Civil Engineering Reference
In-Depth Information
(CONDE NAST TOWER)
(1998)
With virtually no warning, the exterior construction scaffold and personnel hoist on
the exterior of the 48 story Conde Nast Tower in Times Square, New York partially
collapsed. Starting at approximately 8:15 AM on July 21, 1998, a two story section
of the scaffold at the 20
th
to 22
nd
floors buckled leaving almost 30 stories of scaffold
above the collapsed area unsupported in the vertical direction and tilting precariously
outward above the busy Times Square area of Manhattan while partial sections fell
onto the streets and buildings below (Figure 5-21 and 5-22). A number of pieces of
the steel and aluminum system fell onto buildings and the adjacent streets. An 18
meter (60-foot) length of the elevator hoist mast fell across 43
rd
Street, piercing the
roof of an adjacent building and killing one occupant. Weighing over 5.44 tonnes (6
tons), the elevator counterweight fell to the street creating a crater 3 meters (10 ft)
deep in the pavement. Although both cabs of the personnel hoist were in operation
and occupied by construction workers, both groups were able to discharge at different
levels just before the collapse. One cab operator, who went back for his tools, had the
collapse occur around him and was saved only because he was able to grab a piece of
rope and was pulled to safety by others.
The Conde Nast Tower stretches the full block from 42nd Street to 43rd
Street. The initial failure and potential for additional collapse presented an ongoing
danger to pedestrians, vehicle occupants, and those individuals in the buildings in the
Times Square area. New York City Office of Emergency Management was
summoned to the site within minutes of the collapse and a crisis management team
was quickly formed consisting of a number of city agencies as well as private
consultants. LZA Technology, a division of the Thornton-Tomasetti Group was
retained to evaluate the collapse conditions, contain the disaster, create an emergency
stabilization plan, and design and implement the demolition process for the failed
structure. Wiss Janney Elstner (WJE) was retained by the New York City Department
of Buildings to investigate and determine the cause of the failure.
Lessons Learned
Results of the investigation indicated that the mechanism of the failure was buckling
of the scaffold frame legs just below the 21
st
floor. Structural analysis led
investigators to conclude that it was missing bracing (design condition compared to
the as-built condition) that caused the aluminum scaffold frame legs to fail in
buckling. Based on structural analysis, the missing bracing was determined to have
reduced the vertical load capacity of the scaffold in the area of collapse by
approximately 34%. Investigators concluded that the required bracing, necessary for
stability, had never been installed.
