This new pier ends with a five-story building, which called for thick, heavy, post-tensioned concrete slabs that created a challenge in its construction. The fourth-floor slab is 36 inches thick and the fifth-floor slab is 30 inches thick. These decks are supported by only one concrete elevator core to the south, one concrete stair core/elevator core to the north, and three interior concrete columns between those cores. The decks cantilever approximately 30 feet to the east and west past the cores. The minimal support and long cantilevers necessitated the thick post-tensioned decks.
The typical way to support fourth- and fifth-floor slabs like this during construction would be to shore from lower slabs up. But the pier’s marine deck under the building at the pierhead was designed for only a 100-pound-per-square-foot live load for cost efficiency and did not have the live load capacity to support the heavy slabs during construction. As a result, the construction of the pierhead building had to be uniquely engineered.
Andrew Habel, Florida regional director of McLaren Engineering Group out of Orlando, Fla., working with Skanska USA Building, devised a plan. Holes were formed in the pier’s marine deck to allow for steel H-piles to be driven through the deck later. These piles independently supported a system of steel framing to temporarily support shoring for the fourth- and fifth-floor concrete placement.
Steel framing support points were placed on the marine deck, over new permanent concrete piles when possible, thus minimizing the number of steel piles required. During the design process, the team learned that, in addition to avoiding new permanent precast concrete piles supporting the new pier, the temporary steel H-piles also needed to avoid a field of old precast concrete piles cut off below the mud line, leftover from the demolition of the old pier. Avoiding those piles added to the complexity of the temporary H-pile layout and the steel framing supported by the H-piles.
Next, an innovative construction sequence for the building was used by Skanska. The upper floors would need to be constructed prior to the lower floors to avoid interferences with the shoring at the lower floors and to allow clearance for removing the steel framing used for deck formwork support. Instead of constructing the building from the bottom floors up, the supporting concrete elevator shafts, stair cores, and columns were built first, followed by the fourth-floor deck, and then the fifth-floor deck supported by the fourth-floor deck and the steel framing system. The shoring, and the steel framing for the supporting shoring, was then removed and the steel H-piles that went through the marine deck were cut off below the mud line and removed. The second- and third-floor decks, which were a more conventional slab thickness, were then shored from the marine deck during concrete placement.
Once supported by the shoring framing system and the H-piles, construction of the fourth floor took place at night. This placement required 80 workers, 77 concrete trucks, and 767 cubic yards of concrete. Successful concrete placement of the fifth-, third-, and second-floor decks followed. The independent system of steel framing resulted in a cost savings to the City of St. Petersburg of $1.2 million over a more traditional approach of reinforcing the marine deck to handle the construction loads.