Replacing Historic Bear Creek Bridge
Colorado recently rebuilt an 88-year-old bridge along U.S. 550, overcoming historical, geological, and logistical challenges with panache.
In the San Juan Mountains of southwestern Colorado, the stretch of U.S. 550 between the towns of Ouray and Silverton is known as the "Million Dollar Highway." Originally built in the late 1800s, the two-lane highway is a major north-south route that takes motorists past scenic views and historic sites and through steep grades and hairpin curves. Just 2 miles (3.2 kilometers) south of Ouray, the roadway crosses Red Mountain Pass and the historic Bear Creek Bridge over a stream and dramatic waterfall. The Colorado Department of Transportation (CDOT) recently completed a $4.86 million project to replace the bridge.
Bear Creek Bridge was constructed in 1883 as part of a private toll road. In the early 1890s, the owner transferred the toll road to Ouray County, which later conveyed it to the State's division of highways (now CDOT). CDOT constructed a new bridge in 1922 and made improvements in 1962. Today, the bridge carries an annual daily average of 2,200 cars and is an important part of the State's San Juan Skyway.
In 2007, however, the structure was considered functionally obsolete and received a sufficiency rating below 50. The rating made the bridge eligible for Federal Highway Bridge Program funds and State funding from Funding Advancements for Surface Transportation and Economic Recovery (FASTER), legislation developed to address Colorado's structurally deficient bridges and safety projects. The State's decision to provide funding for the project reflected widespread recognition of the importance of replacing the bridge given its location.
"This bridge replacement project came with significant challenges, such as its historic designation, extremely narrow confines, and lack of a reasonable alternate route, not to mention weather and geotechnical concerns," says Ed Archuleta, CDOT Region 5 resident engineer. "We had to approach every aspect of the project with innovation to build a safer, vastly improved structure within the confines and get it done quickly."
Because of the many restrictions and concern about keeping the road open to traffic, CDOT decided to tackle the project with a two-season construction schedule. The first season ran from July through October 2010 and involved rock removal and road widening. After a winter shutdown, required because of the area's heavy snowfall, crews headed back in April 2011 for the second season, which entailed replacing the bridge using accelerated construction techniques. CDOT's innovative decisions and detailed planning helped to complete the remainder of the project in just 6 months and accomplished the goals of a safer and improved structure.
Special Project Considerations
Bear Creek Bridge is part of the San Juan Skyway, which lays claim to several scenic and historic designations, including National Forest Scenic Byway, Colorado Scenic and Historic Byway, and All-American Road. The bridge itself has historic significance as the site of a monument honoring Otto Mears, a road and railroad builder who was responsible for constructing the original Bear Creek Bridge as part of a toll road to connect Ouray and Silverton. The toll booth for the road stood at one end of the original bridge. In 1926, The Silverton Standard newspaper raised the money to place a monument in Mears' honor at the site. The monument and the bridge became local tourist attractions.
The location of the bridge added a significant challenge for CDOT. During construction, closing the road and using the only possible detour would have added nearly 1.5 hours to motorists' travel times, an option that CDOT immediately ruled out. The bridge stands nearly 200 feet (61 meters) above the canyon and directly above a waterfall. CDOT had to deal with space constraints because the bridge and adjacent roadway are narrow -- ranging from 24 feet to 36 feet (7.3 meters to 11 meters) wide and without shoulders at the construction site. Steep road grades and tight curves between Ouray and the project site required CDOT to place size restrictions on all equipment and materials to be used at the site.
In addition, at nearly 8,000 feet (2.4 kilometers) elevation and in a storm path, the project site faces heavy winter and spring storms, meaning the construction season for the Ouray area is brief. The long winter season and heavy snowfall presented a major challenge for scheduling construction. The abbreviated construction season, usually lasting only 5 months (May-September), forced CDOT to look for innovative means for completing the task quickly.
When designing the new bridge, CDOT took each challenge into account. Whether it was obtaining archeological, historical, and paleontological clearances; working hand-in-hand with the U.S. Forest Service to ensure that the design upheld historic and scenic standards; maintaining at least one traffic lane during construction; or implementing accelerated bridge construction techniques to complete the project in two seasons, CDOT left nothing to chance.
"We put a lot of time into the plan--ning of this project," says Archuleta. "We recognized the special issues and spent a lot of time on the design to make it work with the site."
Designing Around The Challenges
Early on in the planning process, CDOT decided to employ accelerated construction techniques such as prefabricated bridge components and a temporary widening of the existing structure to accommodate traffic during the construction. The design concept was to establish the span configuration on a new roadway alignment. CDOT determined that a varied-skew, single-span structure would work best and minimize environmental impacts.
To facilitate the expedited construction, CDOT considered precast prestressed concrete bulb-T girders and steel girders for the project. In Colorado, precast concrete girders are the most common solution to this type of design problem. For this project, the designer proposed solutions that are generally reserved for precast concrete, but instead applied them to a two-section steel girder alternative, because Colorado's standard precast concrete girders were too long to ship to the site. In addition, they would have required two large cranes onsite for placement.
To minimize rock excavation and construction costs, the superstructure design uses five varied two-section steel girder lines, three 6-foot (1.8-meter)-deep welded plate girders for long span length, and, where the span length was shorter, two rolled girders with varied lengths. The girders are Grade 50 weathering steel spaced 8 feet (2.4 meters) apart. The bridge deck is full depth, 8-inch (20.3-centimeter)-thick precast prestressed concrete. The girders are single spanned between the abutments to eliminate a bridge joint.
Using two-section steel girders, standard rolled beams, and welded plate girders offered a number of advantages. They can be shipped in short lengths, require only a small crane because of their low lifting weight, accelerate construction time, remain cost competitive, and have simple fabrication and construction details.
Also during the design phase, CDOT conducted an indepth investigation of the rock upon which the bridge is built, including drilling and rock mapping. The subsurface condition features slate and quartzite hard bedrock layers. As a result of the investigation, CDOT determined that temporary stabilization of the bedrock beneath the abutment foundations was necessary. A rock anchor system was installed to strengthen the bedrock.CDOT worked with the Geotechnical Engineering Technical Services Team within the Federal Highway Administration's Resource Center for onsite review of the investigation, design, and construction plan.
The agency's engineers also considered various alignments for the new bridge design. Most of the alternatives straightened the roadway over the bridge to varying degrees. Given the road's narrow and winding nature, CDOT chose to stay as close as possible to the existing alignment.
According to Steven Cross, CDOT design project manager, "Because of the bridge's historic character and location, we worked to keep the location of the new structure as close as possible to the existing alignment. We worked closely with the U.S. Forest Service on the aesthetics as well, such as rock cuts, bridge color, railing type, and the pedestrian area."
The design also incorporated an adjacent pedestrian overlook with parking for visitors to view the waterfall and canyon scenery safely.
"The design of the new bridge greatly enhances safety on this very narrow, winding stretch of Red Mountain Pass," CDOT Region 5 Transportation Director Kerrie Neet says. "And the improved parking area and overlook are not only a safety enhancement but also a great stopping point for visitors to appreciate the bridge and the beauty of the canyon and falls below."
To maintain at least one traffic lane during the construction and complete all activities in two seasons, CDOT established a plan for the construction phasing mentioned earlier. In the first phase (July-October 2010), CDOT excavated rock on both sides of the bridge to widen the area by approximately 10 feet (3 meters), mostly on the northern side. On the north abutment, workers drilled and installed an anchoring system to stabilize the rock slope. Crews installed 25 anchors, each approximately 3 inches (7.6 centimeters) in diameter and 25 feet (7.6 meters) long. The anchoring system took approximately 3 weeks to complete.
"The rock stabilization efforts were challenging because we were dealing with some of the hardest rock in Colorado," says Cross. "The rock anchoring was necessary to stabilize the rock and slip planes, and ultimately create a safer structure." Cross explains that CDOT took additional precautions by installing netting and a wooden platform directly beneath the construction to catch falling debris from the rock excavation and bridge demolition, as well as prevent the material from going into the creek and canyon below.
As the final part of phase one, CDOT added temporary widening to shift traffic to the bridge's north side to facilitate work on the south side in the early part of phase two. The shift allowed one lane of alternating traffic for the duration of the construction on the south side. Later in phase two, at the completion of the south side of the bridge, CDOT shifted the one lane of traffic to that side and began work on the north side. Flaggers controlled traffic during the day, while a traffic signal did the job at night. Throughout both project seasons, the highway was closed at night from 8 p.m. to 6 a.m. for less than a week to accommodate girder erection and rock bolting.
In April 2011, CDOT and its contractor, American Civil Constructors, began constructing the first part of the bridge by drilling and installing micropile foundations into the bedrock. With the foundations in place, the crews mounted three welded steel plate girders, light enough to be erected by hydraulic crane, on the south side of the bridge. Because of the bell-shaped channel in which the bridge sits, the girders varied in size, ranging from about 54 feet (16.5 meters) to 132 feet (40 meters) long. The steel supplier fabricated the girders offsite and trucked them to the construction site in sections to accommodate a shipping restriction of less than 80 feet (24 meters).
After the girder placement, the crew installed 26 full-depth precast concrete deck panels. The panels also were fabricated offsite -- about 300 miles (483 kilometers) from the construction site -- and shipped when CDOT was ready to install them.
"In addition to the benefit of faster installation using the precast panels, construction offsite also improved quality control due to the controlled manufacturing environment," says Archuleta. "What that really means is we're getting better permanent materials."
With the deck panels in place, the crews then connected each panel with reinforcing steel and cast-in-place grout. They finished the south side of the bridge by paving and adding a waterproofing membrane.
Construction on the north side began in August 2011 and followed the same steps used in constructing the south side. The most notable difference, however, was CDOT's use of rolled steel beams for the last two girders under the southbound lanes. Due to the bell-shaped curve of the channel, the southbound girders along the wider section are longer than those under the northbound lanes.
From the start, CDOT recognized that communicating with the community about the details of the Bear Creek Bridge replacement would be crucial to the project's success. In fact, the department incorporated a requirement in the contract specifications for one full-time public relations professional dedicated to the project. Before the work even began, CDOT and the contractor began sharing project details with the community. They used a variety of vehicles, including a hotline, flyers posted in local businesses, regular email updates, periodic press releases, and information posted on CDOT's Web site. To inform motorists already on the roads, CDOT used variable message signs placed strategically along U.S. 550 north and south of potential alternate routes at Ridgway, CO, and Durango, CO, and four permanent message signs.
"We knew we needed to have a strong public relations plan as part of this project, and it really proved to be beneficial. The project -- especially the end result -- was very well received and supported by the community," says Archuleta.
CDOT's use of two-section steel girders provided an alternative that was cost competitive and easily constructed, and met the design criteria for this complex project. The new Bear Creek Bridge is substantially wider than the old bridge, with 12-foot (3.65-meter) lanes and 6-foot (1.82-meter) shoulders. In addition, the adjacent pedestrian overlook and parking area create a much safer facility for locals and tourists alike.
The design of the superstructure respects its natural and historic surroundings. With its clean lines and natural rock color, the completed bridge aesthetically blends into its high-mountain setting. The deep brown color of the weathering steel girders, obtained without additional cost, provides contrast to the brown-stained concrete deck and abutments. Likewise, the alignment of the new bridge echoes the curves of the road, maintaining the natural integrity of the location. To show respect to the historical significance of the location, the Ouray County Historical Society, with a contribution from the contractor, raised funds to restore the Mears monument, honoring the road's original builder.
"Overall, for a project with so many special issues to consider, it went very well," says Archuleta. "We're pleased with how well our planning and preparation prevented major conflicts, how the design turned out, and with the overwhelmingly positive feedback we've received from the community."
Nancy Shanks is the public relations manager for CDOT's Western Slope regions. She holds communications and journalism degrees from the University of Colorado at Boulder.
Alicia Sindlinger is a contributing editor for Public Roads.
For more information, contact Nancy Shanks at 970-385-1428 or firstname.lastname@example.org.