Existing column foundations were retrofitted to support the new loads of the larger building. Column foundations near or within the footprint of the new core were articulated with a new cast-in-place foundation mat bearing on rock, including rock anchors at key locations to control uplift.
“Now, a new cast-in-place concrete core is threaded through the existing structure from foundation to the new roof, providing lateral stability and adequate stiffness to the new larger building,” Provenza said.
The new steel tower sits atop the existing roof, and the column grid of the new tower is carefully coordinated with the existing column locations below. The steel tower has fewer columns than the existing structure, providing more expansive tenant spaces, thus optimizing the tower footprint.
“To maintain a continuous load path, the columns of the new steel tower are sloped to meet the locations of those in the existing building so that no heavy transfers are required,” Provenza said. “Built-up beams on the ninth floor transmit the horizontal forces from the sloped columns to the core and reconcile elevation differences between the existing roof and the new building. The intersection of these structural elements was carefully detailed to meet the engineering requirements while also being cognizant of constructability aspects.”
The ground floor was partially lowered to allow for new spaces with taller ceilings, including the lobby and additional amenity spaces. The existing slab was reinforced with steel members below, and a new steel structure was installed for the new lowered ground floor area.
“These solutions not only saved the owners considerable time, but millions of dollars as well,” Provenza said.