Project Overview
Built in 1914, Union Station in Kansas City, Mo., has become a destination for citizens and visitors alike. The historic train station is now home to a variety of permanent and temporary exhibits as well as a planetarium, science center, and classrooms.
The main structure was extensively updated in 1999, but parking and pedestrian access was still in need of improvements, explains Julie Sarson, project manager for Burns & McDonnell in Kansas City, Mo. In 2016, Burns & McDonnell, serving as design-builder, was selected by the owner to design and construct a new bridge that could carry both vehicles and pedestrians from the front of this grand building to the third level of the West Yards parking structure.
But this wasn’t a typical design project. “The geometry and framing of the structure was complicated and atypical for a bridge,” Sarson says.
The new structure had to provide vehicle access into the structure and a wide, two-way pedestrian path via a 207-ft length between the historic building and parking structure. The owner preferred a shallow, heavy aesthetic, and wanted a structure that could be assembled quickly and on a strict budget. And because the bridge is located in an urban environment, Union Station management required a “pigeon-proof” design with no ledges, crevices, or other nesting spots that would attract birds.
Burns & McDonnell presented several options, including a “sine wave” structural shape with a gentle curve, but they ultimately chose a prestressed box-beam design with a wide pedestrian path that curves out to a city overlook. The precast concrete design met the structural, budgetary, and “aviary” goals of the project, she says.
Precast Solution
The span arrangement for the bridge was controlled by horizontal clearance to the local access road, and was set to frame the existing arches at the base of the Carriage Pavilion. The designers used precast, prestressed concrete spread box girders in long spans ranging from 33 to 75 ft for the superstructure system. “These were preferred over steel girders or concrete I-girders because of their clean and low profile aesthetic, and because a “heavy” structure was desired to complement the aesthetic of the massive Union Station building,” Sarson says. “The engineer and precast concrete producer worked closely together to ensure a perfect fit in the field for the prestressed box beams and prestressed slab panels.”
To repel the pigeons, the rectangular box-beam shapes were fully enclosed in concrete diaphragms at the pier caps, which delivered a both cost-effective and “nest-free” solution.
Because the Pavilion and the third level of the parking structure are at roughly the same elevation, the owners were concerned about flooding from the bridge into the facility or the parking structure during rain events. The designers felt that extensive drainage piping would visually clutter the clean lines of the structure and no inlets were allowed to be placed in the pedestrian path. Instead they opted to “warp” the bridge deck toward a single downspout hidden behind the south end of one bent. A series of drain inlets were then placed in the vehicular roadway to receive water, including water flowing through open slots in the median barrier from the pedestrian path.
Once the pieces were cast, delivery erection went smoothly, in part due to the fact that the precast concrete pieces were shipped just 10 miles to the project site, Sarson says. She also notes that while the biggest piece was roughly 31 tons, it was “easily handled by the equipment on the small and constrained site.”
The resulting structure is viewed as a big success, providing cars and pedestrians with easy, congestion-free access to this landmark facility. Union Station CEO George Guastello called the bridge “an engineering marvel and aesthetic masterpiece.” “In a most literal sense,” he says, “it’s as if the new bridge were always meant to be.” |