New York, NY
A New Generation of Reinforced Concrete Office Towers
in New York City
5th is a modern reinforced concrete office building which conveys elegance
in the form of a 30 story tower. Located at the corner of 42nd Street
and 5th Avenue in Manhattan, the building gross square footage is 550,000
square feet which includes retail space in the lower floors and typical
office space above the 7th floor.
Designed by Kohn Pederson Fox of New York City, the building
architectural features offer stunning views across the street to the New
York Public Library and Bryant Park. Early on in the design it
was decided that the views from the building were the focus of the layout.
The building columns were kept away from the perimeter and the main façade
was angled to offer spectacular unobstructed views of 5th Avenue.
Stringent architectural requirements posed structural challenges and
required innovative solutions by the structural engineering firm of Rosenwasser/Grossman
of New York City. The floor layout required columns to be spaced at 30'
on center. After many design iterations that the design team elected
an 11" deep conventionally reinforced concrete flat-slab with 22"
deep capitals as the most economical floor framing system. This
floor system also provided an elegant solution for the large interior
spans and eliminated the floor beams otherwise needed to support the slab.
Beam supported slabs typically increase the floor to floor height, adds
cost and complicates formwork.
thickened 15” flat-slab was used to frame the 15’ cantilevers
at the spandrel to meet the curtain wall design requirements while still
allowing an impressive 15’ dual cantilevers at the south-west corner
of the building (See Fig. 3). As is common with cantilevered structures,
deflection at the building perimeter is of paramount concern. A maximum
¾” vertical displacement of the cantilever slab after the
installation of the curtain wall was estimated. As a result, a ¾”
upward camber will be included in the formwork so that after installation
of the curtain wall system this camber is eliminated. The perimeter columns
supported the cantilevers as well as a portion of the interior span. This
structural design solution offered unrivaled architectural flexibility
with spans reaching almost 45 feet from the curtain to the core.
A key advantage of conventional flat slab systems is the reduction of
floor to floor heights which significantly reduced the cost of formwork
and building frame. The flat-slab 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 of selecting this floor system with the simplified
formwork it offers. While an aggressive construction schedule, the two-day
cycle is the perfered 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 11”
flat-slab with a 12" thick concrete shear wall which completely encases
the fire stair above the mezzanine floor. Located at the rear of the property,
the shear wall maximizes the rentable space with views on 42nd St and
5th Ave. Along with gravity load support, the flat-slab serves as the
horizontal diaphragm component of the lateral force resisting system.
Managing the the eccentricity created by the location of the shear wall
elements was easily accommodated without the need for additional structural
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 the 7th floor for the 36-in. round columns that
reduce to 28-in. above the 15th floor.
To further enhance the buildings safety, An 8-in. reinforced concrete
curb used at the first floor and serves as a barrier against vehicular
intrusion. The perimeter columns are encased in a steel shell to maximize
their robustness when subject to blast loads. In the event of local overloading
and failure of a slab-column connection, sufficient steel is provided
to arrest progressive collapse.
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. The 12-in. concrete shear wall completely
encasing the fire stairs is a significant safety feature of this structure.
The design required two independent fire stairs separated by 12”
concrete wall. Floor slab steel reinforcing detailing was modified in
some instances and increased in certain areas to provide the added structural
integrity mitigating the potential for progressive collapse. 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.