Though we started with a conventional concrete shear wall system for the two service and elevator cores, our client, Oxford Properties Group, challenged us to develop a system that reduced the onsite construction duration of the structure.
Precast core construction provides an economical alternative to conventional cast-in-place concrete or structural steel, including composite steel/concrete solutions. The need for large construction tolerances was reduced, the construction schedule was shortened, and worker safety was improved. When used with form liners, the core can be exposed in the final building.
In New York City, segments are erected by union ironworkers, who also erect the structural steel for the column and floor framing. This arrangement permits crane operators to switch from erection of the precast to erection of the steel seamlessly, reducing the overall structural construction duration. The offsite fabrication of segments allows stockpiling of the core elements such that their fabrication may begin well ahead of their need on site.
The design and shop drawing processes are integrated using compatible design, drafting, and detailing software. All embedded items, reinforcing steel, post-tensioning ducts, and grout tubes are fully modelled for clash detection.
The segments are cast on their side with embedded plates and anchors, sleeves, and block-outs for mechanical, electrical, and plumbing, as well embedded elevator rail supports. An enclosed casting yard permits all-weather casting with full access to the casting bed for the installation of reinforcing steel bars and post tensioning ducts. The segments are end-match-cast with the adjacent segments to reduce interfacing tolerance issues.
The segments are lifted onto trucks for shipment to the site in this side position. At the site, a hydraulic “tilt table” rotates the pieces to enable their erection. Custom-built spreader beams are used to keep the segments balanced on the crane hook during erection.
The building has 298 precast segments in the two service cores that provide lateral stiffness and gravity support (along with elevators, stairs, and service risers). In the cellar, a conventional cast-in-place concrete “starter core” is supported on new and existing caissons; a new pressure slab also acts as the elevator pit slabs.
Site work consists of erection, post-tensioning, and grouting. Segments are placed with a thin layer of epoxy at the horizontal joint. Post-tensioning operations include both vertical bars, present in every segment, as well as horizontal monostrand tendons which are present in some segments. Minor shimming may be required at the horizontal joints to maintain proper alignment, but match casting limits this requirement. The vertical post-tensioning ducts are fully grouted for integrity and corrosion protection, generally in two-level lifts.