Out with the old, in with the new—in less than 36 hours. The latest technology in bridge construction has come to Maryland via two structures that were replaced, each over the course of two weekend evenings. KCI provided construction engineering and inspection services for the Maryland Department of Transportation State Highway Administration’s (MDOT SHA) first project using a self-propelled modular transporter (SPMT) to replace two bridges carrying West Nursery Road over the Baltimore-Washington Parkway (MD 295). The new superstructures—the girders and deck—were built in the median and then driven down the parkway to be set in place on the existing abutments, which had been rehabilitated.
LOCATION, LOCATION, LOCATION
Though relatively small compared to spans that have been moved in other states, the West Nursery Road site proved to be an ideal location to test this technology. Built in 1948, the existing 70-foot-long simple-span bridges were weakening. “Our philosophy was to start small and eventually work our way up,” said MDOT SHA Project Engineer Jeffrey L. Robert, PE. “We have been keeping up with this technology and the benefits. At this location, it just seemed like the site conditions were perfect.” Determining factors included traffic volumes, geography and utilities.
While West Nursery Road feeds a prominent business and hotel district serving Baltimore/Washington International Thurgood Marshall Airport, MD 295 is a major connector between two major metropolitan regions. The combination of heavy volumes of traffic and a lack of convenient detours made traditional construction methodologies and extensive lane closures unattractive.
Just north of the interchange, the median offered a clean ingress and egress with a wide enough tract for a staging area and bridge farm, where the new structures were built away from traffic. Minimal utilities allowed contractor G.A. & F.C. Wagman Inc. to drive the piles required to construct 18-foot-tall temporary bents to hold the new superstructures, which had to be fabricated at the same height as the existing spans to facilitate the move. Engineers completed a survey layout to mirror the existing structure in the median, ensuring that the transporter was picking up and setting down the deck in the same orientation.
DOING THE HEAVY LIFTING
SPMTs are multi-axle platform-type vehicles designed to haul and lift oversized and heavy cargo using hydraulic rams. They can be coupled side-to-side or end-to-end with multiple transporters computer controlled as a single unit. The SPMT’s wheels can rotate 180 degrees, affording the flexibility and maneuverability to make the zero-radius turns and small incremental adjustments required to precisely set massive loads in place.
Bridge construction has changed from a focus on new construction to a focus on maintaining the existing bridge inventory. Needed are innovative ways to maintain traffic while quickly replacing deteriorated bridges with bridges that last longer.
Federal Highway Administration
In 2004, representatives from the Federal Highway Administration, American Association of State Highway and Transportation Officials, and the National Cooperative Highway Research Program conducted an international scan of prefabricated bridge elements and systems. Use of SPMTs was the top recommendation. Although Europe has been the leader in using these construction methods for more than a decade, similar bridge moves have been successfully completed in several states.
“There are only a handful of companies in the United States that have this specialty equipment and experience using it,” said Robert. “We held off-site visits with them to review and get input on what we were proposing.” Despite the challenges of unfamiliar technology, accelerated bridge construction utilizing SPMTs lends distinct advantages to the project in terms of improved safety, reduced schedule and cost savings.
Every year, more than 20,000 workers are injured in highway work zones, twelve percent attributed to transportation incidents. The Federal Highway Administration reported that 514 motor vehicle crashes at construction sites resulted in fatalities in 2010.
At West Nursery Road, instead of demolishing and building the bridges in place, much of the work took place in the median, reducing the interaction between construction crews and motorists. “When you build a bridge away from traffic, it’s safer for the workers and the traveling public who have to go through the work zones,” said Robert, who believes that improved safety is the most important benefit of utilizing this approach.
KCI senior inspector Travis K. Root agrees. “Unless you have worked within two feet of a car or tractor trailer going 70 or 80 miles-per-hour, you have no idea what it feels like,” he said. “My family is really grateful for this design.”
ON THE CLOCK
Building the bridges off site not only improves safety, it significantly compresses the overall project schedule. Conventional bridge construction follows an on-site sequence, where SPMTs often collapse in-traffic phases to a single step—moving the span into its final position—while minimizing impacts to motorists.
“Where restrictions typically limit when traffic patterns temporarily can be changed, in the medians you can work as long as you need,” said Root. “Demolition and construction can occur simultaneously.” Mobilizing day and nighttime shifts significantly shortens the project schedule. At West Nursery Road, the overall duration was reduced from two years using conventional methods to nine months with the SPMTs, the pinnacle being two consecutive weekends in November when closures and detours facilitated the actual bridge moves.
An average of 22,000 vehicles cross West Nursery Road each day, while the Baltimore-Washington Parkway carries more than 90,000. “With phased construction, changes to the West Nursery Road traffic pattern would have negatively affected traffic for hundreds of thousands of motorists through the entire construction schedule,” said Root. “The general public has no idea how much inconvenience was avoided.”
DOLLARS AND SENSE
That inconvenience has a dollar value, and the economy of traffic flow along with the potential for reduced maintenance and construction costs, create the financial benefits associated with a bridge-on-wheels approach.
“There’s a cost to the public when they’re delayed or have to travel alternate routes,” said Robert. “On this job the user cost was about $2,000 a day, seven days a week.” That rate is based on a combination of fuel consumption and values computed for lost time. By compressing the schedule and minimizing lane closures, MDOT SHA is generating more than $400,000 in user savings.
Part of this project’s goal is making the next project even more successful. Everybody brought their A-game and most have gone above and beyond their regular duties and responsibilities.
Travis K. Root, CMITSenior Inspector
In more tangible terms, building this bridge through conventional methods could have increased construction costs by between $500,000 and $1 million. “There’s a heavy traffic volume in that area, and analyses indicated that lane reductions and detours were impractical for any significant amount of time,” said Robert. “We would have had to build a temporary bridge during the rehabilitation.”
LEARNING THE ROPES
Another way the project is saving taxpayer dollars is by paying it forward both for the state and the industry as a whole. The West Nursery Road bridges are part of the Federal Highway Administration’s Highways for Life program that strives to improve safety, reduce congestion and boost the quality of longer-lasting highway infrastructure through accelerated construction techniques. MDOT SHA received a $600,000 grant from the program and in exchange is sharing the lessons learned during design, fabrication and placement.
“We all collaborated and looked ahead to identify any problems and solve them ahead of time,” said Robert. “All the players were aware that this project will set the bar for our state and the use of this technology.”
KCI inspectors documented concerns, creative ideas and modifications, ranging from recessing anchor bolts into the bridge deck to modifying deck grooving specifications.
EXPECT THE UNEXPECTED
At no time was that collaboration more evident than when disaster struck during the first scheduled lift. Just after midnight, the existing bridge carrying West Nursery Road over northbound MD 295 shifted off the SPMT and lodged itself on the west abutment.
The project team rallied to first ensure the load was stable and the SPMTs weren’t damaged, and then to identify alternatives that would restore traffic as quickly as possible. “We chose to abandon the idea of moving the existing bridge to the staging area that weekend, and instead focused on developing avenues to put it back in place,” said Wagman project manager Scott A. Miller. “This approach had the least amount of impact to the traveling public and also was the safest way to move forward in regard to worker and public safety.”
With over 20 years of contracting experience including time spent as a crane operator, Root was part of the team suggesting a synchronous lift utilizing multiple 350-ton or larger cranes. He also helped identify what companies might have equipment nearby. By noon, specialty subcontractors had dispatched more than 30 trucks carrying the parts and weights needed to assemble four massive cranes, and by early evening, crews were inching the superstructure back into its original position.
“The recovery went exceptionally well,” said Miller. “The equipment, labor and material resources coordinated in that time frame exceeded expectations, and the actual lift was well planned under duress and executed to perfection.”
What could have been a disaster turned into a shining example of collaboration and quick response to the unforeseen circumstances that often occur when pursuing leading-edge technologies. Less than 24 hours after the incident, state and independent engineers had completed thorough inspections of the existing reset span, and West Nursery Road and MD 295 were both reopened to traffic.
ON THE ROAD AGAIN
Two weeks later, in deference to Hurricane Sandy, the SPMT executed the first of several perfect lifts, moving the existing structure onto temporary bents in the median. With only feet of wiggle room on either side, the new span was maneuvered and lowered into place the following night.
“Crews executed a safe, flawless bridge move and restored traffic ahead of schedule,” MDOT SHA Administrator Melinda Peters told The Baltimore Sun. The agency plans to use the approach in the future.
Although it may have been a bumpy ride near the end, the project is still on schedule for completion in early 2013. The lessons learned, both during construction and the crisis, will go a long way toward ensuring accelerated bridge construction methods find their place in the nation’s and the state of Maryland’s transportation program.