While green roof technologies have existed for over 30 years, Knoxville decided to give an unusual twist on these intensive roof systems on their 2010 Knoxville Station Transit Center (KSTC). I met with Ben Pethel of The Penland Studio - Landscape Architecture to learn more about the structural design and about the role of the Landscape Architects in this project.
Thirteen years ago, the City of Knoxville began laying the foundation for what would become Knoxville’s first Silver LEED certified public facility. The realization of this dream took place on August 16th 2010. After considering multiple locations throughout downtown Knoxville, the City favored the location of a 2.6 acre site on the corner of E. Church Ave. and S. Hall of Fame Dr. The only problem was that the ideally-located site would not be large enough to host a transit center which housed twenty plus buses, trolley services, over 3.5 million annual riders, and more. The solution to this problem came in the form of a $29 million 108,000 s/f new two-bridge system.
The project was awarded and taken up as a joint venture between Knoxville firms McCarty Holsaple McCarty (MHM) and Bullock Smith & Partners (BSP). Ben Pethel was consulting with BSP at the time. The design utilized air rights to build two additional bridges which span over James White Parkway. The structure on grade houses a lobby, trolley stop, restrooms, service center, interpretive signage about the designs, and administrative spaces. The upper bridge adds 380’ feet of length to the 710’ long bus platform. The busway includes solar shades from rain and sun for those waiting and heating rooms for winter use. Solar panels and geothermal units are also present on site. The second bridge, known as the Pedestrian Plaza is actually an intensive green roof system according to Pethel. It is one of two built green roofs on the site.
While I was aware of green roofs on rooftops (known as Extensive Green Roof Systems,) this was the first time I encountered an intensive green roof system, not to mention it was on the ground. Since the Pedestrian Plaza was constructed on one of the two new bridges, little vertical depth was available to accommodate vegetative root depth for trees. Pethel explained that unlike the simple, thin-profiled vegetated roof which spans the length of the building, a more complexly designed system would need to be designed. The bridge green roof required the Landscape Architects to design the hardscape, drainage systems, soil systems, irrigation, site furnishings, lighting and plantings in a highly coordinated effort.
This was no easy feat as the new bridge would parallel the existing, sloping E. Church Ave. viaduct and allow for vertical and lateral expansion and contraction between the two while providing ADA compliant pedestrian connections. With limited space, Pethel, Curtis Catron (BSP principal) and Valerie Friedmann (BSP intern) worked the design to meet strict grade restrictions. The new bridges had to meet with the existing bridge which was sloped at 4%. To be code compliant no pedestrian walks on the site could exceed 5%. Above the soil the design sought to recognize the interior space of the building, provide shade, and reduce the heat island effect of the surrounding paving while minimizing maintenance and water requirements. A simple plant palette and a plant massing style reflects the typical scene that one would see along roadsides while traveling by bus, according to Friedmann.
Paving patterns were designed to match flooring on the inside of the transit center. This concept was originally planned to extend onto the green roof above the building as well by intersecting sedum species with bands of blue fescue. The sidewalk and planting beds curve to highlight pedestrian traffic flow through the plaza. Pethel pointed out that the design made good use of engineering by locating trees over structural pillars.
The transit center was to originally have a third green roof at the end of the bus platform sidewalk, but this was cut due to financial or other reasons. The green roof on the busway was going to be a demonstration of the extensive system used on top of the building, available for safe view by the general public.
In spite of the many limitations which come with a project like this (political, financial, structural), the John J. Duncan Jr. Knoxville Station Transit Center turned out to be an amazing facility which has redefined the way Knoxvillians travel and set a precedence for truly sustainable design in Knoxville. Since its design and completion, the facilities and designers have received numerous awards, further strengthening the perception that Knoxville is looking forward to a more brightly designed future.
Landscape Architects: Bullock Smith & Partners (BSP) : Curtis Catron, The Penland Studio - Landscape Architecture: Ben Pethel; BSP Intern: Valerie Friedmann
Architects: Joint Venture Architects - McCarty Holsaple McCarty and Bullock Smith & Partners
Engineer: Wilbur Smith Associates (now CDM Smith)
General Contractor: Johnson & Galyon Construction
Orchid Award - Keep Knoxville Beautiful
Tennessee ASLA- 2011 Merit Award
East TN AIA - 2012 Excellence Award for New Construction
Award of Excellence - Metropolitan Planning Commission
Grand Iris Award - American Council of Engineers Companies Tennessee
Plant Species: (Selected species)
'Aeryn' Trident Maple
Winter King Hawthorn
Oak Leaf Hydrangea
Hidcote Blue Lavender