A Springfield, Virginia, home improvement store utilizes a multi-level parking structure to accommodate patrons. The main entrances to the store are located on the first elevated deck while vehicular entrances are located on the ground level as well as first elevated deck. The garage is a precast prestressed concrete construction that consisted of 60 ft double tees supported on 30 ft inverted tee beams.
Due to a limitation of the original design, a 9,500 lbs gross vehicular weight restriction was imposed, severely affecting the ability of the store to serve the contractor type business. To resolve this challenge, the Owner aimed at increasing the capacity to approximately 20,000 lbs GVW. Evaluation of the garage by the Engineer-of-Record revealed that achieving this new capacity required flexural upgrade of the structural components. Several strengthening options were evaluated by Structural Group and led to a unique and cost-effective solution that was least disruptive to store operations and pedestrian and vehicular traffic.
The strengthening solution consisted of a bonded reinforced concrete overlay, external post-tensioning systems, and externally bonded FRP, to increase the load carrying capacities of the slab, double tees, and main girders of the garage, respectively. Several mockup tests were performed until an optimum balance between the concrete surface profile, concrete mix, and placement technique was achieved and ensured a bonded overlay, thus the composite behavior of the deck slab.
External post-tensioning, an active strengthening technique, was selected for upgrading the double tees that provided active force to offset effects of the new bonded overlay and produced a strengthening system compatible with the existing construction. This unique system consisted of two loops of 0.6" strands, one on each side of a stem, that were anchored using heavy-wall steel pipes installed at the ends of the double tee stems. The system is stressed at the center of each stem using special steel anchoring block and stressing rams. The post-tensioned system was specially detailed and manufactured to be completely watertight. Additional capacity of the inverted tee beams was achieved using an externally bonded carbon based system that was installed using the wet lay-up technique. In addition, steel brackets were installed on the columns to provide additional support to the free ledges of the inverted tee beams. The project was completed on time, within budget, and with minimal disruptions to store clients.