The Opal
Kew Gardens, Queens, New York
Buildings
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 The
Opal is the first post-tensioned concrete residential building since
the early seventies 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 that
required an innovative solution.
 The
structural engineering firm of DESIMONE Consulting Engineers of
New York City 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”
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 sixties 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”
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.
12000-psi concrete is used up to reduce the column sizes to its
smallest practical size.
 Also,
cast-in-place concrete construction provided this building better
acoustic properties, fireproofing at no additional cost and enhanced
robustness in the event of terrorist attacks. With minor additional
engineering effort and cost to the owner, concrete structures have
always been considered the natural solution to anti-terrorism, force
protection and fire resistance.
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