The thesis study performed examined the structural redesign of Three PNC Plaza from a steel frame building with composite slabs to a strictly concrete building. The structural depth consists of the design of the new one way post tensioned floor system in long direction of the building, supported by wide shallow post-tensioned girders spanning in the short direction. The new system was designed to have minimal impact on the buildings architecture, as a result original column locations were maintained for the redesign to keep the 42.5’-0” by 30’-0” bays intact. The design utilized an 8” post-tensioned slab with 60” by 18” girders spaced at 30’ center to center. The slab was broken up into 5 distinct zones, based on the number of spans as well as span distances, and each were designed individually using the ADAPT-PT program. Girders were also broken up into typical girders A, B, C and D, and each type was designed using the same program.

The existing lateral system utilized 7 distinct concrete shear wall cores located throughout the center of the build. These cores rise up until the 14th story where the steel moment frame structure assumes full capacity of lateral loads. The structural redesign follows the same premise only with a concrete moment frame. The shear walls had to be relocated resulting in 2 larger sets of core walls instead of the former 7. The new system is classified under a Dual System Design with both ordinary concrete moment frames and shear walls. The code does not specifically address post-tensioned lateral systems so several conservative assumptions were made during analysis.

Relocating the core walls of the building also resulted in the movement of vertical circulation wells. This aspect was explored in an architectural breadth to see the impact on floor plans. The mix use nature of the building makes floor plans vary greatly throughout the building. The first 14 floors of the building are mainly office level and did not see much of an impact by the change. However, the hotel/condo portions of the building through levels 15 to 23 were affected. Limited floor plans were provided but in a typical hotel/condo floor plan one of the hotel rooms had to be removed due to elevator shafts. This effect could cause potential issues for the condos on the top floors of thae building that take up more floor space.

The second breadth topic focused on the construction management aspects of the redesign. A cost estimates and new schedules were composed for each system. It was found that the systems were around the same bare material cost however the new concrete redesign required additional overhead and production costs. The concrete building was also found to take longer to construct over the original steel framed building.

To view a PDF version of the full report please click HERE.