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Transcript
Group 8
John Hancock Centre - Building Façade Failure
Introduction:
The John Hancock Tower is a 60-storey, 790-foot-tall (241metre) skyscraper located in
Boston, USA, it is an office building whose main tenants are the John Hancock Insurance
group. It was first completed in 1976 and is, to this day, the tallest building in New
England, it is also the 112th tallest building in the world. The building’s glasssy exterior
was a landmark in minimalist skyscraper design. The building has been praised for
consorting agreeably with other landmarks in an 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.
History:
The tower was designed by the architects I.M. Pei and Henry N. Cobb who were of the
modernist, minimalist school of thinking. It was the aim of this group to construct tall,
skinny towers with a façade that appeared to be entirely of glass. These designers were at
the forefront of a movement, which, during the first half of the 20th century, saw building
facades transform from massive load-bearing walls to relatively thin, lightweight, nonload-bearing "curtain walls." The transition from masonry load-bearing walls allowed
designers to include greater areas, and larger panes of glass. For this building the largest
possible pane sizes were used. This design approach garnered many plaudits for the
designers (Cobb was presented by the AIA with a National Honor Award for the design
in 1977), but it also contributed to the buildings most major problem.
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,
delaying completion from 1971 to 1976 and causing the total cost is to increase from 75
to 175 million. During the initial site excavation there was a failure of temporary
retaining walls, they caved-in under the weight of clay causing damage to surrounding
streets and buildings. The tower was also found to sway to an unacceptable degree in the
wind, causing motion sickness effects for occupants and according to the leading
Structural Engineer Wolfgang might even have caused the building to topple over in
certain rare wind conditions. What Turlimann found was that the natural sway of the
building was very close to its period of torsion, so instead of simply swaying back and
forth in the wind, it would bend in the middle and eventually collapse. To correct this two
300-ton weights have been placed at opposite ends of the 58th floor, these are used as
dampers to stop swaying of the upper floors in high winds, steel bracing has also been
added to stiffen the building.
The most notable engineering failure of the building, and the one we are most interested
in is in relation to the buildings windows. Glass is a quintessentially brittle material
which has its own set of challenges. Glass is usually installed in metal framing systems,
which means that thermal movement is inevitable: Aluminum frames subjected to a
change in temperature can move about 2.5 times more than glass subjected to the same
change. Failure can result if the design does not accommodate this differential movement,
as we will see in this case. The John Hancock Tower has 10,344 windows and they began
failing almost from the start. The failures reached a crisis level during a winter gale on
the night of Jan. 20, 1973, while the tower was still under construction. Gusts reached 75
miles per hour at the upper floors. Huge panels of glass, each weighing 500 pounds,
shattered and dropped like sequins off a dress, smashing into other windows on their way
down. In all, at least 65 fell.
Myths of Façade Failure:
Many Myths began to spread among the working profession about the reasoning behind
the falling glass most of which, as you will see below have been proved wrong. However,
a legal settlement and “gag order” prevented the findings on the glass failure from being
released until 17 years later. 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.
One myth - still believed even by former Hancock executives - is that the windows fell
out because the tower was swaying too much in the wind. This isn’t true, the tower did
sway as we have seen, but not enogh to cause the failures.
Another myth is that the glass was sucked out by bizarre wind forces at "hot spots"
caused by the sharp angles of the tower's rhomboid shape. Again, not true - although
there were such hot spots, and the tower's shape did prove to be a critical factor in its
problems. Yet another myth is that the windows broke because they were stressed when
the tower's foundations settled. Again, not true - yet there really was a terrible problem of
settlement.
Cause of Façade Failure:
Samples of the Hancock window panels have being exhaustively
tested in a wind tunnel in Ontario. They were subjected to
vibration and oscillation until they failed. Eventually it was found
that the whole problem was right there in the window unit itself.
Each panel was a sandwich: two layers of glass with an air space between, all held in a
metal frame. To cut the glare and heat of the sun, a coat of reflective chromium was
placed on the inside surface of the outside pane of glass. (This layer of chrome was what
gave the building its mirror effect.) The window frame was bonded to the chrome with a
lead solder. During the testing, it was noticed that when a window failed, the failure
began when a tiny J-shaped crack appeared at the edge of an outside pane of glass.
What was happening was this: The lead solder was bonding too well with the chrome so well, so rigidly, that the joint couldn't absorb any movement. But window glass always
moves. It expands and contracts with changes in temperature, and it vibrates with the
wind. So the solder would fatigue and crack. The crack would telegraph through to the
glass, and the cycle of failure would begin.
Investigators found that similar failures had occurred with this same window type, by
this same window manufacturer, in other, less conspicuous buildings. All 10,344 of the
Hancock's double-pane windows were replaced with single sheets of tempered glass. The
window maker paid the cost, which was $7 million.
Conclusions:
The factors that allowed this engineering disaster to occur were both human and
managerial. Some of the materials used in this project were relatively new in the
construction world (e.g. Double-Glazing) and had not been used on large-scale projects
such as this one. Therefore there was a high level of risk involved. The properties of these
materials, especially the glazing, required more scrutinising in order to ensure that this
building would be safe once completed.
It was the strength of the bonds between solder and outside sheet that caused the problem,
from these events engineers had learned that both the window bonds and the slenderness
ratio had caused these problems and as a result of this the building face was changed to
single, thicker glazing which was split to reduce the slenderness ratio.
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. As a great LeMessurier once said “Any time
you depart from established practice, make ten times the effort, ten times the
investigation. Especially on a very large –scale project”.
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