A triple effect

Aug. 15, 2007

The U.S. 75 underpass at Churchill Way in Dallas—a three-span continuous, tangent, steel-tub girder bridge—not only demonstrates the efficiencies of collaboration between architecture and engineering disciplines, it also serves as a prime example of context-sensitive solutions (CSS).

The U.S. 75 underpass at Churchill Way in Dallas—a three-span continuous, tangent, steel-tub girder bridge—not only demonstrates the efficiencies of collaboration between architecture and engineering disciplines, it also serves as a prime example of context-sensitive solutions (CSS).

CSS has been defined by the Federal Highway Administration as “a collaborative, interdisciplinary approach that involves all stakeholders in developing a transportation facility that complements its physical setting and preserves scenic, aesthetic, historic and environmental resources while maintaining safety and mobility.” It is a key element in achieving sustainability in highway and facility solutions.

While still focused on moving traffic efficiently and safely, today’s departments of transportation also want to preserve and protect their environmental, social and economic systems as they develop and upgrade their transportation infrastructure. This means that CSS are applied to projects to mitigate the impact on the environment and reflect the context of communities through which transportation systems pass. These conjoined concerns are often referred to as the “triple bottom line.”

A corollary to CSS is context-sensitive design (CSD), which applies more narrowly to the engineering side of a project. CSD refers to specific design features and often to environmental mitigation measures that may be required so that the overall CSS integrates with the community and the natural environment. At its most effective, architects and engineers collaborate throughout the design process to develop appropriate CSD solutions. Examples of CSD might be contouring slopes to blend into the surrounding areas or staining concrete to match a local hue.

Strong resemblance

The U.S. 75 underpass at Churchill Way was a joint venture between the Dallas District of the Texas Department of Transportation (TxDOT) and the city of Dallas. The east-west bridge spans 14 lanes of mainlane, frontage road and ramp traffic traveling north-south on North Central Expressway (U.S. 75), a major highway linking downtown Dallas with the northern suburban cities of Richardson, Plano, Allen and McKinney. The bridge is located directly south of a new five-level interchange at North Central Expressway and I-635, known as the Dallas High Five interchange. Prior to the construction of the bridge, Churchill Way terminated at the southbound frontage road paralleling the North Central Expressway.

The new bridge allows eastbound Churchill Way traffic access to the North Central Expressway northbound frontage road, and northbound North Central Expressway traffic access to westbound Churchill Way. The project also included the design of a new “jug-handle” off-ramp connected to the northbound frontage road.

The underpass at Churchill Way is situated in an architectural transition zone lying south of the High Five interchange and north of the North Central Expressway corridor through Dallas. The engineering consultant’s responsibility was to look at the urban design of the section to the south and tie it structurally and aesthetically to that section as well as to the design of the Dallas High Five interchange.

From an engineering and architectural viewpoint, the bridge structure of the underpass at Churchill Way captures many of the same details as other bridges in the North Central Expressway corridor. The design team incorporated many aesthetic themes from North Central Expressway, but took some liberties in their interpretation; because this project was located between two different aesthetic designs, the team wanted to include elements of each theme, creating a transitional element. Details borrowed from the North Central Expressway include square columns with tapered capitals integrated into the bent caps, the double-sloped outside face of the bridge barriers, bold street name lettering and pipe rails on the bridge barriers and the overall color scheme. Meanwhile the trapezoidal girder shapes used in the superstructure echo the predominant theme of the Dallas High Five interchange bridges.

The on-ramps and off-ramps along North Central Expressway to the south of the project feature “suspended” beams and columns. These beams and columns are consistent with a detailing pattern in the retaining walls featuring solid brown panels framed by tan beams and columns. The team captured these ideas on the Churchill Way project and incorporated them into the bridge design, picking up a datum line elevation at the top of the railing and continuing it horizontally in a circular form. The design includes four solid panels that copy the detail of the walls to the south, which are transitioned into four transparent panels where the tan beams and columns frame a brown screen mesh that was installed to prevent people from walking along the rail opening. There also are four open panels where the beams and columns cross over the sidewalk/bike lane. This feature allows the pedestrian to interact with the design rather than just observing it.

The Texas Star pattern, a feature of the High Five interchange, also is incorporated into the columns and the pedestrian walkway railing columns that the designers included in the Churchill Way structure.

The project also included a pedestrian walkway/bike lane across the bridge, generally running parallel and adjacent to the alignment of the jug-handle vehicular off-ramp. Because the vehicular off-ramp exceeded a 5% slope, the designers increased the length of the pedestrian ramp to conform to Americans with Disabilities Act accessibility guidelines, which they did by separating it from the alignment of the vehicular ramp and adding a couple of meanders. This created the opportunity to allow the walkway/bike lane to cross under the circular suspended beam en route to its connection with a new walkway/bike lane adjacent to the frontage road. The walkway/bike lane will link up with a walkway/bike lane that is currently being designed along Cottonwood Creek east of the bridge, providing connectivity to a larger bike lane network that spans the Dallas area.

Moving in the tub

Design challenges included designing a roadway profile that provided sufficient vertical clearance over the North Central Expressway while simultaneously providing for a jug-handle ramp connection that ties into the frontage road. As mentioned above, different grades were required for pedestrian and bike traffic and for vehicles. Designers needed to meld all these requirements. The designers also had to account for a possible future extension of Churchill Way to the east of a bridge that TxDOT may add at a later date. The team provided an abutment integrated into a retaining wall for a future bridge that would allow Churchill Way to cross over Cottonwood Creek and provide access to neighborhoods east of the creek.

Turning to the underpass itself, the structural challenges were far from insignificant. The Churchill Way Bridge is a trapezoidal steel box girder bridge (also known as a steel-tub girder bridge), which is more complicated than a plate girder bridge in terms of design, detailing and fabrication. Each girder features two sloped webs connected by a wide bottom flange, hiding much of the girders’ framing and resulting in a clean, smooth soffit appearance for the superstructure which helped to blend the bridge’s appearance with strong aesthetic themes for the North Central Expressway to the south (which features multiple adjacent box beam bridges) and the Dallas High Five interchange to the north (which features numerous trapezoidal concrete box beam and U-beam bridges).

The bridge’s three spans run 139, 133 and 100 ft. The aesthetic requirement of providing a smooth soffit appearance precluded the use of I-shaped girders, while lane-closure limitations and span lengths eliminated precast U-beam and cast-in-place concrete box girder options. Steel-tub girders, on the other hand, offered ready answers to multiple problems, providing the right architectural appearance, more than adequate spanning capacity and the significant constructability advantage of fewer field sections and a minimized number of girder erection operations over the heavily traveled North Central Expressway.

The 34° skew of Churchill Way as it crosses North Central Expressway added a significant complication to the design and detailing of the bridge. The structural design team conducted a three-dimensional finite-element analysis to ensure that skew effects were adequately quantified and, based on what they learned, decided to use paired, staggered right pier diaphragms to simplify detailing and erection. Although top flange lateral bracing loads would be greater, the design team decided against using external intermediate diaphragms in order to simplify and streamline erection of the underpass. They also denoted two of the four field splices as “optional.” In this way, a contractor could erect fewer, larger field sections and thus avoid lane closures as well as associated “lane rental” costs stipulated in the construction contract.

From both the engineering and architectural perspectives, the U.S. 75 underpass at Churchill Way typifies CSS. Significant energy and effort went into harmonizing this structure with its surroundings. From the smooth, clean appearance of the bents and girders and the careful treatment of pedestrian and bicycle needs to the Texas stars, gently flowing lines and regional hues, it is an expertly coordinated, aesthetically satisfying, environmentally responsible project. Construction was recently completed and the bridge is now open to traffic. It has received numerous compliments from local citizens and motorists regarding its functionality and how well its appearance fits into the North Central Expressway corridor.

About The Author: Coletti, the project’s bridge engineer, can be reached in HDR’s Raleigh office at domenic.­[email protected]. Krueger, the aesthetic design architect, can be reached at Carter & Burgess at [email protected]. Renton, roadway engineer, can be reached

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