The Opal in Kew Gardens, Queens, New York, is the first post-tensioned concrete residential building since the early 1970s which conveys elegance and modern architecture in the form of twin,15-story towers in an L-shaped configuration with a building gross square footage of 550,000 square feet.
Built on a site which is partially underlain by rock as well as soft soils and where the buildings’ foundations straddle the different substrata posed a unique challenge to the structural engineer and required an innovative solution.
The structural engineering firm of DESIMONE Consulting Engineers of New York, New York, designed a combination of deep foundations to work with shallow foundations consisting of spread footings. Sixty foot long caissons were used to bypass the overlying “muck” to safely transfer the loads to deep found rock while the spread footing foundation was used where the rock was shallow.
After extensive studies and cost comparison that also included filigree floor system, the structural engineer opted to use a 7.5-inch post-tensioned flat plate.
There are several important reasons why post-tensioning was selected for this project:
- Post-tensioning is able to span longer distances and as a result it provided the structure with a regular column pattern which allowed the architect more freedom in laying out the interior living spaces.
- The 7.5-inch post-tensioned slab required less mild reinforcement and it also reduced the spandrel beam size and reinforcement.
- The foundation required fewer and shorter caissons because of the reduced dead load of the structure.
- Less deflection was achieved for the nearly 30-foot spans as compared to the other framing systems evaluated, namely Filigree and conventional flat plate.
The flexibility to design a post-tensioned floor as a flat plate allowed the reduction of floor to floor heights which significantly reduced the cost of formwork and building frame. The flat-plate lends itself to the use of conventional plywood construction while the lower floor to floor heights allows for the use of conventional stick shoring. Building each floor on a two-day cycle is facilitated by selecting this floor system with the simplified formwork it offers.
The two-day cycle is the preferred method of construction in New York City. A trend set in the late 1960s and made possible by the moderate spans and lower floor to floor heights common in residential hi-rise flat-plate construction.
The lateral force resisting system efficiently incorporated the 7.5–inch flat-plate with concrete shear walls which completely encase the fire stairs from the ground floor. These vertical structural elements also maximize the rentable floor space and provide economical drift control of the structure for occupant comfort.
Higher strength concrete was implemented by the design team to reduce the size of the tower columns and increase rentable space. To reduce the column sizes to the smallest practical size, 12,000-psi concrete is used.
Finally, cast-in-place concrete construction provided this building better acoustic properties, fireproofing at no additional cost, force protection, and enhanced robustness in the event of terrorist attacks.
Gruzen Samton LLP., New York, New York
DESIMONE Consulting Engineers, Inc., New York, New York
Concrete Hi-Rise Contactor (Superstructure):
DiFama, Brooklyn, New York
Ferrara Brothers, Flushing, New York
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