NYT: Glass that doesn't break
MATERIAL WORLD
By HENRY FOUNTAIN
CHICAGO — To truly appreciate how glass can be used structurally, make your
way to 233 South Wacker Drive in downtown Chicago. More precisely, make your
way 1,353 feet above South Wacker, to the 103rd floor of the Sears Tower.
Once there, take a few steps over to the west wall, where the facade has
been cut away. Then take one more step, over the edge.
You’ll find yourself on a floor of glass, suspended over the sidewalk a
quarter-mile below. If you can’t bear looking straight down past your feet,
shift your gaze out or up — the walls are glass, too, as is the ceiling.
You’ve stepped into a transparent box, one of four that jut four and a half
feet from the tower, hanging from cantilevered steel beams above your head.
The glass walls are connected to the beams, and to the glass floor, with
stainless-steel bolts. But what’s really saving you from oblivion is the
glass itself.
The boxes, which opened last week as part of an extensive renovation of the
tower’s observation deck, are among the most recent, and more outlandish,
projects that use glass as load-bearing elements. But all glass structures
have at least a bit of daring about them, as if they are giving a defiant
answer to the question: You can’t do that with glass, can you?
You can. Engineers, architects and fabricators, aided by materials
scientists and software designers, are building soaring facades, arching
canopies and delicate cubes, footbridges and staircases, almost entirely of
glass. They’re laminating glass with polymers to make beams and other
components stronger and safer — each of the Sears Tower sheets is a
five-layer sandwich — and analyzing every square inch of a design to make
sure the stresses are within precise limits. And they are experimenting with
new materials and methods that could someday lead to glass structures that
are unmarked by metal or other materials.
“Ultimately what we’re all striving for is an all-glass structure,” said
James O’Callaghan of Eckersley O’Callaghan Structural Design, who has
designed what are perhaps the world’s best-known glass projects, the
staircases that are a prominent feature of some Apple Stores.
Through it all, they’ve realized one thing. “Glass is just another
material,” said John Kooymans of the engineering firm Halcrow Yolles, which
designed the Sears Tower boxes.
It’s a material that has been around for millennia. Although glass can be
made in countless ways to have any number of specific uses — to conduct
light as fibers, say, or serve as a backing for electronic circuitry, as in
a laptop screen — structural projects almost exclusively use soda-lime
glass, made, as it has always been, largely from sodium carbonate, limestone
and silica.
“For years, the basic composition of soda-lime glass has not changed much,”
said Harrie J. Stevens, director of the Center for Glass Research at Alfred
University. It’s the same glass, more or less, that is used for the windows
in your home and the jar of jam in your fridge — and that old elixir bottle
you bought at an antique store.
It’s basic stuff, but far from simple. “Of course, glass is an unusual
material,” said James Carpenter of James Carpenter Design Associates, who
has designed glass facades and other structures and was a consultant for the
glassmaker Corning in the 1970s. “Since we don’t really know what it is.”
Although there has long been debate as to whether glass is a solid or
liquid, it is now usually described as an amorphous solid (there is no
evidence that it flows, extremely slowly, over time as a liquid). The
noncrystalline structure is achieved by relatively rapid cooling below what
is referred to as the glass transition temperature, around 1,000 degrees
Fahrenheit for the soda-lime variety.
Cooled further and cut, pristine glass is very strong. But like a new car
that plummets in value the moment it is driven off the lot, glass starts to
lose its strength the instant it’s made. Tiny cracks begin to form through
contact with other surfaces, or even with water vapor and carbon dioxide.
“If you take the freshly made surface and blow on it with your breath,
you’ve reduced the strength of glass by a factor of two,” said Suresh
Gulati, a mechanical engineer and self-described “strength man” who retired
in 2000 after 33 years at Corning but still works for the company as a
consultant.
Even one gas molecule can break a silicon-oxygen bond in glass, generating a
defect, said Carlo G. Pantano, a professor of materials science at
Pennsylvania State University. While glass is very strong in compression,
tensile stresses will make these tiny fissures start to grow, bond by bond.
“That’s what makes glass break,” Dr. Pantano said. “And if it doesn’t break,
it weakens it.”
Protective coatings are one way to avoid new cracks, although they can
affect transparency, which is the main reason for using glass in the first
place. Changing the glass recipe can also make it harder for cracks to form
and propagate. “There is some evidence that you can modify the composition
to make it innately stronger,” Dr. Stevens said, although that risks
altering other properties or making the glass too costly. (And glass
projects are not cheap to start with; the glass in the Sears Tower project
cost more than $40,000 per box.)
The manufacturing process can be modified, too, to keep the surfaces of the
glass as pristine as possible. In one technique, used for laptop glass,
molten glass is pumped into a V-shaped trough, spills over on both sides and
flows down the outside of the V, joining together at the bottom into a sheet
that continues to move downward as it cools. This way, each side of the
sheet is a “melt surface,” exposed only to the air and not touched by any
part of the equipment.
For structural purposes, glass is often strengthened the old-fashioned way —
by tempering. This puts the surface under compression, so that even more
tensile force is needed for cracks to grow.
For flat glass, heat tempering is most often used. William LaCourse, a
professor at Alfred, said the process took advantage of one property of
glass — that when it cools slowly it becomes denser. By rapidly cooling the
exterior of a sheet (usually with air), the surface stays less dense.
“Inside it’s still hot, and tries to cool to a more dense structure,” Dr.
LaCourse said. “This pulls the surface into compression.”
In chemical tempering, sodium ions in the surface are replaced with
potassium ions, which are about 30 percent larger. It’s like taking a
suitcase full of summer-weight clothes and replacing the top layer with
winter-weight items; the suitcase will bulge at the seams when you try to
close it. Glass cannot bulge at the seams, so the surface becomes
compressed.
Tempered glass may take longer to crack, but it can still break. Because
surface compression must be balanced by interior tension, when tempered
glass does break it forms many more smaller pieces than untempered glass, as
more fracture lines release more energy. “The more it is strengthened the
more pieces it will fly into,” Dr. Gulati said. An extreme example of this
is a Prince Rupert’s drop, a small glass ball with a long tail formed by
dropping molten glass into water. You can pound on the ball end with a
hammer and it will not break, but snip off the tail and the ball will
explode into tiny pieces as the tensile forces are released.
In structural applications, breaking into smaller pieces is often preferred,
because these have less chance of causing injury. But tempering alone is
usually not enough.
A primary concern when building with glass is what happens if and when a
component breaks — what engineers call “post-failure behavior.” Unlike steel
or other materials, glass does not deform or otherwise give advance warning
of failure. If breakage occurs, maintaining the integrity of the structure
is paramount so that people on or below it are safe.
That’s where lamination comes in. In a typical project, glass sheets
(one-half-inch thick in the Sears Tower project) are bonded with thin
polymer interlayers. The interlayers add strength and, should one of the
glass layers break, keep the structure together, and the pieces from
falling.
But lamination makes fabricating glass for structural uses very difficult.
Since cutting into tempered glass causes it to break, each sheet must be
polished and drilled for the connecting fittings before it is tempered.
Tolerances are extremely small, to avoid potentially destructive stresses in
the assembled structure.
“It’s doable,” said Lou Cerny of MTH Industries, who managed the
installation at the Sears Tower, where the tolerances were one-sixteenth of
an inch. “There’s just not a lot of people who want to get involved in it.”
No wonder, then, that those who build with glass look forward to a day when
their structures will be unencumbered by metal or other materials.
“My goal has always been to reduce the amount of fittings in glass,” said
Mr. O’Callaghan, whose Apple staircases use stainless steel and,
occasionally, titanium to join the glass components.
Already, some engineers are using different glass shapes to reduce the
dependence on metal. Rob Nijsse, a professor at the Delft University of
Technology in the Netherlands and a structural engineer with the firm ABT
Belgium, has used large sheets of corrugated glass, mounted vertically, for
window walls in a concert hall in Porto, Portugal, and a museum being built
in Antwerp, Belgium. The shape helps stiffen the glass against wind loads.
Other designers think about using different kinds of glass. “There are so
many amazing types of glass available,” Mr. Carpenter said. “There’s an
enormous potential to transfer some of their characteristics into
architectural uses.”
Using a glass that does not expand much when heated, for example, would
enable components to be welded together, forming, in effect, a continuous
piece of glass. Conventional soda-lime glass expands too much, so welding
introduces stresses that can lead to failure.
Researchers at Delft have experimented with welding glass components. But
low-expansion glass is much costlier than soda-lime glass.
Other engineers are starting to use adhesives to join glass directly to
glass. Lucio Blandini, an engineer with Werner Sobek Engineering and Design
in Stuttgart, Germany, used adhesives to create a thin glass dome, 28 feet
across, for his doctoral thesis in a clearing in Stuttgart. “I think
adhesives are the most promising connection device,” Dr. Blandini said. “It
allows glass to keep its aesthetic qualities.” His firm is using adhesives
in parts of structures being built at the University of Chicago and in
Dubai.
But the long-term strength and reliability of adhesives has not been proved,
so most people who work in glass think an all-glued structure is a long way
off.
“We have way too many lawyers in this country,” said Mr. Cerny, the
installer at the Sears Tower. “It’ll be awhile before we see that.”
July 7, 2009
Copyright 2009 The New York Times Company
http://www.nytimes.com/2009/07/07/science/07glass.html
By HENRY FOUNTAIN
CHICAGO — To truly appreciate how glass can be used structurally, make your
way to 233 South Wacker Drive in downtown Chicago. More precisely, make your
way 1,353 feet above South Wacker, to the 103rd floor of the Sears Tower.
Once there, take a few steps over to the west wall, where the facade has
been cut away. Then take one more step, over the edge.
You’ll find yourself on a floor of glass, suspended over the sidewalk a
quarter-mile below. If you can’t bear looking straight down past your feet,
shift your gaze out or up — the walls are glass, too, as is the ceiling.
You’ve stepped into a transparent box, one of four that jut four and a half
feet from the tower, hanging from cantilevered steel beams above your head.
The glass walls are connected to the beams, and to the glass floor, with
stainless-steel bolts. But what’s really saving you from oblivion is the
glass itself.
The boxes, which opened last week as part of an extensive renovation of the
tower’s observation deck, are among the most recent, and more outlandish,
projects that use glass as load-bearing elements. But all glass structures
have at least a bit of daring about them, as if they are giving a defiant
answer to the question: You can’t do that with glass, can you?
You can. Engineers, architects and fabricators, aided by materials
scientists and software designers, are building soaring facades, arching
canopies and delicate cubes, footbridges and staircases, almost entirely of
glass. They’re laminating glass with polymers to make beams and other
components stronger and safer — each of the Sears Tower sheets is a
five-layer sandwich — and analyzing every square inch of a design to make
sure the stresses are within precise limits. And they are experimenting with
new materials and methods that could someday lead to glass structures that
are unmarked by metal or other materials.
“Ultimately what we’re all striving for is an all-glass structure,” said
James O’Callaghan of Eckersley O’Callaghan Structural Design, who has
designed what are perhaps the world’s best-known glass projects, the
staircases that are a prominent feature of some Apple Stores.
Through it all, they’ve realized one thing. “Glass is just another
material,” said John Kooymans of the engineering firm Halcrow Yolles, which
designed the Sears Tower boxes.
It’s a material that has been around for millennia. Although glass can be
made in countless ways to have any number of specific uses — to conduct
light as fibers, say, or serve as a backing for electronic circuitry, as in
a laptop screen — structural projects almost exclusively use soda-lime
glass, made, as it has always been, largely from sodium carbonate, limestone
and silica.
“For years, the basic composition of soda-lime glass has not changed much,”
said Harrie J. Stevens, director of the Center for Glass Research at Alfred
University. It’s the same glass, more or less, that is used for the windows
in your home and the jar of jam in your fridge — and that old elixir bottle
you bought at an antique store.
It’s basic stuff, but far from simple. “Of course, glass is an unusual
material,” said James Carpenter of James Carpenter Design Associates, who
has designed glass facades and other structures and was a consultant for the
glassmaker Corning in the 1970s. “Since we don’t really know what it is.”
Although there has long been debate as to whether glass is a solid or
liquid, it is now usually described as an amorphous solid (there is no
evidence that it flows, extremely slowly, over time as a liquid). The
noncrystalline structure is achieved by relatively rapid cooling below what
is referred to as the glass transition temperature, around 1,000 degrees
Fahrenheit for the soda-lime variety.
Cooled further and cut, pristine glass is very strong. But like a new car
that plummets in value the moment it is driven off the lot, glass starts to
lose its strength the instant it’s made. Tiny cracks begin to form through
contact with other surfaces, or even with water vapor and carbon dioxide.
“If you take the freshly made surface and blow on it with your breath,
you’ve reduced the strength of glass by a factor of two,” said Suresh
Gulati, a mechanical engineer and self-described “strength man” who retired
in 2000 after 33 years at Corning but still works for the company as a
consultant.
Even one gas molecule can break a silicon-oxygen bond in glass, generating a
defect, said Carlo G. Pantano, a professor of materials science at
Pennsylvania State University. While glass is very strong in compression,
tensile stresses will make these tiny fissures start to grow, bond by bond.
“That’s what makes glass break,” Dr. Pantano said. “And if it doesn’t break,
it weakens it.”
Protective coatings are one way to avoid new cracks, although they can
affect transparency, which is the main reason for using glass in the first
place. Changing the glass recipe can also make it harder for cracks to form
and propagate. “There is some evidence that you can modify the composition
to make it innately stronger,” Dr. Stevens said, although that risks
altering other properties or making the glass too costly. (And glass
projects are not cheap to start with; the glass in the Sears Tower project
cost more than $40,000 per box.)
The manufacturing process can be modified, too, to keep the surfaces of the
glass as pristine as possible. In one technique, used for laptop glass,
molten glass is pumped into a V-shaped trough, spills over on both sides and
flows down the outside of the V, joining together at the bottom into a sheet
that continues to move downward as it cools. This way, each side of the
sheet is a “melt surface,” exposed only to the air and not touched by any
part of the equipment.
For structural purposes, glass is often strengthened the old-fashioned way —
by tempering. This puts the surface under compression, so that even more
tensile force is needed for cracks to grow.
For flat glass, heat tempering is most often used. William LaCourse, a
professor at Alfred, said the process took advantage of one property of
glass — that when it cools slowly it becomes denser. By rapidly cooling the
exterior of a sheet (usually with air), the surface stays less dense.
“Inside it’s still hot, and tries to cool to a more dense structure,” Dr.
LaCourse said. “This pulls the surface into compression.”
In chemical tempering, sodium ions in the surface are replaced with
potassium ions, which are about 30 percent larger. It’s like taking a
suitcase full of summer-weight clothes and replacing the top layer with
winter-weight items; the suitcase will bulge at the seams when you try to
close it. Glass cannot bulge at the seams, so the surface becomes
compressed.
Tempered glass may take longer to crack, but it can still break. Because
surface compression must be balanced by interior tension, when tempered
glass does break it forms many more smaller pieces than untempered glass, as
more fracture lines release more energy. “The more it is strengthened the
more pieces it will fly into,” Dr. Gulati said. An extreme example of this
is a Prince Rupert’s drop, a small glass ball with a long tail formed by
dropping molten glass into water. You can pound on the ball end with a
hammer and it will not break, but snip off the tail and the ball will
explode into tiny pieces as the tensile forces are released.
In structural applications, breaking into smaller pieces is often preferred,
because these have less chance of causing injury. But tempering alone is
usually not enough.
A primary concern when building with glass is what happens if and when a
component breaks — what engineers call “post-failure behavior.” Unlike steel
or other materials, glass does not deform or otherwise give advance warning
of failure. If breakage occurs, maintaining the integrity of the structure
is paramount so that people on or below it are safe.
That’s where lamination comes in. In a typical project, glass sheets
(one-half-inch thick in the Sears Tower project) are bonded with thin
polymer interlayers. The interlayers add strength and, should one of the
glass layers break, keep the structure together, and the pieces from
falling.
But lamination makes fabricating glass for structural uses very difficult.
Since cutting into tempered glass causes it to break, each sheet must be
polished and drilled for the connecting fittings before it is tempered.
Tolerances are extremely small, to avoid potentially destructive stresses in
the assembled structure.
“It’s doable,” said Lou Cerny of MTH Industries, who managed the
installation at the Sears Tower, where the tolerances were one-sixteenth of
an inch. “There’s just not a lot of people who want to get involved in it.”
No wonder, then, that those who build with glass look forward to a day when
their structures will be unencumbered by metal or other materials.
“My goal has always been to reduce the amount of fittings in glass,” said
Mr. O’Callaghan, whose Apple staircases use stainless steel and,
occasionally, titanium to join the glass components.
Already, some engineers are using different glass shapes to reduce the
dependence on metal. Rob Nijsse, a professor at the Delft University of
Technology in the Netherlands and a structural engineer with the firm ABT
Belgium, has used large sheets of corrugated glass, mounted vertically, for
window walls in a concert hall in Porto, Portugal, and a museum being built
in Antwerp, Belgium. The shape helps stiffen the glass against wind loads.
Other designers think about using different kinds of glass. “There are so
many amazing types of glass available,” Mr. Carpenter said. “There’s an
enormous potential to transfer some of their characteristics into
architectural uses.”
Using a glass that does not expand much when heated, for example, would
enable components to be welded together, forming, in effect, a continuous
piece of glass. Conventional soda-lime glass expands too much, so welding
introduces stresses that can lead to failure.
Researchers at Delft have experimented with welding glass components. But
low-expansion glass is much costlier than soda-lime glass.
Other engineers are starting to use adhesives to join glass directly to
glass. Lucio Blandini, an engineer with Werner Sobek Engineering and Design
in Stuttgart, Germany, used adhesives to create a thin glass dome, 28 feet
across, for his doctoral thesis in a clearing in Stuttgart. “I think
adhesives are the most promising connection device,” Dr. Blandini said. “It
allows glass to keep its aesthetic qualities.” His firm is using adhesives
in parts of structures being built at the University of Chicago and in
Dubai.
But the long-term strength and reliability of adhesives has not been proved,
so most people who work in glass think an all-glued structure is a long way
off.
“We have way too many lawyers in this country,” said Mr. Cerny, the
installer at the Sears Tower. “It’ll be awhile before we see that.”
July 7, 2009
Copyright 2009 The New York Times Company
http://www.nytimes.com/2009/07/07/science/07glass.html
Joel
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