Group 8
Building façade failures : John Hancock Building
Introduction
The John Hancock Building, consisting of 2 million square feet of space, is a sixty
storey structure sheathed in reflective glass. The building was praised for consorting
agreeably with landmarks on a historic square and for being “not so much an object as
a spectacle of changing light, an intensified comment on the qualities of the sky itself
.”— Donlyn Lyndon. The City Observed: Boston. New York: Vintage Books, 1982.
p195. This was, of course, due to its’ reflective glass, which up until then had never
been used on such a large building.
Background
In the first half of the 20th century, building facades underwent a transition from
massive load-bearing walls to relatively thin, lightweight, non-load-bearing "curtain
walls." Before the transition, walls consisted of layers of masonry ("wythes"), which
formed part of the load-bearing structure of the building. Floors were supported by the
exterior walls and moved with the walls. But by today's standards, these walls were
expensive, energy inefficient, and limited in design flexibility. Something else was
needed — something that offered improved thermal performance, greater design
freedom, and lower cost.
Curtain walls proved to be the answer. The transition to curtain walls began in the late
1800s and continued until the middle of the 1900s. In the early part of the transition,
walls were still massive, but they no longer supported the structure's floor loads. In
the latter part of the transition, lightweight walls were "hung" on the structural frame.
The transition from masonry load-bearing walls in many cases meant a transition to
glass, now one of the most common cladding materials. The quintessentially brittle
material, glass has introduced its own set of challenges. Glass is usually installed in
metal framing systems, which means that thermal movement is inevitable: Aluminium
frames subjected to a change in temperature can move about 2.5 times more than glass
subjected to the same change. Breakage can result if the design does not
accommodate this differential movement. This was the case with The John Hancock
Building.
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Façade Failure
In 1972 and 1973 the mirror-glass windows of the building began to fracture, and was
replaced with more than an acre of plywood.
A "gag order" imposed on the parties to the resulting legal dispute prevented the
release of the facts regarding the cause of the breakage, giving rise to many theories
and myths, some of which exist to this day.
Initially, many design professionals thought the reason for the breakage lay in the fact
that the tower swayed excessively in the wind. Although it was indeed swaying
substantially, this was not the reason for the glass breakage.
Another hypothesis was that wind forces at "hot
spots," which resulted from the rhomboid shape of
the tower, caused overstressing of the glass.
Substantial "hot spots" did exist, but only a small
percentage of the glass was subject to anything near
the load for which it had been designed.
Still another myth was that the windows broke
because of the stress they endured from the
settlement of the tower's foundation.
Causes
But in fact, extraordinary external forces and the building's structural design were not
the cause of the failure. The problem actually lay in the insulating glass itself.
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The insulating-glass units that made up the facade
were fabricated with a thin lead-tape spacer to
separate the two panes of glass. The tape was
soldered to the glass after the edge of the glass was
coated with a film of copper to make it more
receptive to the solder. This created a tenacious bond
between the spacer and the glass, which constituted
the product's greatest strength as well as the source
of its demise.
Conclusions
We now realise that innovation can lead to reduced predictability and increased risk.
To meet the challenge that innovation presents, we must use the lessons of this case
study, coupled with sound technical fundamentals.
References
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www.greatbuildings.com
www.architectureWeek.com
www.pubs.asce.org
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