Built Into a Steep Idaho Hillside, Parking Deck Mimics Office Look
Other than fitting within the context of the nearby Regence customer service Call Center, built earlier, the biggest challenge for the parking designers and engineers was the extremely sloped site. Four levels of the garage are exposed on one end, including an open roof-top parking deck. At the other end, the structure looks like a two-story building, with two levels of exposed parking and three levels fully below grade.
Given height restrictions on the high end of the site, which is adjacent to residential development, and the number of parking stalls required, stair-stepping the garage up the hillside was not feasible. That left the option of building the garage into the hillside, said Josh McDowell, Director of Structural Engineering at the multidiscipline Group Mackenzie architectural and engineering firm.
“This required a lot of close coordination between structural engineers, the contractor, the geotechnical engineer, the shoring specialist and the precaster,” McDowell said. “We got those four parties involved [with us] to discuss how to best proceed.
“Should the precast panels directly retain the soil or should we create a gap between the structure and the hillside? We did a cost analysis of both scenarios and discussed the long-term maintenance requirements with Regence. The decision was made to provide a permanent shoring wall separated by a gap from the precast walls of the parking structure.”
After the hole was dug, mesh was placed directly against the soil walls and a hose sprayed fast-setting gunite over the mesh, followed by installation of horizontal friction anchors with plate washers and nuts. Precast concrete wall panels, columns and beams, from Hanson Structural Precast, were erected 2 feet in from the gunite mesh system. In essence, a void was cut into the hillside and stabilized with the shoring system. A precast garage was then built in that void as if it were erected on a level field.
The first 3 or 4 feet of the gap were backfilled with pea gravel. The rest of the gap is open to allow drainage and air flow, and is capped with a grating that provides access to clean out leaves and debris.
The system is designed to drain water from behind the shoring
wall, water in the gap, and subsurface water around the foundation. Water migrates through the pea gravel and down to a below-grade “French drain.”
The parking structure is built on a spread-footing foundation and cast-in-place slab. Precast pre-topped double-tees, light walls, spandrels, interior walls, shear walls, landings and stair stringers make up the garage. The double-tees typically span 62 feet. Shear forces are handled by the three precast stair towers and the center precast wall, which supports the ramps. Precast erection took just 45 days.
Inset into the hillside, the garage required ventilation. It has two points of entrance and exit, one at the high end and one at the low end. A spiral loading system is used for circulation within the deck, with a stairway-type configuration of level landings and sloped rises that include parking stalls.
Mimic the look of an office building
The second major consideration, said Richard Mitchell, AIA, Principal of Group Mackenzie, was aesthetics. Though zoned for corporate development, the property is next to the Regence Call Center, as well as nearby residences and an elementary school. It was important that the structure had the appearance and character of typical campus buildings.
“To achieve this,” Mitchell said, “we utilized brick veneer that matched the brick on the other nearby buildings and supported it off the exposed precast spandrels.” The exterior precast walls included cast-in embeds and masonry ties were used to connect the masonry products to the precast panels. To further provide the look of an office building, “window” openings on the garage were designed proportionately similar to those on the Call Center. Lighting for the parking deck is controlled by sensors, so that the level of lighting changes with the time of day and dims when the garage is not being used.
The corporate fitness center, on the top floor of the garage, helps to project the look of a campus building. This location also allows the parking structure below to be closed when the Regence office is vacant to prevent after-hours skate boarding and vandalism.
The center was weatherized by applying insulation and a false ceiling to the underside of the space between the stems of the double-tees. In addition, this location allow for a uniform floor-to-floor height for the parking structure below and offered great views of downtown and the river from the fitness center.
Fortunately, McDowell added, the architectural and engineering team that had worked on the Call Center was the same team that worked on the parking garage.
“We took a lot of the same design language and aesthetics from that structure and incorporated them into the upper elements of the parking garage, the stair towers, and the fitness center,” he said. “We also carefully considered how the brick veneer was to be laid out on the garage. We used all those elements to mimic the look of the office building.”
Location: Lewiston, ID
Owner: Regence Blue Shield of Idaho, Lewiston
Architect: Group Mackenzie, Seattle
Engineer: Group Mackenzie, Seattle
Precast Engineer: Stapley Engineers, Boise
Geotechnical Engineer: TD&H Engineering, Lewiston
General Contractor: Kenaston Corp., Lewiston
Precaster: Hanson Structural Precast, Mountain Region, Caldwell, ID
Roy Diez wrote this article on behalf of Hanson Structural Precast. He can be reached at email@example.com.