pieces of campaign literature, a lavishly illustrated brochure, prominently featured an aerial view of the “Trinity of Bridges” overlaid with a photo of American Institute of Steel Construction’s 1928 award to the Sixth Street Bridge.’59 AISC named the structure “The Most Beautiful Steel Bridge” constructed in 1928, adding to the fame of the Allegheny County Department of Public Works and publicizing the potential of steel for enhancing not only strength but attractiveness. One of five judges, Charles Evan Fowler, commented on the choice of winners: The new Sixth Street Bridge at Pittsburgh was selected because in my opinion it combined the essential elements of beauty, simplicity, symmetry, harmony, and proportion, and also because of its originality of design, it being a self anchoring eye-bar cable suspension bridge. The cables have enough section to give a very satising appearance of stability, combined with simplicity of construction. The towers are very chaste in design, yet heavy enough to give every appearance of strength and solidity. The large arch over the roadway is surmounted by an unequal number of portal openings, yet very harmonious with the entire design of the towers, and the graceful curve of the cables.’6° The award enhanced the city’s image as a forward-thinking municipality with an economy based on a material of the future, but the bridges also healed friction that had festered between business interests in former Allegheny City and Pittsburgh since the mid-nineteenth century. Roush noted, “As has already been expressed by some business interests on the north side of the river, ‘There is a feeling that a barrier had been removed between the two business sections of the City.”6’ The Three Sisters Bridges embody not only a unique construction method and design but also a specific response to local and national political configurations. The structures represent a larger process of social conflict and cohesion that began half a century before, as well as advances in material technology and design that became available only within the decade of its construction.

105 D. B. Steinman, A Practical Treatise on Suspension Bridges: Their Design, Construction and Erection, 1st ed. (New York: John Wiley & Sons, 1922), 53 et passim.

107 Waddell and Steinman waged a turf war in professional literature during the 1910s, arguing that comparative cost of the two bridge types indicated different uses. Waddell complimented the less established Steinman on attempting to calculate costs — primarily as a way of setting his opponent up for criticism in using approximations of weights when calculating the span lengths at which both types proved equally economical. J. A. L. Waddell, Economics of Bridgework: A Sequel to Bridge Engineering (New York: John Wiley & Sons. 1921), see especially 8-9, 104-106, 268; D. B. Steinman, Suspension Bridges and Cantilevers: Their Economic Proportions and Limiting Spans, 2nd ed. (New York: D. Van Nostrand Co., 1913); see also C. B. McCullough, Economics of Highway Bridge Types (Chicago: Gillette Publishing Co., 1929). Despite Waddell’s attack, Steinman may have had the last laugh. He proved more open than Waddell to new ways of categorizing bridge structures, noting in 1922 the existence of new technologies, such as eye-bars and self-anchored suspension spans, which eventually blurred the boundaries of the debate. (Steven J. Fenves of Carnegie-Mellon University’s Department of Civil Engineering kindly noted Steinman’s contributions to the technological diffusion discussed above.)

Splitting the Three Sisters

(Note, this portion of the conversation was moved from Talk:Hot Metal Bridge)

Which I for one would prefer to see the Three Sisters broken apart again. You see each individual bridge on a map of Pittsburgh. That's not the case for Hot Metal. ClarkBHM 21:34, 30 March 2006 (UTC)

My long term plan (as the guy that merged the Three Sisters) is/was to have articles for each separate bridge, all linked to from the Three Sisters article which would then become an overview and history/setting sort of article. Then the nav (crossings box) can weave through the separate bridge articles instead of through it, and the separate bridge articles can carry all the history of the previous bridges (which all is in the history of the 3 sis article as deleted material since it wasn't formatted very well) I confess I may have bitten off more than I can chew because while that's my plan I haven't done it yet which I feel guilty about! More comments to that talk page, perhaps? ++Lar: t/c 01:50, 2 April 2006 (UTC)

I agree with your assessment. The Three Sisters as an article provides a nice overview of the unique nature of having three similar bridges being built within the same time frame. I think that each individual bridge should have its own article though. Identical information for each bridge can refer back to the Three Sisters page... ClarkBHM 02:34, 2 April 2006 (UTC)

There is plenty of source material on the individual bridges as well as their predecessors out there. I OCRed in about half of the 40 some pages of the HAER writeup referenced (which is all public domain text). Plus pghbridges.com has good material on each bridge and the predecessors. It's just a matter of making time to do it! The older Roberto Clemente Bridge article is still around but the other two, Rachael Carson Bridge and Andy Warhol Bridge do not exist (never created?) The RC article does not have much at all which is why I was thinking maybe of making it a redirect. So ya, if you have time and want to take it on, please do! I suggest a trawl through the history of this article, the info is there... Or, I'll try to do it soon (I am about to go on holiday for a while), perhaps with some help? I confess I am not a Pittsburgh native, I just happened to back into these bridges doing research on Self anchoreds and thought it neat. I don't think there is anywhere else in the world that has three near identical suspension bridges all built at the same time. I confess... I started this and then left it hanging, my bad. At least I got Coraopolis Bridge done though. ++Lar: t/c 02:47, 2 April 2006 (UTC)