The Ingenieur Vol 59 July-Sept 2014 The Ingenieur Vo. 59, July-Sept 2014 | Page 58

INGENIEUR

DESIGN FLAW IN
NEW YORK
The Citicorp centre, a 59-storey tower in
Manhattan, New York designed by William J. Le
Meassurier, would have faced a major disaster if a
serious error in its design had not been detected
in time.
The Citicorp centre was the seventh tallest building
in the world at that time in 1997. The tower had
25,000 individual steel jointed elements behind its
aluminium skin. It was supported on four massive
278 m high columns, which were positioned at the
centre of each side allowing the building corners
to cantilever 22m out. Its wind bracing system
consisted of 48 braces (in six tiers of eight),
arrayed like giant chevrons. A tuned mass damper
was also provided to dampen the wind-induced
vibrations.
Problem - Diagonal winds
The problem was posed by diagonal winds, which
would result in a 40% increase in strain in four out
of the eight chevrons. Moreover despite the welded
joints specified, bolted joints were provided by the
contractor as the welded joints were considered
to be expensive and stronger than necessary. But
if the bracing system was sensitive to diagonal
winds, so were the joints that held it together. The
joints must be strong enough to resist the moment,
which was the difference between the overturning
moment caused by wind forces, and the resisting
moment provided by the weight of the building.
In the Citicorp tower, the 40% increase in
stress produced by diagonal winds caused a 160%
increase in stress on the bolts at some levels of
the building. The assumption of 40% increase
in stress from diagonal winds was theoretically
correct, but it would go higher in reality, when
the storm lashed at the building. This fact was
completely disregarded by the design team. The
weakest joint was discovered at the 13th floor and
if that one gave way, catastrophic failure of the
whole structure would have resulted.
Professional Ethics
Le Meassurier learnt of these design faults after
the building was completed and handed over.
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