| 140 West 42nd
Street
New York, New York
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Studies: Office Buildings > 140 W. 42nd St.
A New Generation of Post-Tensioned Concrete Office Towers in NYC
 140
West 42nd Street is the first post-tensioned concrete office building
since the early seventies which conveys elegance and modern architecture
in the form of a 24 story tower with a building gross square footage approximately
200,000 square feet which includes retail space at street level and typical
office space above. The tower is located between two landmark buildings,
the Knickerbocker Building and Bush Tower and is an important addition
to the ongoing transformation of the Times Square District of Manhattan.
Designed by Gruzen Samton Architects of New York City, the building architectural
features and function required that office space be shared between the
existing Bush Tower and the new proposed space. This requirement meant
that the new tower’s floor to floor height would have to match the
Bush Tower’s floor to floor height of 13’-9”.
 This
architectural challenge required innovative solutions by the structural
engineering firm of DESIMONE Consulting Engineers of New York City. Also,
the floor layout required interior column free space with all building
columns to be located at the perimeter . After many design iterations
that included a composite floor system using W36x (36” deep) steel
girders as part of the lateral force resisting system, the design team
elected to use a 12" deep flat plate with post-tensioned bands as
the most economical floor framing system. Also, by using the banded-beam
system, a considerable amount of architectural flexibility was achieved.
The post-tensioned “bands” were designed to fit within the
12” slab profile which eliminated the need for floor beams otherwise
needed to support the slab. Beam supported slabs typically increase the
floor to floor height, adds cost and slows construction due to formwork
complexity. As a result, most of the slab is conventionally reinforced
with the post-tensioning limited to these “bands” which allows
for future retrofitting of the floor slab such as inclusion of slab openings
for interior staircases without compromising the structural integrity
of the post-tensioning system.
 This
structural design solution offered unrivaled architectural flexibility
with spans reaching almost 40 feet from the curtain to the core and allowed
the designers to achieve the same floor to floor height to match the Bush
Tower’s floor to floor height. The alternative composite floor system
added approximately 3 feet to each floor which would have made this architectural
requirement impossible.
A key advantage of conventional flat plate systems is 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. While an aggressive construction schedule, 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.
Unprecedented in office building construction, the two-day cycle will
reduce the floor completion schedule by 50% compared to an equivalent
structural steel floor system.
The lateral force resisting system efficiently incorporated the 12”
flat-plate with a 16" thick concrete shear wall which completely
encases the fire stair from the ground floor. Located at the rear of the
property and in keeping with safety and security in post 9-11 construction,
all means of egress will be protected with 16-inch impact resistant concrete
shear walls for occupant safety. 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. 12,000-psi
concrete is used up to reduce the column sizes to its smallest practical
size. In addition, 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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