As Nashville's population has soared, so has the multi-story steel expansion of the city's main library parking lot.
Nashville is one of America's current up-and-coming cities.
Census data shows that the city added nearly 100 new residents per day in 2018, and the Metropolitan Planning Department estimates that by 2040 Middle Tennessee will add 1 million new residents.As the city booms, space (especially parking) in the city center becomes at a premium.
Due to the surge in business, tourism, and convention traffic (the city has hosted NASCC: Steel Conference twice since 2008!)), the Downtown Nashville Partnership and the City of Nashville recognized the need for more parking and asked structural engineer Gresham Smith to expand the city's main library parking lot vertically by three levels.The garage itself is operated by SP+ and maintained by Block by Block for use by downtown employees, daily visitors to the library and nearby attractions, and guests of the Renaissance Hotel across the street.

Because of the site's long history and versatility - having been the site of Nashville's original synagogue, then a shopping center, and then the main library (as of 2001) - there is not a lot of documentation on the site beyond basic floor plans.The structural team needed to learn as much as possible about the structural condition of the site before they could determine if a vertical expansion was feasible, and worked with multiple geotechnical engineering firms to conduct a multi-phase forensic analysis, providing enough information at each stage to determine feasibility before proceeding to the next phase.
During the course of the investigation, the team discovered that the existing four-story garage had at least three different structural systems:cast-in-place conventional reinforced concrete on the first three elevated floors, cast-in-place post-tensioned concrete on the top floor, and composite structural steel infill on the roof portion.In addition, the structure appeared to have been altered at least three times previously.After a year of testing the site's bedrock, garage foundations, and concrete structure, the team gathered the information they needed to confidently move forward with the design of the garage's vertical expansion.

In the absence of existing drawings and on a compact, busy urban site, the team chose to use structural steel (660 tons in total) rather than concrete to build the extension.Because structural steel is much lighter than concrete and has a higher strength-to-weight ratio, overloading of existing foundations and structures was avoided.The design utilized a composite floor system that limited the amount of concrete required, so sections of the floor could be poured in a single day without worrying about cold joints.In addition, steel could be used for smaller columns, resulting in improved interior visibility and driver circulation.(Typical beams are W30x99, infills are W14x22, and columns are W14x120, W14x176, and W18x106.) And because the steel is exposed, it's easier for managers to perform direct inspections and maintenance.

In terms of protecting the exposed steel structure, the team considered both painting and galvanizing, ultimately choosing the latter, which they determined would have the longest life and lowest maintenance requirements.In addition, the team reinforced the floor slabs to limit the number of construction joints and potential locations for moisture intrusion.In addition to mild steel reinforcement, macrofiber reinforcement, shrinkage-compensating admixtures, and crystallizing admixtures were used to reduce the initial shrinkage of the concrete and minimize cracking.