The engineers behind the replacement of the Portageville Bridge in Letchworth State Park had to contend with challenges ranging from nesting eagles and dens of rattlesnakes to freight trains passing just 23m away from the construction site. Each year over one million people visit Letchworth State Park in Livingston County, around 400km northwest of New York City, to take in the stunning views of the three large waterfalls in the Genesee River as well as the beautiful gorge that the old Portageville Bridge traversed. The structure, one of the oldest iron rail bridges in the USA, was recently replaced by American Bridge for Norfolk Southern Railway Company.
This historic connection had been the centrepiece of photos for almost two centuries, whilst also serving as an important connector for the Southern Tier line that runs between Binghamton and Buffalo, in New York State. More than 166 years ago, the first of three Portageville Bridge structures was built as a vital link for the Erie Railroad across the Genesee River. The 244m-long bridge opened in August 1852 after 13 months of construction. It was a massive timber trestle, with the track sitting 71m above the river, and at the time it was believed to be the largest timber bridge ever built.
When the bridge burned down unexpectedly in May 1875 due to embers from a passing train, a replacement was needed immediately. In just three months, a new bridge made of iron was constructed on the same alignment; an enormous feat even by today’s standards. Local lore had it that the fire may not have been an accident, thus explaining how the new structure could be devised, built, and opened to rail traffic so quickly. Despite its rapid construction, the bridge was still in use more than a century later.
The iron structure had 13 open-deck spans totalling 250m, which carried the single track across the gorge, 72m above the Genesee River. It consisted of six towers, four girder spans, and three truss spans. Apart from a strengthening project in 1903, in which the shorter superstructure spans were replaced with steel deck girder spans and the longer spans were replaced with pin-connected steel deck truss spans, and some additional maintenance repairs, the structure had remained largely the same since the late 1800s.
However, monthly inspections on the historical bridge had become a costly and time-consuming effort for Norfolk Southern Railway. As the last bridge on this route to be replaced or rehabilitated for modern freight loads, it was the weakest link and one that was not able to handle modern transportation needs. The structure was not capable of carrying the 130t freight cars, and weights had had to be reduced by nearly 6t to cross the bridge. Even with the weight restriction, train speeds had to be reduced to 16km per hour to prevent overloading, creating a bottleneck in this main east-west corridor. With anticipated expansion along the Southern Tier Route, the bridge was no longer able to keep up with expectations.
Plans to replace it date back to the late 1990s, when Modjeski & Masters was hired to evaluate both repair and replacement options. Replacement was deemed the most viable option, since a new bridge was essential to enable Norfolk Southern Railway to operate safe, reliable and efficient rail operations on the Southern Tier Route. Nearly ten years later, in 2007, Modjeski & Masters started design research and provided the best options to complete the enhancement of the route.
Because of the high visibility of the bridge and the large number of visitors to the area, the National Environmental Policy Act required public input into the design. Parties agreed on a spandrel-braced arch bridge that would span the entirety of the gorge, framing the beautiful backdrop. This popular option would leave no physical footprint in the water below and remove obstructions to enhance the views in the park.
The new arch bridge is a single-track railroad crossing with a ballast-filled concrete deck; it has a 147m-long arch main span and there are three 24m-long girder approach spans at each end, making a total length of 294m. The deck sits 72m above the Genesee River and contains 3,332t of steel.
Construction began in November 2015, just 23m south of the existing structure. This proximity made the project site difficult to command, and as it was a linear project with limited lateral space due and surrounding state park property, the American Bridge team had to address obstacles in terms of space for materials, construction staging and housekeeping. This meant that the construction method had to be carefully considered. To avoid the need for falsework and to keep the impact on the Genesee River to a minimum, the cantilevered erection method with a tie-back system was implemented for the arch erection.
To begin this process, two Manitowoc 4100W Ringer cranes with 91m-long booms were built, one on each side of the gorge. This allowed each half of the arch to be constructed simultaneously using the cantilevered erection method, until they met in the middle. To support the segments during construction, a temporary tie-back system was used.
The first two approach girder spans and the first four panels of each arch cantilever were erected using the Ringer cranes. These cranes then lifted rough-terrain cranes onto each cantilever to erect the remaining members. This enabled the two cantilevers to be joined together so that the bridge could function as a complete arch.
The Ringer cranes fed material to the rough-terrain cranes and erected the final approach span once the arch was completed. The temporary tie backs were then released once the two cantilevers were connected. This method was possible because a spandrel-braced arch with temporary tie backs is a self-supporting structure when cantilevered. The temporary works consisted of two major items: the stage one support tower and the arch tie-back system.
The support tower on the east side was created from falsework towers used on previous American Bridge projects such as the US 69 Missouri River Bridge. The restricted footprint of rock available on the west side for the support tower required the innovative use of F1 falsework. The towers were then used to erect the first three arch members. Once they had been put into place, the tie-back system was installed and engaged, ultimately releasing the stage one assembly from the support tower, and allowing the support tower to be removed. The cantilever arch erection then proceeded from both the east and west to eventually close the arch.
The arch tie-back system was an integral part of the cantilever erection and was designed specifically for the arch construction stage. This required 900t per truss line – or 1,800t in total – to hold back the arch. The tie-back system consisted of cables that tied into each gusset plate and was anchored by a series of guy towers and backstays. The delta plate also served as an erection aid and each tie-back cable pinned into a plate that connected to the jacking rod clevis.
The jacking rod fed into the tie-back tensioning device equipped with one centre-hole jack and a lock-off nut. The tensioning device adjusted the cable lengths and thus the arch geometry, during the erection and arch closure. Four guy towers each received three tie-back tensioning devices, transferred the demands to the back-stay members, and directed the vertical components into the permanent approach-span abutment. The backstays were pin-connected to a grillage system that was anchored by 43m-long pretensioned rock anchors.
As erection progressed, the deflection of the arch as well as the tension in the tie-back system strands were monitored. The engineering group provided the field group with guidelines for the tie-back cable tension demands and panel point deflections according to American Bridge’s structural analysis model. The engineering group also provided jacking stroke distances for the adjustment stages.
Through a coordinated effort between the east and west tie-back system, the arches were adjusted until the bolt holes in the lower panel point were aligned and the field team was able to zip up the mid-span panel point connections starting from the bottom, using the tie-back system for further adjustment.
American Bridge’s scope of work also included skewback excavation, rockfall protection/fencing, site work, micropile foundations, form/pour concrete and mass concrete, and landscaping and site restoration. Great care had to be taken because the work was in very close proximity to the original foundations of the existing structure when performing skewback excavation.
As the construction process for the new bridge took place alongside a live railway line, American Bridge had to cooperate closely with Norfolk Southern Railway to plan operations around train schedules.
Once the structure was complete, with waterproofing applied to the concrete decks and the approaches leading to the bridge final-graded with sub-ballast, the bridge and new alignment was handed over. Norfolk Southern Railway then installed the ballast, laid the rail, made the tie-ins, and switched the traffic.
The new structure was opened in December 2017, allowing Norfolk Southern Railway to increase the loads of its freight trains and to run them at 40km per hour over the new bridge.
Work since then has been focussed on the demolition of the old structure. Most of the metal elements have been removed using cranes, but central sections were taken down with explosive charges over the course of three blasts in February and March this year.
There has been a strong demand for sections and elements of the old bridge.
A portion of one tower has been given to Letchworth State Park for a permanent historical display within the park, but many others have ended up in rail museums as well as in the hands of local residents. A request for such a memento was granted to a site visitor that had become engaged to her husband while standing on the old bridge, many moons previously; the site workers were rewarded with a consignment of 5,000 home-cooked biscuits the following week.
As with any project of this size and magnitude, particularly a project through a state park, a large number of permits were required. Two of the more unusual contract permit requirements for this project involved compliance with a bald eagle protection plan, as well as a timber rattlesnake protection plan. Approximately 366m upstream from the Portageville Bridge sits an occupied eagle’s nest. Because the eagles return to the same nest every year, Norfolk Southern Railway was required by the US Fish and Wildlife Service to monitor the eagles’ use of the nest as well as their behavioural responses to construction activities.
Each year from mid-January until the eaglets fledged the nest, Norfolk Southern Railway engaged an eagle monitor to observe and record their behaviour. American Bridge was required to take some precautions.
A concern raised was that construction noise might cause the eagles to abandon the nest and relocate. This was addressed through the installation of the micropiles via drilling rather than driven piles. The only other restriction related to limiting the number of blasts that could be carried each week during the skewback rock blasting and excavations; no more than three blasts could take place per side of the gorge every week. This never impacted the planned schedule and, since the project started, the nest was never abandoned; eagle chicks have successfully hatched in both years that American Bridge has occupied the site.
Timber rattlesnakes are also a concern in the area. Dens within Letchworth State Park are located several kilometres north of the bridge and the nearest sighting was within 1km of the existing structure. As existing or expected site conditions appeared to be suitable for summer migration, American Bridge had to comply with the protection of this species. A rattlesnake barrier was installed around the project’s perimeter, consisting of a silt fence tilted outward at a 15° slope to prevent snakes from entering the work zone, much to the contentment of the employees. Training on timber rattlesnakes, mandatory for all employees and subcontractors, involved instruction on what to look for as well as steps to take if one was found on the site. Since the project began, there have been no recorded sightings on the site.
Work is currently under way to remove the old foundations of the metal towers and the old railroad embankments, as well as creating extensive landscaping that includes re-establishing roads, hiking trails, and construction of a new viewing area on the west side.
The entire Portageville Bridge project is expected to be complete at the end of 2018. The latest update confirms that the eagles and their eggs are still there, and that they have made it through the latest demolition processes undisturbed.
This is an updated version of an article that first appeared in American Bridge Connections magazine, issue number 1008