A CLEAR VIEW A project lets visitors see all angles from the 103rd floor of the Sears Tower in Chicago. Builders are experimenting with new materials and methods to expand the use of glass in construction. More Photos >
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.
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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 every Apple Store.
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.”
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