Building Façade Failures – John
Hancock Tower
Group 8
Building Façade Failures – John Hancock Tower, Boston, USA
Background:
The John Hancock Tower is a 60-storey, 790-foot-tall (241metre) skyscraper located in
Boston, USA. It was designed by the architects I.M. Pei and Henry N. Cobb and
construction of the tower was completed in 1976. To this day it remains the tallest
building in New England and is also the 112th tallest building in the world. The tower is
a prime example of minimalist and modernist architectural design with its parallelogramshaped floor plan and the large, double-glazed, blue-tinted glass panels that cover almost
its entire surface. Cobb was presented with the National Honour Award for the John
Hancock Tower by the American Institute of Architects (AIA) in 1977.
In spite of the architectural genius of the building, the John Hancock Tower is perhaps
most famous for the embarrassing engineering flaws that plagued its construction from
the very outset. These included retaining wall cave-ins during the initial site excavation,
the addition of a damper on the 58th floor to stop the sway of the upper floors in high
winds, the addition of steel bracing to prevent a possible toppling of the tower and also
the massively inflated costs of construction, from $75 million to $175 million. However
the most notorious and indeed dangerous flaw associated with the skyscraper was in the
design of its glass façade. During construction in January 1973, a strong gale hit Boston
which caused a large proportion of the tower’s 10,344 windows to fracture. Some of
these 4' x 11', 500 lb (1.2x3.4 m, 227 kg) panels even blew out of the building, cracking
other windowpanes on the way down as they crashed onto the pavements below. The
surrounding streets and sidewalks were hastily cordoned off by the police, a measure that
was repeated whenever the wind speeds reached 45 mph (72 km/h).
Figure 1
Glass panels continued to break in the ensuing months leading to the temporary
replacement of around one third of the John Hancock Tower’s windows with plywood
panels (see Figure 1). This prompted jokes about it being “the world’s tallest plywood
building”. Exhaustive wind tunnel testing was carried out on samples of the glass used in
the panels in order to identify the problem. Starting in October 1973, all 10,344 windows
were replaced by the glass manufacturer with single sheets of heat-treated glass at a cost
of $5 million - $7 million. However, a legal settlement and “gag order” prevented the
findings on the glass failure from being released until 17 years later. This led to many
theories and myths in the meantime, most of which were later proved wrong. In 1990,
the glass manufacturer and the insurance company involved settled their dispute and it
eventually emerged that the insulating glass panels themselves were responsible for the
failure of the façade.
Reasons for Failure:
The initial consensus among many design professionals was that the excessive sway of
the tower under heavy wind loading caused the window panes to fracture. Although it
was true that the Hancock did sway significantly, this was not the reason for the façade
failure. Another theory was that the breakage was due to an unforeseen increase in stress
in the glass brought on by the settlement of the building during construction. The tower
did indeed have settlement issues but again this idea was ruled out as the cause of glass
failure. It was also thought that unusual wind forces at “hotspots” on the skyscraper led
to overstressing of the glass panels which in turn caused excessive pressure to build up at
certain points on the tower. These “hotspots” were apparently a result of the tower’s
rhomboid-shaped plan.
However, it turned out that none of these reasons were the cause of the mysterious façade
failure. The truth was revealed after samples of the Hancock window panels were tested
in an Ontario wind tunnel to study how they behaved under increasing levels of
oscillation and vibration. Each panel was composed of a metal frame holding two layers
of glass with an air gap in between them. While the two panels were identically
supported, it was always the outer panel that cracked first.
In order to manufacture the double-glazed window panels, a thin lead-tape spacer was
placed between the two sheets of glass to separate them. After coating each piece of
glass with a bonding copper film, the lead spacer was soldered onto the glass panels.
However, the outer panes of glass also had a film of highly reflective chromium on their
inner surfaces in order to reflect the sun and thus reduce its glare and heating effects.
Through testing, it was discovered that the solder bonded very well with this reflective
coating, too well in fact. Normally, window glass vibrates when subjected to wind
loading and also contracts and expands with temperature changes. However, in the John
Hancock Tower the glass in the windows had nowhere to move therefore causing fatigue
and cracking in the solder. Due to the strong bond, these cracks would then transfer to
the outer sheet of glass. Finally, under wind loading, stress would concentrate at these
cracks and cause failure to occur.
Conclusion:
The factors that allowed this engineering disaster to occur were both human and
managerial. Some of the materials used for this project were relatively new in the
construction world (eg. Glazing) and had not been used on large-scale projects such as
this one. This was a large risk. The properties of these materials, especially the glazing,
required more scrutinising in order to ensure that this building would be safe once
completed.
Despite satisfying all the building regulations, it was understood that the large sway in
the building (up to 0.9m) was a large factor contributing to the disaster. Again, more
study and scrutiny was required as to what problems could arise due to this large
movement. As well as this, study into the facing of the building and how the glazing
might behave was poorly researched. It is very easy to point out these problems, as we all
know now what went wrong, however, it is also easy to see that for such a massive
project, no stone should have been left unturned and investigations and research should
have been of the highest standard. This was not the case.
It is not all doom and gloom, however, as much has been learnt from this disaster and
building regulations have been changed because of research into this disaster. A large
amount of investigations into the stability of high-rise buildings has occurred since. With
the use of wind tunnels and models, we have begun to understand the difficulties that can
be encountered in buildings of this nature, in terms of flexibility and how structural steel
frames behave. Lateral and twisting movements can have a massive affect on the façade
should the facing of the building be very rigid and not allow much movement.
Also, the glass sheeting had a slenderness ratio that was too high and should have been
slit into smaller sections to reduce this ratio. Along with the slenderness ratio, another
problem was the strength of the bonds between the solder and outside sheet. As a result,
the face of the building was changed to single, thicker glazing that had been split to
reduce the ratio.
References:
www.greatbuildings.com
http://www.time.com/time/magazine/article/0,9171,910824,00.html
www.wikipedia.org
http://www.pulitzer.org/year/1996/criticism/works/CRIT-MAR3.html
http://www.architectureweek.com/2001/0425/building_3-2.html