Today’s competitive economic climate, along with the public’s demand for faster delivery, is pushing designers and contractors to stretch innovation to its limits.
The Nevada Department of Transportation (NDOT) satisfied both priorities last January by lifting an 1,100-ton, fully constructed bridge and sliding it 60 ft across I-15 into its permanent location in a matter of hours—and without closing I-15.
The innovative design-build project was Nevada’s first use of accelerated bridge construction (ABC) technology. The choice to use ABC on the $15 million project was no gamble. It meant NDOT would build the bridge six months faster and for millions less.
Cost more, save more
The I-15 bridges were part of the West Mesquite Interchange project to widen Falcon Ridge Parkway in Mesquite, Nev., and increase capacity for future traffic demand. The improvements, which were completed in April 2012, widened the parkway, increased the bridge length, added roundabouts at interchange ramps, replaced 1,200 ft of existing mainline pavement and featured new landscaping and lighting to create a welcoming entrance to this popular tourist destination.
NDOT wrote the request for proposals with the understanding there could be significant closures and impacts on I-15 to rebuild the interchange. In addition, NDOT assumed the interchange would be built in a new location, which would add to the cost. Utah-based contractor W.W. Clyde and designer Horrocks Engineers presented a plan that used ABC to realign the interchange and keep it in its existing location. These ideas reduced the original project estimate of $25 million to less than $15 million.
“We knew that ABC would cost more, but we could make up the difference saving on lane rentals, building the crossovers, limiting excavation and reducing time and overhead,” said Mike Dobry, lead structures engineer with Horrocks. “Once we demonstrated the construction costs and user savings, NDOT immediately recognized the opportunity to try ABC. The team went in with the attitude of making it work.”
After making the choice to move forward with the new design, the team had to work through some initial challenges, according to Marty Strganac, NDOT’s resident engineer on the West Mesquite Interchange project. The team successfully addressed concerns of technical unknowns, oversight of precast materials and other learning curves since none of the partners, including W.W. Clyde and Horrocks, had ever slid a bridge before.
NDOT was not late to the nationwide ABC party; rather, they were waiting for the right opportunity to try this technology in their state. NDOT believes the Mesquite interchange was a prime location for the state’s first bridge slide.
“We had ample existing right-of-way adjacent to the final location where the bridges could be built on their temporary foundations,” said Strganac. “The interstate was also an overpass on the bridges at this location, which meant we could route traffic down the ramps during demolition and the slide and avoid any closures on I-15.”
Other elements that made this location ideal for ABC included the high traffic volume that justified paying more for faster construction, the lack of viable alternate routes and the fact that precast materials could be used.
“We can’t do ABC every time, but this location definitely had the right formula,” said Strganac.
A primary concern for keeping I-15 unrestricted during construction was the effect closures could have on freight mobility.
“Freight accounts for 25% of the vehicles that go through this corridor every day,” said Strganac. “Traditionally we would have had lane closures for up to a year on I-15 to construct the bridge, which would have affected commerce. Instead, we found a way to replace the bridge with no delays for trucks or commuters.”
NDOT reported Mesquite residents and I-15 commuters saved nearly $13 million in time and fuel by eliminating the need for closures, detours and reduced speeds. The local residents also benefited from the Falcon Ridge Parkway widening being shortened from months to days and avoided 10 minutes of additional travel time to the city’s only hospital.
NDOT worked in close coordination with the city of Mesquite, which procured the federal funding for the project. Kurt Sawyer, interim city manager, recognized how reducing the closure time would immediately benefit drivers and local businesses.
“There was such a low impact to motorists and businesses compared to typical construction methods,” he said. “We especially appreciated NDOT’s efforts in ensuring there was access during special events in Mesquite, preserving the economic vitality of the businesses.”
Teams recognized the added safety benefits of building the bridges adjacent to the freeway and out of live traffic.
According to Strganac, “Public safety is the No. 1 priority on any NDOT project. It drives every decision we make. By constructing the bridge in this innovative manner, we didn’t have to divert traffic like we would on a traditional project. This kept the construction area safer for vehicles, pedestrians and the workers.”
Upholding NDOT’s safety commitment meant extra planning for the team. From design, scheduling, surveying and construction, every step took additional time to prepare for the move.
Alan Preston, construction project manager for W.W. Clyde, said, “The project team had contingency and safety plans in place for various potential scenarios during the move. These scenarios ranged from disruption of material delivery to equipment failure to the guardrail not being completed
More than 150 engineers, designers, craftspeople and safety personnel were on-site for each 56-hour start-to-finish move. Each was ready to put the plans for any potential scenarios into action.
“The night of the slide, we had enough equipment on-site to excavate both sides of the road at the same time, plus backup machines and extra craftspeople, like welders and mechanics, just in case,” said Preston. “We couldn’t take a chance with a hard and fast window.”
The extra planning paid off when the second bridge did not slide as smoothly as hoped and was coming in askew. Personnel were able to reset the bridge and start again without losing much time.
Preston also ensured safety provisions were put into place so workers and the public were able to negotiate the detour. Constructing the bridge using ABC eliminated the need for crossovers and detours. This kept the construction area safer for workers, motorists and pedestrians. Crews installed temporary concrete barriers on either side of I-15 to prevent live I-15 traffic from exiting into construction areas and also to prevent construction equipment from crossing over into I-15 traffic during the slide.
Fast and steady
Lessons and benefits continued to be learned by the groundbreaking project team during construction and the bridge slide itself.
Bridge construction began in mid-November 2011 adjacent to the freeway. Crews accelerated the work using precast concrete girders and partial-depth deck panels. The actual construction of the single-span bridge followed the same steps as a traditional build-in-place construction, although extra care was necessary to ensure a quality final product.
According to Dobry, “Building a bridge in place can be forgiving and all dimensions are relative until you build the substructure. When the bridge has to slide onto another substructure, we had to do extra survey and triple-check our dimensions to be sure it would fit exactly in the right spot.”
Also a first for NDOT, temporary hollow-bar soil nails were used to support the existing structure while the permanent foundations were constructed underneath the existing bridge. Installing the soil nails was a viable solution, but also one that required additional monitoring.
“We were nervous the existing bridge would settle because it was under live traffic and we didn’t want any interruption for drivers on the existing bridge,” said Dobry. “We monitored the existing abutment to make sure it didn’t settle during the installation of the soil nail wall because it was built very close to the existing abutment spread footing. We made sure to back up everything with our calculations.”
To expedite the move and to avoid using a complicated and time-consuming precast alternative, the design team decided to move the approach slabs with the bridge. The approach slabs were built on temporary falsework and shoring along with the rest of the superstructure. All three pieces were elevated together, tied with reinforced cables and rebar and coordinated with the jacking system. Approach slabs were slid onto horizontal steel grade beams.
The bridge slides were done in two separate moves (I-15 southbound on Jan. 10 and I-15 northbound on Jan. 24) over two 56-hour periods with careful sequencing.
First, crews rerouted traffic on I-15 through the interchange on- and off-ramps that had been temporarily widened to two lanes for the closure. Next, the existing bridge was demolished, which took about 12 hours.
“At the same time the structure was being torn down, we were tearing up concrete and breaking it up on-site just to keep on schedule,” said Preston. “The concrete was hauled to a local golf course that used it for a new flood-control dike. We also recycled the removed asphalt as aggregate base in new road sections of the project.”
About 24 hours into the closure, crews began the slide. Using specialized hydraulic jacks operated by Mammoet, an international company that specializes in heavy lifting, the bridge was lifted 3 in. into the air from its temporary foundations to clean and lubricate the bearings below. Crews applied gallons of lubricating dish soap to Teflon-coated elastomeric bearing pads and locked the bridge into the slide rails. When pushing from the temporary support, the hydraulic ram had “ears” that slid into notches cut into the slide rails, forcing the bridge to move in only one direction. The jack stayed within this frame, moving the bridge from notch to notch, about 38 in. every 2 minutes, until it was in place.
“Even though it was a single-span bridge,” said Dobry, “it seemed like a three-span because of the falsework and shoring and extra coordination needed for the jacks. The steel grade beam system was essential to slide all three pieces as one.”
The final hours were spent replacing 300 ft of asphalt on both sides of I-15, tying the road back into the bridge and replacing the entire guardrail.
The actual slide on the south half took one hour and 15 minutes, and the north half, a more complicated effort, still only took an impressive five hours. The bridge reopened to I-15 traffic after each slide, and the permanent concrete tie-ins were completed in early February.
Putting on a show
ABC technology is gradually being adopted by other departments of transportation (DOTs) across the country. The NDOT bridge slide gave more than 150 representatives from 24 different DOTs, the Federal Highway Administration (FHWA) and industry the chance to see the innovation firsthand. The esteemed group included FHWA Administrator Victor Mendez. The group watched on-site before attending an FHWA-sponsored showcase to discuss the design and engineering associated with the feat. The FHWA supported travel to the showcase through its Highways for LIFE program.
Preston was grateful for the support of the national colleagues and for the chance to learn from them, although he admits, “Having the FHWA showcase be part of the project did add pressure to not only perform the night of, but to also stay on schedule for the entire project to ensure we were ready to slide that night.”
More than 200 residents also came out to view the moves.
While not every Nevada bridge replacement will be a candidate for ABC, NDOT is looking to apply the technique to other projects where it makes financial sense and there is enough space to build the bridge on the side of the roadway.
For now, NDOT is happy to hang its hat on its first successful use of ABC.
According to Strganac, “We shortened the bridge construction from months to days, an important east-west connection was kept open for Mesquite residents and hospital emergency services, an aging bridge was replaced and a vital freight corridor was unaffected. Given this option, I don’t know why we would have done it any other way.” R&B