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Federal Highway Administration Research and Technology
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|Publication Number: FHWA-HRT-05-007 Date: September/October 2005|
Publication Number: FHWA-HRT-05-007
Issue No: Vol. 69 No. 2
Date: September/October 2005
A recent rehabilitation project in Colorado bridges the gap between maintaining historical integrity and meeting current safety standards.
|(Above) Workers are beginning substructure work on the north approach to the Red Cliff Arch Bridge in Colorado's Rocky Mountain region.|
According to his peers in the transportation industry, many considered Colorado bridge design engineer King Burghardt to be ahead of his time. Today, more than 60 years after Burghardt designed one of his State's most aesthetically notable structures, bridge design and construction engineers at the Colorado Department of Transportation (CDOT) challenged themselves to "think historic" when planning to rehabilitate the Red Cliff Arch Bridge.
Historic preservation was a major consideration, as the bridge is located on a section of highway that represents an important link along Colorado's Top of the Rockies National Scenic and Historic Byway. In addition, the bridge itself is a historic structure and was added to the National Register of Historic Places in 1985. CDOT's historian and the Colorado Historical Society provided consultation early in the design phase, offering feedback on construction drawings. In addition, prior to the start of the work, the Colorado Historical Society photographed and videotaped the bridge to document the historical aspects of the structure.
While preserving the bridge's historical features, CDOT officials needed to update the structure to meet current Federal safety standards. "After 60 years of service and enduring the effects of weather at 8,790 feet [2,680 meters] above sea level, the bridge required major rehabilitation," says Peter Lombardi, CDOT project engineer. "The concrete, reinforcing steel, and paint on the structural steel were decaying to the point where they were straining maintenance forces' ability to keep up with the repairs."
The 64-year-old Red Cliff Arch Bridge-which carries U.S. Highway 24 over Eagle River in Colorado's Rocky Mountain region-is one of the State's two remaining steel arch bridges. CDOT officials expected the rehabilitation to be a daunting project, one that would require 21st-century design engineers to come to the table with historical sensitivities, and construction workers to come to the work site with the guts to deal with the inherent hazards of the project.
According to historical documents, the original work on the structure required long days of hanging from a cantilevered platform-61 meters (200 feet) above the canyon floor-in sometimes subzero temperatures. Burghardt had written in his journal, "In the morning, each gang was lifted to its scaffold on a platform hung from the high line. They took their lunches with them and spent the entire day in the air with the winter wind continually blowing up the canyon."
The rehabilitation project began in early March 2004, and the ribbon-cutting occurred the following November (176 working days later), just before the early winter snows began to fall. Even without having to contend with the subzero temperatures typical of winter in that region, crews still had their work cut out for them.
"We knew the rehabilitation project would present challenges yet be very rewarding," says CDOT Resident Engineer Keith Powers. "The structure hangs above a deep gorge, creating a tenuous situation for workers at times, environmental considerations to address, and impacts on the Red Cliff community."
The $3.6 million rehabilitation work focused primarily on replacing and widening the bridge deck as well as repainting the steel portions, while maintaining the historic structure's appearance. The project also involved extensive work on the abutments, girders, and bridge rail. By comparison, the contract for constructing the original concrete abutment, piers, and arch pedestals in 1939 totaled $218,817, and fabricating and erecting the massive steel arch cost $153,590.
|The contractor uses a backhoe to remove the architectural rail for refurbishment.|
|Using heavy equipment, workers remove the existing deck from the bridge.|
A number of concerns made the rehabilitation necessary and timely. Potholes in the bridge deck were increasing in size and becoming more commonplace. Due to the deteriorated condition of the wearing surface, maintenance repairs were not always effective, increasing the urgency of replacing the deck. In addition, the structural steel was corroding due to the failure of the paint system. The bridge had been painted three times since its original construction and required additional attention. But this time the entire paint system would need to be removed, down to the bare metal, and replaced with a new coating.
|Scaffolding installed under the bridge deck (above) provides a safe and efficient platform for the workers and inspectors. A view of the bridge from below (below) shows the scaffolding in place beneath the arch, where it helps protect the county road and river from falling objects.|
CDOT also completed preventive maintenance on the bridge piers and foundations. A shoulder retention wall near the bridge was secured to solid rock below by rock anchors to accommodate the widened template of U.S. 24 and prevent further sloughing of the fractured surface rock. This work included placing concrete around the toes of the bridge piers to curb the erosion that had occurred over the last 60 years. Workers removed the deteriorated surface concrete on the piers and, after dowelling new reinforcing steel into the sound concrete below, added concrete to bring the pier back to its original shape.
Other maintenance safeguards included rehabilitating the old bridge rail, which was cleaned of paint and rust and then hot-dipped galvanized for corrosion resistance. Micropiles were used for the new retaining walls north of the bridge to protect the roadway from further erosion.
"Accomplishing this project while keeping crews safe was a primary concern for us," says Matt Cirulli, project superintendent for Lawrence Construction, the primary contractor on the rehabilitation project. "One of the biggest challenges in the work was, of course, the height of the structure, which rendered conventional construction methods impractical."
The contractor used an innovative work platform to provide a safe and efficient area for the workers and inspectors, contain falling objects, and protect the county road, Eagle River, and Union Pacific Railroad tracks below. Instead of erecting traditional rigid scaffolding, cables were strung from brackets and hangers attached to the flanges of the girders and stringers. The crews then strung two spans of corrugated steel panels to form working platforms and tightened the cables for support. One scaffolding system provided access beneath the deck, and the other beneath the structural steel members of the arch.
Removing the existing paint and recoating the structure presented another safety concern. Given the age of the bridge, CDOT and the contractor assumed that the existing paint system included lead-based paints. Removing the lead-contaminated paint required crews to encase the work areas completely and provide proper venting and containment of all paint removed. The workers used negative vacuum pressure to ensure containment as they sandblasted the paint, ran it through a filter, and bagged it for safe disposal in a landfill. Because of these safety procedures, crews ultimately were successful in removing the existing paint without harming the environment or their own health.
"This work was no small task," Powers says. "Our crews had to labor in a tight canyon, dealing with the effects of high altitude, fast-moving storms, high winds, and tough access. All this meant that containment was constantly being attended to."
To protect the bridge from corrosion, the coating contractor removed the old lead-based paint down to the bright metal and repainted the structure with a new three-coat paint system that retained the same green color of the existing arch.
Like the new paint, the bridge repairs and upgrades also needed to blend visually into the structure to maintain its historic integrity. "When designing the rehabilitation, we minimized the visual changes to the bridge and adjacent roadway," says CDOT Bridge Designer Andy Pott. "All architectural elements such as the wingposts and pylons were duplicated from the original drawings."
The new exterior curb approximates the original curb with similar rail supports. Workers refurbished the original rail and reused it on the widened 1.8-meter (6-foot) deck, placing it outside a new crash-tested Type 10 bridge rail (a two-tube steel rail with concrete curb), so views of the bridge from a distance remain the same.
"In removing the concrete deck, crews used 92-centimeter (36-inch) diamond-blade concrete saws to cut away portions from the bridge. Each portion could weigh no more than 2,270 kilograms (5,000 pounds). The workers used track excavators with hydraulic thumbs to peel off pieces of the crumbling deck. The work proceeded from the middle span so the bridge was unloaded evenly from alternate sides, keeping the structural loads balanced. To protect the structure, placement of the cast-in-place deck also was sequenced in a similar manner.
|With the concrete deck and abutment removed, the bridge awaits placement of the new deck.|
After the concrete decking was removed, crews jacked up the girders and replaced the concrete abutments on both ends. Working within tight tolerances and maintaining a high degree of craftsmanship also were among the challenges. According to project manager Cirulli, deciding on a concrete form for the pylons took a lot of time and effort. The contractor could not find a ready-made form on the market to cast the four corner pylons, and building the form in the field individually for each corner would have required additional time. Instead, the contractor commissioned a single, strong hand-built form that could be reused for the four pylons. The form was in two pieces, clamped together to create a pylon, which allowed the concrete to be poured in place monolithically. Once the concrete had enough strength, the form was removed and used to pour the next pylon. Workers used a forklift to move the forms. This procedure expedited the project, as the form was reused instead of rebuilt.
|During the paint removal process, shown here, workers enclose the work area in vacuum-sealed plastic to contain and collect lead-based paint debris for safe disposal offsite.|
To increase the safety of the girder supports and strengthen the bridge to handle heavier loads, the contractors welded and bolted steel brackets to the structure and straightened the steel components that had been struck over the years by falling rocks. Workers employed heat-based straightening techniques to remove the dents. To avoid future damage, a subcontractor draped a cable net on the rock face of the north abutment, cable-lashed the large outcroppings, and removed the loose rocks on the south abutment slope. Brackets added to the girder connections supplement the support provided by the existing clip-angle connections. To strengthen the bridge to carry heavier loads, shear connectors were added to the existing girders to make them composite with the deck. The girders and deck now act as a single unit to resist loads. This modification increased the load rating of the bridge enough to remove load restrictions.
The new cast-in-place deck features a silica-fume concrete, which is a newer CDOT mix (Class H) selected for its durability and ability to be left as a bare deck without a waterproofing membrane and overlay. The dense silica-fume mix resists water intrusion more effectively than ordinary concrete and provides additional protection to the deck's reinforcing steel.
|A worker wearing a safety harness and protective clothing is spraying a new coat of paint on a bridge girder.|
Aside from the challenges involved in the construction process, CDOT needed to manage the important issue of media outreach and public relations. The rehabilitation work necessitated a full closure of the bridge from May to July 2004, affecting not only travelers but also residents and business owners from the town of Red Cliff, which is nestled into the landscape below the bridge. Though Red Cliff remained accessible via secondary roads near both ends of the bridge, messages regarding the bridge work and highway closures were sure to confuse some motorists.
"Traffic was a significant inconvenience for motorists, as well as the residents of Red Cliff, who ultimately endured increased through-traffic through their narrow town streets," CDOT's Lombardi says. "Although residents did not want detoured commuter traffic passing through town, they did not want to discourage the tourists from visiting. Trying to convey these two polar messages was a challenge."
|Workers use a screed and finishing machine to smooth the concrete applied during the second deck pour on the north end of the bridge.|
CDOT met with community members prior to the project start date to work through the details of communicating the bridge closure without discouraging visitors to Red Cliff's restaurants and art studios-the lifelines of this small tourist community. Press releases, variable message signs, public announcements, and flyers all were relatively successful, according to CDOT officials, but a predominant message prevailed: ROAD CLOSED.
"We definitely took a hit when the bridge closure first began," says then-Town Manager of Red Cliff Guy Patterson, who contacted several area chambers of commerce anonymously to pose the question, "Can I get to Red Cliff from I-70?" "It took some additional, individual public relations to relay the message that Red Cliff was open for business-and we did okay after the bridge reopened. I have to say, the bridge looks great-we were glad it was done."
Another critical issue for the town was the wear-and-tear on its local roads due to the detoured traffic, including construction vehicles. When the bridge work was completed, construction crews patched and applied a chip seal to the road through Red Cliff.
|In October 2004, as the project neared completion, workers put the final touches on a new name plaque mounted on a concrete wingpost, an architectural feature that was removed and replaced during construction.|
Although the Red Cliff Arch Bridge received quite a facelift, its "surgeons" were careful to hide the evidence. The major changes were made primarily where safety was concerned: The structure was widened to accommodate modern traffic volumes, new bridge railing was added, and new materials were used where possible to enhance the bridge's service life and durability. But as far as its appearance is concerned, only the most discerning eye or astute historian will detect any modern updates. Most will simply see that the bridge looks brighter, perhaps cleaned up a bit. And that was the goal.
"The most important visual aspect of the bridge-the arch itself-was left the same," Pott says. "We merely cleaned and repainted the bridge to maintain it for future generations to enjoy."
|This custom-built form enabled the contractor to pour the four pylons quickly without having to rebuild the form, saving valuable time on the project.|
Note: The Red Cliff Arch Bridge was placed on the National Park Service's National Register of Historic Places in 1985, and the survey is archived at the Colorado Historical Society, Office of Archaeology and Historic Preservation, in Denver.
The author would like to thank Carol Carder, a contract writer for CDOT and FHWA, for contributing to this story.
Nancy Shanks has worked in the CDOT Public Relations Office for 8 years. In 2002, she relocated to the department's Durango office in the southwestern corner of the State. She is the public relations manager for CDOT's two western slope regions.