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Christopher Long, Walker Parking Consultants

The Metropolitan Government of Nashville-Davidson County in Tennessee, working with Walker Parking Consultants and other partners, created an underground parking structure in Nashville Public Square. Completely below grade and designed to accommodate and support a beautiful public park on its street-level roof, this uniquely designed structure is the primary parking facility serving employees and visitors of the Historic Courthouse and City Hall on the Public Square, along with those who visit the public park situated atop its roof.

In an effort to accommodate a high volume of vehicles, while preserving beauty and fostering ongoing commerce, entertainment and a sense of integrated community among residents and visitors of downtown Nashville, Tenn., the Metropolitan Government of Nashville-Davidson County decided to evaluate its parking needs and balance them with the goal of preserving an aesthetically pleasing downtown area.

Officials selected Walker Parking as the functional design/consultant and structural engineer, along with additional consultants and an architect, to assist with the design and construction of a parking facility that would be cost-efficient, provide long-term durability and minimize maintenance. The project would need to be completed without compromising the ambience of Nashville Public Square, a beautiful public park which had recently replaced a surface lot that stood in front of the courthouse for 31 years. The decision was quickly made to build the entire facility underground.

The design team chose a cast-in-place post-tensioned structure system, with proper drainage and a high performance concrete mix design to provide durability. The concrete mix included a low water-cement ratio and air entraining admixtures. In a further effort to reduce maintenance measures, the amount of steel connections to be maintained was minimized and exposed connections were galvanized.

The parking facility met the American Concrete Institute standards for hot weather placement (ACI 305R) and for cold weather placement (ACI 306.1). In addition, the project was in full compliance with the American Society for Testing and Materials standards. These included sampling and testing for water soluble chloride ion and concrete raw materials (ASTM C1218); practice for microscopial determination of airvoid content and parameters of the airvoid system and hardened concrete (ASTM C457); and test method for length change of hardened hydraulic cement, mortar and concrete (ASTM C157).