Astride the iron horse

Aug. 14, 2009

The new Fourth Street Bridge in Pueblo, Colo., will cross over the vast Pueblo Railroad Yard with 28 sets of active rail tracks while maintaining a critical route through the city.

The new Fourth Street Bridge in Pueblo, Colo., will cross over the vast Pueblo Railroad Yard with 28 sets of active rail tracks while maintaining a critical route through the city.

The bridge is currently under construction by Flatiron Constructors Intermountain for the Colorado Department of Transportation (CDOT) and is scheduled to be completed in early 2011. FIGG Bridge Engineers Inc. designed the concrete segmental solution with long, open spans and a minimal footprint. From yard constraints and closely spaced tracks emerged a record-setting span—Colorado’s longest highway span at 378 ft. Building from above allows for uninterrupted railroad operations during construction, a key component to achieve a successful project at this site.

Motorists, pedestrians and cyclists on the new bridge will all benefit from improved safety with wider travel lanes and better pedestrian and bicycle facilities. Initially, the bridge will be striped for two lanes of traffic in each direction and is expandable to three lanes to accommodate future traffic demands. The 54-ft width also accommodates a 10-ft multiuse path in each direction and wide safety shoulders. During construction, the width accommodates four lanes of traffic for construction phasing.

Fourth of Fourth

State Highway 96A (Fourth Street) is a major east-west route through Pueblo, connecting western residential neighborhoods with businesses, downtown Pueblo and I-25. West of I-25, Fourth Street passes through historic downtown Pueblo before crossing over a local road, the Pueblo Railroad Yard with 28 tracks, a flood-control levee, the Arkansas River and the river trail. The new Fourth Street Bridge will be the fourth crossing at this location since the late 1800s, and will replace an existing structure built in the late 1950s.

The Pueblo Yard is jointly operated by the Union Pacific Railroad (UPRR) and the Burlington Northern Santa Fe Railway Co. (BNSF). It is a major switching yard and hub for western railroad operations. Coal and freight moving along the front range of the Rocky Mountains are stacked and redirected from this point. One BNSF and two UPRR mainlines are among the 28 continuously operating tracks at the crossing. Because of the constant activity, neither closure nor track removal was feasible during construction or as a long-term solution. In addition, the boundaries of the rail yard, with the Arkansas River flood-control levee on one side and the Pueblo business district on the other, restricted yard expansion over the years to within the yard. The result was minimal track spacing and clearances to existing bridge piers.

Current railroad design requirements, including temporary and permanent clearance envelopes, had to be satisfied with the new design. In addition, the bridge solution had to minimize temporary and permanent impacts to tracks, yard roads, utilities, railroad operations and facilities.

The city of Pueblo and the Army Corps of Engineers recently completed the Arkansas River Restoration Project with the goal of restoring the natural riverine environment in this region. Among other features was construction of a whitewater kayak park just downstream of the bridge site. Maintaining recreation through the project site both during and after construction was an important consideration. Building from above and utilizing long, open spans atop a slender pier shape for a minimal bridge footprint created many benefits for this recreational area.

Previous bridges at this location followed an alignment parallel to and north of the existing bridge. Following the same alignment, the new bridge minimizes right-of-way acquisition requirements, maintenance of traffic demands and provides the best opportunity for geometric improvements.

Continual traffic flow during construction was one of the major goals for CDOT. With the exception of the easternmost span of the new eastbound structure, the entire bridge is outside of the current alignment. To keep traffic moving through the corridor, the westbound bridge structure will be built first, and then all four lanes of traffic from the existing bridge will be temporarily transferred to the new westbound structure while the eastbound bridge is completed.

Missing the trains

Details of the railroad yard, its proximity to the river and local businesses and the need to maintain busy yard operations during construction required a unique design and construction solution for the new Fourth Street Bridge. After carefully studying many different layout and structure alternatives, a long-span, cast-in-place concrete segmental bridge was selected. Careful pier placement and longer spans ensure that required clearances are met, while building from above in balanced cantilever construction allows rail operations and river use to continue. In addition, the concrete-segmental balanced cantilever solution also provides a low maintenance and sustainable structure with bidirectional compression of the concrete superstructure that enhances the long life of the bridge.

Pier locations within the yard were chosen to satisfy railroad clearance requirements. To span the 23 UPRR tracks, a single long span was required. One main-span pier was located beyond the westernmost track along the toe of floodwall and the other along the edge of the east yard road, adjacent to the UPRR mainline track. These pier locations led to the Colorado-record 378-ft clear span over the UPRR.

Span balancing for the most efficient design and additional site constraints dictated pier locations outside of the yard. Side spans of approximately 230 ft provide clear spans over the BNSF yard with five tracks to the east and the floodwall and Arkansas River to the west. A pier between the river and adjacent river trail protects the environment and keeps recreational opportunities open. At the east end, a pier is located outside the yard fence. As such, there are no ground impacts to the BNSF portion of the yard, and a clear span over the county road is provided.

The resulting layout reduced the number of piers in the railroad yard from five to just one, enhancing safety and greatly opening the yard. Slender pier dimensions also create more openness below the bridge.

Concrete-segmental balanced cantilever construction from above eliminates the need for large, ground-based equipment within the rail yard. Train movements and yard activities can continue unimpeded while the superstructure is built seamlessly overhead. In addition, the segmental construction method completes entire bridge sections at once and eliminates the need for secondary operations, speeding construction.

Cantilever construction is used over the railroads and river. To build the remainder of the bridge, the end spans are cast on falsework to take advantage of the readily available ground access. Side spans cantilever out to meet the end spans, and a closure pour joins the two, completing the bridge.

City of the art

The Fourth Street Bridge has a long history, being a critical link and focal point of the Pueblo community for more than 100 years.

“The new bridge will be a landmark structure defining the landscape and providing a gateway to Pueblo. Throughout the design process, the team worked closely with city representatives, community groups, local artists and other stakeholders to capture their vision in the aesthetics of the new bridge,” said Linda Figg, president, CEO and director of bridge art for FIGG. The community selected aesthetic features during the FIGG Bridge Design Charette process.

The stakeholder group selected a group of themes representative of the different aspects of the crossing. Natural Environment, Pueblo Heritage and Contemporary Sculpture were chosen by the group and melded together to guide design. The contemporary bridge shape was combined with natural material treatments. Pueblo’s local art focus will be represented in special pier medallions, bridge end monuments and pedestrian treatments. Bridge sidewalk overlooks at river piers will provide a resting place for pedestrians and a unique view of the Kayak Park and river environment. Final art elements will be achieved by a local artist.

Building in Fourth gear

The Fourth Street Bridge contract was awarded in October 2007 to Flatiron Constructors Intermountain of Longmont, Colo. Flatiron’s $27.7 million bid for the segmental alternative saved CDOT approximately $5 million against the steel alternative and also was less than the engineer’s estimate. Construction is progressing on schedule thanks to the collaborative efforts of CDOT, Flatiron and FIGG, noted Karen Rowe, CDOT resident engineer.

“I am very pleased with the progress of the construction of the Fourth Street Bridge,” said Rowe. “The team we have working on this project from the CDOT construction inspection staff to Flatiron Constructors to FIGG is amazing. I’ve never been so pleased with any of my construction projects.”

Construction is currently proceeding across both the rail yard and Arkansas River. The majority of substructure work was completed as of Sept. 30, 2008, and upon removal of the existing bridge, the remaining substructure for the eastbound bridge will be completed by about June 2010. Segmental construction for the first cantilever is scheduled to be completed by July 2009, as well as forming and concrete placement in end-span regions. Three of four main-span pier tables are planned for completion this summer as well.

All piers are supported on just two drilled shafts, providing necessary flexibility and minimizing the bridge’s footprint, especially in the railroad yard.

Abutments and end spans utilize 4- and 5-ft-diam. drilled shafts, respectively, while main-span piers utilize 8-ft-diam. shafts. The shafts are drilled through an alluvial sand layer and socketed 25 to 43 ft into hard rock. Overexposure of the shale bedrock layers found along Colorado’s Front Range can lead to lower capacity. Consequently, the contractor chose to drill the larger 8-ft shafts in stages, first drilling smaller pilot holes followed by the final required diameter. Tip elevation was not drilled until the contractor was satisfied that concrete could be placed within the required CDOT time frame.

Footing and column construction followed completion of drilled shafts at each pier location. Careful design and close coordination with the railroads during construction ensured efficient moving of materials, forming and concrete placement. At Pier 4, adjacent to both the UPRR and BNSF mainline tracks, substructure construction occurred without requiring any track closures.

Bridge end spans are being constructed cast-in-place on ground-based falsework. To speed construction and minimize materials, falsework is removed once the spans are completed and all post-tensioning stressed. Once the adjacent cantilevers are complete, they are joined to the end spans with closure placements.

Pier tables on top of the main-span piers are being cast in place on temporary falsework that has been designed to maintain required railroad clearances. Tracks remain open while pier table construction occurs overhead.

Superstructure construction of the main-span cantilevers advances rapidly, with Flatiron able to achieve a production rate of two segments per week. As the first cantilever is completed, operations move to the adjacent cantilever to complete the new westbound bridge. Later, forms will be relocated for construction of the eastbound bridge.

A temporary support prop used during cantilever construction shares construction loads with the permanent pier and provides added rigidity for better geometry control. This allows the bridge piers to maintain a slender shape and minimal footprint.

When this concrete-segmental bridge is completed in 2011, it will be a testament to the careful planning and design that considered all project goals and objectives, with emphasis on the importance of constructability for this unique site. The city of Pueblo will receive a new, modern landmark bridge in the heart of their city that reflects a special sense of place and a community with a passion for art.

About The Author: Sandoval is with the Colorado Department of Transportation. Fultz and Dvorak are with FIGG.

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