Feature | Phil Christian, P.E. | October 07, 2016

This is how we roll

NJ Transit employs ABC techniques to replace two commuter rail bridges in less than 60 hours each

Replacing bridges on busy commuter rail lines is a challenging proposition. Not only do owners have the physical cost of bringing these aging assets into states of good repair, they also must consider the user costs—the economic burden of impaired mobility and service disruption—that did not exist decades ago when the bridges were built. Lengthy track outages and construction phases can be devastating to surrounding communities and commuters, not to mention fare boxes.

 

New Jersey Transit saw an opportunity to renew its Park Avenue and Watchung Avenue bridges using accelerated bridge construction (ABC) techniques. The historic bridges on the agency’s busy Raritan Valley line carry 27 trains and more than 20,000 passengers per day over congested Park and Watchung avenues in downtown Plainfield. The pair of steel, through-girder bridges were part of New Jersey Transit’s ongoing capital program to bring all bridges on the rail system into a state of good repair.

 

Each bridge was nearing its 100th year in service and both were showing their age. The superstructures were in poor condition and low vertical clearances were pockmarked from truck impacts. Replacement would mean giving each bridge a new main roadway span, bordered on each side by new sidewalk spans, and, if engineers could figure out a way, a higher vertical clearance.

 

The Park Avenue and Watchung Avenue bridges were replaced with ABC.

 

After exploring replacement options, New Jersey Transit landed on an innovative ABC technique using self-propelled modular transporters (SPMTs). SPMTs roll on multi-axle platforms operated through state-of-the-art, computer-controlled systems. The motorized vehicles are capable of pivoting 360° to lift, carry and set very large, heavy loads. Once the payload is delivered, the SPMT exits the site, opening the area to traffic within hours.

 

They would be perfect, transit officials felt, for removing and delivering large bridge spans in Plainfield’s constrained environment.

 

“Extended service outages would have caused severe mobility issues—not only on our commuter line but also in the surrounding community,” said Lisa Fanning, project manager for New Jersey Transit. “ABC presented the opportunity to keep the rail service running continually, reserving two weekends for quick superstructure replacements. We had used other ABC methods, but the Park Avenue and Watchung Avenue bridges would be New Jersey’s first use of SPMTs on a rail-transit project.”

 

Crews repaired abutments and piers in addition to replacing existing superstructures.

 

The in & out method

HNTB Corp., the project’s engineer of record, recommended the roll-in/roll-out replacement after reviewing the project’s unique circumstances.

 

The use of a traditional lateral slide-in/slide-out on temporary bents in the roadway was not feasible. The proposed bridges could not be built, and the existing bridges could not be accommodated side-by-side on the existing alignment. It would have created significant impacts to local businesses and restricted station access in an already congested downtown.

 

Both bridges are directly adjacent to Plainfield Station, with one bridge at each end of the station platforms. Staging construction by switching tracks between structure bays was not a viable option. With SPMTs, the majority of demolition and construction could take place at an off-site staging area, safely away from the motoring public.

 

Moreover, restricting access to downtown Plainfield for weeks or even months would have been a tough sell. Instead of “spreading out the pain” of bridge replacement across several weeks or months, SPMTs meant New Jersey Transit could take a surgical approach, like receiving a vaccination in the doctor’s office: A quick stick and then it’s over. And with the SPMT, crews could plan exactly when the switch out would take place, timing it specifically when there was the least amount of traffic in and out of downtown.

 

New Jersey Transit also operates buses in the Plainfield Station area. It would have been a challenge, a potentially expensive one at that, for the agency to reconfigure bus service for months on end to accommodate rail commuters around the Plainfield Station in addition to maintaining its normal bus routes in the area.

 

Because SPMTs are cumbersome and slow—they move at walking speed—for them to be effective, the off-site construction area needed to be near the bridge. New Jersey Transit was able to procure a portion of a city street across from the station, just 400 ft from either bridge. As a result, the agency had an opportunity to bundle projects. With the Park Avenue and Watchung Avenue bridges being in close proximity to each other, mobilizing workforces only once—instead of twice—could result in a big savings.

 

The self-propelled modular transporter allowed crews to quickly make the switch out.

 

Planned to the minute

New Jersey Transit agreed to a single-weekend two-track outage for each bridge. The Watchung Avenue Bridge’s superstructure would be replaced over the 2015 Halloween weekend. Park Avenue would follow three weeks later in November, in advance of the Thanksgiving holiday. Each 53-hour construction window would begin at 12:01 a.m. on a Saturday morning and end at 6 a.m. the following Monday.

 

Replacing a bridge in a single weekend only works when each final-stage construction task is planned out to the minute and everything is set up well in advance. Crews would need to take the tracks out of service, remove them, pick up the bridge’s superstructure and move it out, move the new superstructure in and restore the tracks to finish on time.

 

Each superstructure span—the two individual sidewalk spans and the main roadway span—was stick-built in its entirety on temporary bents at the off-site location. During the weekend outages, all the contractor had to do was load the main roadway span onto the SPMTs and transport it to the bridge site. The shorter, lighter end spans were delivered by truck to the bridge site and erected into place by crane.

 

HNTB designed the new bridges to mimic their predecessors, which were steel through-girder structures. The new superstructure consisted of A709, Grade 50 steel, which was painted to minimize corrosion and match the appearance of the original bridges. Additionally, A325 high-strength bolts were used for all connections. Tension-control bolts were employed, because their rounded button heads, visible to the public, resemble the look of the old bridges’ riveted construction.

 

As soon as the last trains came through on Friday night into Saturday morning, NJ Transit forces swarmed the bridge, removing the rails and ties from the bridge deck before turning the tracks over to the contractor. The contractor then had the very tedious task of removing all of the ballast from the bridge in order to make the spans as light as possible for removal and transport, a process that took several hours.

 

Following that, the contractor cut the sidewalk end spans from the main roadway span and plucked them out as single units using a crane. Each end span was placed on a waiting flatbed truck and carried off-site for further demolition.

 

Once the end spans were removed, the contractor signaled for the SPMT. The SPMT, carefully positioned under the existing main roadway span, lifted the span off the pier bents and rolled it to a construction staging area adjacent to the off-site location. This construction staging area was necessary, as the primary off-site location was occupied by the replacement bridge superstructure units awaiting transportation to their new location.

 

Meanwhile, back at the bridge site, crews completed the final substructure repairs and prepared the substructures to receive the new superstructure. Substructure work included pier rehabilitation, repairs to the concrete abutments and rehabilitation of the bearing pedestals. Additionally, to accommodate an 8-in. increase in vertical clearance, pier column extensions, bearing grillages and abutment backwall extensions were installed.

 

Just as crews were finishing up the substructure repairs, the SPMT arrived carrying the new main roadway span. After it was set into place, flatbed trucks returned with the new sidewalk end spans, which were lifted into place as complete units by crane and connected to the main roadway span. Crews then prepared the bridge decks to receive the ballast by installing the final sections of spray-on waterproofing membrane and ballast mat.

 

The bridge was then turned back over to NJ Transit crews, who used the remaining time to bring in new rails and ties and set everything to final grade. The new Watchung Avenue Bridge was open by 6 a.m. On the second weekend, the Park Avenue Bridge was opened several hours earlier than scheduled because of lessons learned and efficiencies that were gained on the first bridge and applied to the second bridge.

 

NJ Transit agreed to a single-weekend two-track outage for each bridge.

 

In the mainstream

Both bridges were replaced for approximately $8 million. On average, ABC may cost slightly more than conventional construction, but it becomes much more economical when owners factor in the user costs. It is not correct to say ABC saves an owner money, but if you wanted to put a value on the inconvenience to passengers and the public, as well as the time-dollar value it would cost a community to have an important rail line out of commission for weeks on end, then owners save big time.

 

ABC is fast becoming a go-to methodology for smaller, urban bridges—railroad bridges, in particular. It is really the only approach commuter railroads can take, because passengers depend on service to be active daily. In the coming years, every Northeast transit agency will face the circumstance of needing to replace its bridges, especially if the assets are more than 100 years old, as many are in this pocket of the country. Keeping the transit lines in service will dictate owners find new, faster replacement methods. Once the public experiences the benefits of ABC, as it did in Plainfield, N.J., it becomes difficult for owners to go back to the business of replacement as usual.

 

 

About author: 
Christian is a project manager for HNTB Corp.
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