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Civil Engineering Magazine THE MAGAZINE OF THE AMERICAN SOCIETY OF CIVIL ENGINEERS

By Daniel B. Irwin, P.E., and Kevin W. Johns, P.E.

Since 1852, the Portage Viaduct has carried a rail line across the Genesee River Gorge, near the town of Portageville, New York. First made of timber, then replaced with a wrought iron structure after a destructive fire, the crossing became an icon for the community. Tasked with replacing the viaduct, bridge engineering firm Modjeski and Masters worked with the community and the Norfolk Southern Railway Co. to create a new iconic arch bridge for the gorge.

The Genesee River begins its northward flow in north central Pennsylvania and eventually empties into Lake Ontario in New York near the city of Rochester. Approximately halfway in between, about 50 mi from Buffalo, New York, and Rochester, near the town of Portageville, New York, the Genesee River flows into a deep, narrow gorge for about 18 mi. Because of the size of this gorge, it is commonly referred to as "The Grand Canyon of the East." This area is now encompassed by Letchworth State Park, a popular tourist destination known for its scenery, which includes three large waterfalls in the Genesee River. Located just to the south of the first waterfall, known as the Upper Falls, and crossing the gorge is a rail line and bridge now owned by Norfolk Southern Railway Co. (NS).

It is this crossing that has been known as the Portage Viaduct since 1852. The first crossing at this location was an 800 ft long, 234 ft high wooden trestle founded on sandstone piers. It opened in 1852 but was only in use for a few decades before it burned down in 1875. Within three months, a new wrought iron replacement crossing was completed and carried its first train across the gorge. A renovation of the superstructure took place in 1903, but by the 1990s, the bridge's aged condition required both a load restriction and speed restriction—to just 10 mph.

In 1998, bridge engineering company Modjeski and Masters Inc. (M&M) was hired by NS to explore methods for improving the capacity of the existing Portage Viaduct. These included rehabilitation and strengthening alternatives as well as replacement options. The three most feasible replacement options were determined to be a trestle-type span similar to the existing crossing, a truss span, and a spandrel-braced arch.

In 2006, the railway asked M&M to perform an in-depth inspection and load rating of the Portage Viaduct to better determine the scope of rehabilitation required if the existing structure were to be retained. Nearly all bending and tension members in the structure were found to have exhausted their calculated fatigue life, and most members also required significant strengthening to be brought up to Cooper E80 load capacity, which is the current design loading for railroads in North America. Therefore, rehabilitation was found to be impractical, and in 2006 M&M recommended replacing the structure.

NS directed M&M to begin a replacement study shortly thereafter. The focus was on the three alternatives previously determined to be most feasible, and all were evaluated on an alignment that was parallel to the existing bridge and 75 ft from it, to allow rail traffic to continue uninterrupted while the new bridge was constructed.

As a historical transportation route that predated the state park in which it is located (see "Bridging the Genesee River Gorge Has a Complex History," page 46), the rail line and crossing had become an important icon to the region; images of the bridge could be seen on the walls of local businesses, in tourism brochures, and even on the class rings of a local high school. Because of the proximity of the bridge to the park's scenic Upper Falls, the bridge was also part of the backdrop of countless photos taken over the decades. Recognition of this importance mandated that the replacement alternatives be compared not only on construction costs and methods but also on environmental and visual impact. Renderings of the alternatives were to be released for public input at numerous public meetings held in the area. NS was open to conveying the existing bridge to the park or any other interested parties for preservation, so renderings were produced both with and without the existing bridge in the event that it was retained instead of demolished upon completion of the new crossing.

Upon weighing the evaluation criteria and public input, NS chose the spandrel-braced arch as the preferred alternative. The rock walls of the gorge were ideal for an arch span. Additionally, the span could be constructed without placing any falsework in the river and required no permanent structure in the river, and there was a general consensus that it was the most visually appealing option. While the public was sentimental toward the Portage Viaduct, no parties were interested in taking ownership of the structure; the public was ready for a new icon in Letchworth State Park, and the Portage Viaduct would be removed once rail traffic was transitioned to the new structure.

With the preferred alternative selected, M&M began final design of the new Portageville Bridge, as it was dubbed: a 483-ft, two-hinged spandrel-braced arch spanning the Genesee River with three 80 ft multigirder deck spans flanking each end of the arch.

M&M worked with NS to develop project-specific design criteria. The design of the new bridge would follow the guidelines in the Manual for Railway Engineering (American Railway Engineering and Maintenance-of-Way Association [AREMA], 2018), NS's Public Projects Manual (2013), and New York State Department of Transportation (NYSDOT) standards. NS requested that the new bridge utilize the railway's standard concrete trough—which holds the stone ballast to support the track. In addition, NS requested that the bridge be designed with the provision to carry 12 in. of additional ballast below the track to account for potential increases in depth over time. Live load was designed per the AREMA manual.

While evaluating the wind load to be applied to the structure during the development of the project-specific design criteria, M&M noted that the commentary section in the AREMA manual states that while the manual-defined loading for wind on a train is 300 lb per linear ft, applied 8 ft above the top of rail, this is only adequate for lines that do not carry double-stacked equipment. The manual encourages the engineer to consider increasing this force if double-stacked cars are to be operated on the line, which is the case for freight operations on the Southern Tier Line. M&M therefore began an evaluation of wind loading to develop a revised load to use for design. Per the AREMA manual, wind loading was based on a 30 lb per sq ft pressure—a practical maximum for the operation of trains; the risk of equipment being overturned by wind becomes a concern beyond this level. M&M determined that a conservative approach would be to use the same wind pressure but applied to a 20 ft high car, representative of typical doublestacked car, resulting in 600 lb per linear ft. This would be applied at 10 ft above the top of the rail, equivalent to midheight of a 20 ft high car.

An important task during the final design process was evaluating the suggested construction staging procedure for the bridge. From the replacement alternatives study, M&M determined that the spandrel- braced arch could be built piece by piece onsite—that is, stick-built—as two cantilever halves with temporary tiebacks on either side of the gorge. Upon meeting at midspan, the tiebacks could be released to allow the bridge to function as an arch. M&M calculated an assumed live loading for the contractor equipment and performed a 3-D nonlinear finite element analysis with 15 stages to represent the entirety of construction. In addition to the contractor's live loading during construction, the analysis included wind loads and the weight of the bridge itself. Nonlinearity was required due to the removal of the tiebacks and redistribution of forces at the stage when the two cantilever halves would begin to function as an arch. The results of the analysis revealed that none of the members had designs that were controlled by the suggested construction staging procedure, so their design did not have to be revised to accommodate the construction process.

M&M needed to overcome two noteworthy site development challenges during the final design. Both involved an access road and a parking lot. The road led to the park's southern entrance, the Portageville Entrance, and the parking lot was adjacent to the west approach of the existing Portage Viaduct. The Portageville Entrance is the busiest entrance to the park. However, the access road was hazardous because of the constraints of the gorge and the existing bridge; it had a substandard width and a blind curve. Additionally, with many hiking trails converging in this area, the road was frequently used by pedestrians. The parking lot, though mostly on NS property, served as an access point for the hiking trails and for snowmobile riders in the winter and was a key part of the park infrastructure.

When evaluating the earthwork and excavation required for the new structure, M&M saw that the park access road would be severed by the west skewback excavation, where the new arch would abut the gorge, and the parking lot would be partially cover by the new west abutment backfill. The optimal solution was determined to be a westward realignment of the access road, while increasing the road width and eliminating the blind curve. This realignment allowed the creation of a new and larger parking area over top of part of the old road alignment. Although this would require closure of the access road for the duration of the project, New York State Office of Parks, Recreation and Historic Preservation personnel agreed to the closure because of the enhanced safety and capacity benefits that the solution offered.

M&M delivered the completed plans, specifications, and estimate package to NS on January 31, 2013. The new Portageville Bridge was set to be the first spandrel-braced arch built for railroads in North America in approximately nine decades. Of the others built for railroads that are still in use, it has few peers. M&M's expertise and long history meant it had designed two of them: the Crooked River arch bridge in Oregon (1911) and the Hurricane Gulch arch bridge in Alaska (1921). In New York State, there is the Whirlpool Rapids Bridge in Niagara Falls (1897), with the adjacent Michigan Central Railroad arch currently unused.

Some of the more noteworthy quantities from the engineer's estimate for the new Portageville Bridge are: 8.8 million lb of Grade 50 structural steel, 7.3 million lb of which were in the arch span; 3,800 cu yd of concrete; and 16,000 cu yd of rock excavation. Each end of the arch ribs bears on a 24 in. diameter forged steel pin, reacting into bearing shoe castings on the skewbacks that are 9 ft by 10 ft in plan. The estimated cost for the project was $64.7 million.

Construction commenced in February 2015 when tree removal began on what was to become the new right-of-way for the bridge. In fall 2015, American Bridge Co. (AB), of Coraopolis, Pennsylvania, was awarded a $58-million contract for the new structure. A formal groundbreaking ceremony was held on October 28, 2015, with representatives from NS, M&M, and NYSDOT present. By November 2015, AB had mobilized to the site.

Understandably in a location as scenic and serene as Letchworth State Park, there were some unique environmental concerns. Timber rattlesnakes, a protected species in the area, were known to have dens in the park; therefore, a silt fence was installed to create a physical barrier to prevent rattlesnakes from entering the work area. Bald eagles presented another challenge: less than 0.25 mi from the work area there is an occupied eagle's nest. Once a nest is established, an eagle pair will return to that same nest each year. The U.S. Fish and Wildlife Service required monitoring the eagles to ensure that noise and other disturbances from construction activities did not cause them to abandon the nest. Efforts to mitigate noise and disturbances included limiting the number of blasts per week during skewback excavation, and drilling, rather than driving, micropiles. The efforts proved successful; the nest remained in use, and eaglets were hatched during the multiple-year project.

Test blasting for skewback excavation began in early 2016, with regular drilling and blasting operations for the stepped excavations starting in March. By December 2016, the east skewback concrete was placed and ready to have the arch bearings set into place, with the west skewback ready by March 2017. Micropile installation for the abutment and pier footings began in early 2016 as well, and by July 2016, pier shaft reinforcement was being installed in preparation of forming and placing concrete. An innovative addition inside of the approach span piers was corrugated steel pipes to act as "chimneys" to help dissipate heat from mass concrete placement to avoid cracking.

On January 5, 2017, a pair of girders between the east abutment and the first pier were set into place, marking the first steel to be erected. Within a few weeks, the first segment of the arch was also set into place at the east skewback. The arch erection methods utilized by AB were largely unchanged from those proposed by M&M on the contract drawings. Tieback cables were framed into the upper ends of the arch and connected to frames anchored to rock at the abutments. Large "ringer" cranes on each side of the gorge built the two cantilevered halves of the arch piece by piece until they came to the limit of their reach. At this stage, rubber-tired cranes were set onto cribbing on the arch cantilevers to continue stick-building the arch to its completion.

The first members to close the arch were installed on July 26, 2017, and by August 2017, the tension in the tieback cables was released, allowing the new Portageville Bridge to begin functioning as an arch span. The arch erection progressed smoothly, and the two cantilevered halves aligned with only minor adjustment in the tieback systems—a testament to the skill of the fabricator, Canam-Bridges, of Quebec City, Quebec, Canada, and of the AB team erecting the steel.

The flanking girder spans directly adjacent to the arch span were placed in September 2017, followed by the last of the arch span stringers. Formwork and reinforcement for the cast-in-place concrete ballast trough were installed thereafter. The approach alignment was graded, ballasted, and laid with track while concrete was placed for the ballast trough on the new bridge. The concrete ballast trough received a spray-on waterproofing membrane before ballasting and track placement, at which point the new Portageville Bridge was ready for service.

At 1:40 a.m. on December 11, 2017, the final train crossed the Portage Viaduct. Shortly afterward, the track was taken out of service to begin the task of cutting the line over to the new Portageville Bridge. At 2:20 p.m., the first train crossed the new bridge: NS 36T, which was traveling from Buffalo to Allentown, Pennsylvania. Demolition of the existing bridge began almost immediately, starting with the removal of rails and bridge ties, followed by removal of the spans and bents outside of the gorge. Three controlled blast demolitions were utilized to safely drop the tallest of the bents into the Genesee River, where they were removed for salvage. By May 2018, nearly all traces of the viaduct had been removed and site restoration efforts were under way. The removal of the last of the river piers, all of which dated to the original wooden Portage Viaduct, marked the first time that the Genesee River flowed unimpeded through this area since 1851.

On May 24, 2018, NS and the New York State had a dedication ceremony for the Portageville Bridge, during which it received its official new name: the Genesee Arch Bridge. Celebrated was the fact that rail traffic was no longer load restricted or speed restricted because of the bridge, the economic benefits of which were immediately felt. Final painting of the arch connections was completed by July 2018, with all focus then turning to site restoration. On October 1, 2018, the Portageville Entrance and the region of the park surrounding the bridge was reopened to the public, truly marking the beginning of a new era for the park, the railroad, and regional commerce.

A key centerpiece to the new parking area by the Genesee Arch Bridge is a piece of a tower bent from the 1875 Portage Viaduct, replaced on the location where it had stood for the previous 143 years, to serve as a monument for future generations to remember the past.

Bridging the Genesee River Gorge Has a Complex History

In the early 1850s, the Buffalo and New York City Railroad, a subsidiary of what would later be known as the Erie Railroad, expanded westward from the New York City area toward Buffalo, New York. As part of this expansion, it encountered the Genesee River Gorge. Silas Seymour, the chief engineer of the railroad, decided to cross the gorge at a location known as Portage, and an 800 ft long, 234 ft high wooden trestle founded on sandstone piers quarried from the river banks was built for that purpose. Construction of the Portage Viaduct, as the bridge was to become known, started on July 1, 1851, and the first train crossed the trestle on August 14, 1852; the total cost was approximately $4.3 million in today's dollars.

Seymour's design recognized that timber exposed to the elements would degrade and thus allowed for the removal and replacement of individual members without disruptions to rail operations. At the time of construction, the Portage Viaduct was reported by The Civil Engineer and Architect's Journal to be the largest and highest timber trestle in the world. Quantities of material utilized in construction were 9,200 cu yds of masonry, 1.63 million board ft of timber, and 49 tons of wrought iron. Guards were employed day and night to dispatch any fires that might break out from cinders and ashes thrown from passing trains.

From the very beginning, the viaduct was recognized for the breathtaking views it afforded of the Genesee River Gorge and its waterfalls. The Erie often stopped its passenger trains on the viaduct, allowing passengers to walk along the deck to admire these views. In early 1858, one of these passengers was a man by the name of William Pryor Letchworth. A businessman from Buffalo, Letchworth was in search of a rural estate where he could relax and escape from the stresses of life in the city. Upon seeing the views of the gorge below, Letchworth decided to acquire the surrounding land to establish his estate. In 1859, he began purchasing land and by 1871, he was able to retire and permanently move to the site, which, because of the frequent sight of rainbows in the mist above the area's waterfalls, he named Glen Iris in recognition of the Greek goddess of rainbows. Letchworth devoted the remainder of his life to charity and humanitarian pursuits, and in a gift to future generations, he gave his Glen Iris Estate and its surrounding 1,000 acres of land to New York State in 1907. Letchworth State Park was created that same year, and over the decades, nearly 13,000 additional acres of land have been added to create a tourist destination currently visited by more than 1 million people per year.

When the original Portage Viaduct was destroyed by fire on May 6, 1875, the decision to rebuild was made immediately and just four days later, on May 10, 1875, a contract for the new wrought iron bridge was awarded to the Watson Manufacturing Co., of Paterson, New Jersey. George S. Morison, principal assistant engineer to Octave Chanute, the Erie's chief engineer, prepared the design of the new structure. Morison designed the tower bents to accommodate two tracks of rail traffic in anticipation of a future widening of the Erie through this region. According to Morison's 1875 paper presented in the ASCE Transactions, the first ironwork was erected on June 13, 1875, and by July 29, 1875, all ironwork was in place. On July 31, 1875, the new Portage Viaduct carried its first train-a turnaround of just 86 days.

The cost of the new viaduct was approximately $2.1 million in today's dollars—less than the cost of the original timber viaduct. Noteworthy is that this structure used salvaged bolts from the timber viaduct, and repairs to the masonry piers were performed with concrete, representing an early use of this material for such purposes in the United States.

In 1903, the superstructure of the Portage Viaduct was replaced to accommodate increased rail loadings. The tower bents, having been originally designed for two tracks of loading, were able to accommodate this increase, although the viaduct remained single-tracked. The bents remained largely unaltered throughout the life of the bridge, only requiring minor strengthening in the 1940s.

In the years following the construction of the Portage Viaduct, Morison went on to become one of the pioneers in long-span steel bridge design, overseeing the building of many noteworthy spans, the 1892 Memphis (now the Frisco) Bridge being the largest of those. Very similar to his time understudying with Chanute, Morison went on to mentor many young engineers, including Ralph Modjeski, with Modjeski starting employment with Morison in 1885 and concluding with his work on the Frisco Bridge in 1892. And much like Morison, Modjeski went on to become a pioneer in the next generation of bridge engineering, ultimately founding his own engineering practice in 1893 that last year celebrated its 125th anniversary as Modjeski and Masters Inc., the company responsible for the design of the current Portage Viaduct's replacement crossing.

Changes in the railroad industry occurred in the following years as well: the Erie merged with the Delaware, Lackawanna, and Western Railroad in 1960 to form the Erie Lackawanna Railroad, which became part of Conrail in 1976. In 1998 Conrail was jointly acquired by Norfolk Southern Railway (NS) and CSX Transportation, with the Portage Viaduct and its respective line being conveyed to NS as part of its Southern Tier Line across New York State. By the time NS began operations over the Portage Viaduct, it was becoming the key operational bottleneck on the Southern Tier Line. The bridge was not capable of carrying the modern standard 286,000 lb railcars and was speed-restricted to just 10 mph. The Southern Tier Line served many local industries, and more were expected to come to the region, further increasing demands on the aging structure.—DBI/KWJ

PROJECT CREDITS

Client Norfolk Southern Railway Co., Norfolk, Virginia
Engineer Modjeski and Masters Inc., Mechanicsburg, Pennsylvania
Contractor American Bridge Co., Coraopolis, Pennsylvania
Fabricator Canam-Bridges, Quebec City, Quebec, Canada

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