External post-tensioning was the ideal solution to strengthen an existing slab under a structure in Baltimore, Md. in order to accommodate a new lobby and escalators for the building's main tenant, T. Rowe Price. In this building, the post-tensioned (PT) tendons act similar to beams running through the concrete slab. When the tension is removed from the cables of the PT tendons and locked-off, the result is similar to the floor losing its support beams. The typical solution to this situation is to place shoring under the affected areas to replace the support. However, in this case, shoring to support the adjacent bays would interfere with the tenants' business operations.

Because of the project constraints, Structural Preservations Systems (SPS) could only place shoring directly under the slab opening, not under adjacent slabs, as that would result in less capacity as the cables were de-tensioned. This required the design team to create a phasing plan in which they determined which cables were allowed to be de-tensioned and in which order. This lock-off phasing plan allowed the crews to detension individual PT tendons, while maintaining the structural integrity of the entire floor system. The project was phased in small sections in order to minimize disruption to the existing businesses and most of the work was performed at night. Once the tendons were retensioned, the crews would begin the concrete demolition for the next lock-off phase. The total square footage of the project area was 3,600 square feet.

Key to success was adherence to a very tight schedule in which a slight delay could cascade into a logistical nightmare. The project began in May 2006 and was completed 70 days later in July - a total of 49 working days. In addition to the strict phasing requirements, because of the area businesses, it was imperative to minimize dust and debris, as well as maintain a safe environment for the building inhabitants. The owner did not want any outside the building to know construction work was occurring on the inside. The solution was to have the work-space completely enclosed by building interior walls with metal studs and drywall. Additionally, SPS built a transition room at their access point, which had two doors and poly sheets hung within it in order to create a dust blocking effect. Further, any tasks that resulted in noise were performed between the hours of 10 p.m. and 6 a.m. during the week. No noise could be made from Friday night through the end of the weekend because of the increased business hours of one of the tenants -- a seafood restaurant.