Legends of Tuscaloosa Mat Foundation and Elevated Slab
The Legends of Tuscaloosa is a six-story elevated structure situated in close proximity to the football field at the University of Alabama. It provides a premier and rare location that gives residents a "stadium-front" view.
The concrete "skeleton" of the building was awarded to a concrete contractor with their turnkey contract requiring the design and construction of a complete concrete frame. Initial design called for a rammed aggregate foundation system. Using this system, shafts are drilled and filled with compacted aggregate. While effective, the concrete contractor, who had previous experience, knew that a post-tensioned mat foundation system could be installed more quickly. Subsequently, they contracted VSL, who was also providing the design and material for the post-tensioning of the elevated slabs, to redesign the foundation of the Legends. The main focus for the design switch was the enhanced construction schedule; however, the mat foundation also allowed for improved soil pressure distribution which reduces differential settlement - a factor that can be important in building construction. Additionally, a post-tensioned foundation allows for improved durability, crack resistance, and water-tightness.
VSL worked closely with its engineer on the design solution and ultimately provided the materials and drawings for the construction of the 22,000 square foot foundation.
The concept of the mat foundation, in and of itself, is a common post-tensioning application. This project, however, presented some unique opportunities. Although mat foundation systems are typically designed on-grade, this one was constructed five feet below grade in order to accommodate utilities that would be situated below the building. Accordingly, the engineer had to design the foundation to bear the additional load created by the backfill soil. Once all strand, anchorages and conventional reinforcement were placed, the entire foundation was completed in one monolithic, 2,000 yard pour.
The next step, the stressing sequence, involved another challenge. The construction of the structure required stressing in stages, so while some strands were stressed before the five feet of soil were placed - others could not be until two or three levels of the structure were completed. To accommodate this schedule, VSL provided a stressing plan that allowed for most of the strand that was going to be covered with soil to be stressed before the foundation was backfilled. VSL grouped the tendons that were required to be stressed during the second stage to minimize the locations that would have to be blocked out for access to the tendon tails.
Because of the desired length of the spans and ease of construction, post-tensioning was also specified for the 120,000 square feet of elevated slab. VSL again worked with an engineer to provide the design for this post-tensioning application and supplied all material.
In total, the foundation and frame were successfully constructed in five months.