How to Build a Glass Tower that Doesn't Fall Down

A skyscraper of glass had long been one of modernist architecture's dreams. Boston's Hancock Tower was supposed to idealize that dream. Instead, its 500 pound windows started crashing to the street. What went wrong then, and what goes right now?

When the Sky Started Falling

Giant glass towers have become commonplace to the point of unimpressive—maybe even something of a corporate HQ cliche. But in the early 70s, the John Hancock tower was something particularly stellar. Sixty floors, wrapped in stark, muted glass—10,344 pieces of glass, to be exact. The building, tallest in all of Boston, has a peculiar wedge shape, like a giant sail in the skyline. It was, at the risk of sounding like a stodgy architecture professor, a triumph of design. It was enormous, beautiful, and novel. At first. That is, before it started to completely fall apart.

The Hancock tower wasn't even finished before it started to fail. One by one, in the winter of 1973, its mammoth glass plates started to pop out and plummet hundreds of feet to the sidewalk, where they, of course, terrifyingly shattered—sometimes after crashing through other windows on the way down. With broken glass everywhere, the area around the building was roped off in a panic. It was like someone's drunk uncle had knocked into a 60-story, very expensive Christmas tree. But no one knew why it was happening. Some thought it was the wind. Some thought it was the shape. Nobody was right.

How to Build a Glass Tower that Doesn't Fall Down

Then, it got a lot worse. A Swiss engineer brought in to help the broken tower noted something slightly more significant than tumbling, half-ton windows—the entire structure was at risk of falling over. Edge-wise. The tower had been reinforced along its broad sides to brace it against the wind—but this had left its skinny ends inadvertently vulnerable, and meant the entire thing could topple over itself in a spectacular implosion of ruptured glass and steel, like a scene from Independence Day, as directed by Mies van der Rohe.

Two massive, 300-ton counterweights had to be installed in the tower's guts to compensate for its sideways motion. All this for only an extra $3 million! Oh, and by this point, over an acre of the building's mangled face was boarded up with plywood, like some neighborhood crack house. Bostonians eye-rollingly called it the "Plywood Palace."

How to Build a Glass Tower that Doesn't Fall Down


Today, it's beautiful again. A survey of architects conducted by the Boston Globe named it the third best building in the history of Boston (a pretty city with a lot of history, mind you). So what changed? How did the architectural equivalent of your pants falling down at the Oscar podium turn from disgrace to gorgeous monolith? And how do architects design similar buildings so the same shattertastic popping-apart doesn't repeat?

Building a Better Window

Fixing the horrible mess of the Hancock Tower was a long, expensive process. It was also a secretive process—all members involved were sworn to secrecy as part of a legal compact. Details have leaked out through interviews over the years, but rather than rely on the architectural grapevine, we went straight to an authority.

Nicholas Holt is Technical Director at Skidmore, Owings and Merrill—the architectural firm behind a couple of minor buildings you may have heard of. Like the world landmark Sears Tower, and the Dubai's Burj Khalifa—you know, the largest skyscraper in history. Suffice it to say Nicholas knows how to put together a building that stays up, and more importantly for us—the man knows his damn glass. I talked to him about how one, in 2010, designs a glass structure to avoid 70s blunders of the Hancock Tower.

The use of glass in a given structure is more than a way of dazzling us as we look up. It looks incredible, yes. But modernists loved it beyond its handsomeness—the stuff is useful, and wears its function on its sleeve.

Why Glass?

Justifying this simple material is an important place to start.

A modern building has needs. Beyond not disintegrating (we'll get back to that), an architect of today wants their building to use as little energy as possible while serving the little people inside all day. People work indoors. People need light to work. People like natural light. Construct a building out of glass? Let the sun shine in. Beautiful. More real light means less artificial light, which means less electricity, which, well, you get it. Everyone wins, and the thing looks beautiful in the process.

But let's not forget—the sun isn't just a big glowing orb. It's shooting down heat along with all that light. And lest your office turn into a giant sweaty greenhouse, a building ought to let light in, while turning heat away at the door. The trick, Nicholas explains, is to just bounce the heat off. It never crosses through. Low-emission (or Low-E) glass works like a thermal bouncer. Let in the beautiful light, give heat the boot. Modern buildings run in what Nicholas calls "constant cooling mode." The stuff inside a skyscraper makes heat. A lot of it. People, PCs, lights—all of it radiates warmth, and it adds up. Quick. So even in the winter, the last thing you want, surprisingly, is more warmth pouring it from outdoors. Low-E glass is treated with a thin reflective coating that reflects thermal radiation—heat. You keep what you want, you push back what you don't. The best of both worlds.

You Need the Right Glass

Reflective glass isn't a new principle. But, when they tried to slap it across the Hancock Tower, it was an imperfect design method. Really imperfect. The fatal flaw of the Hancock's windows wasn't wind, or torque, or being built on an Indian burial ground—it was the way they tried to bounce off heat. The windows were doubled paned, with a layer of lead applied between chrome-treated glass panels. It was a dangerous mix of things that can flex and things that really, really shouldn't—glass, namely. And what happens when a surface is subjected to day after day of hot sunlight? It expands and contracts. These gradual thermal undulations warped the lead, which spread the damage into the glass panes themselves. Too much stress, and pop—out went the window, hurdling down to the street.

Today, architects ditch the lead. They frequently ditch metals altogether, instead using flexible silicone that won't spread its nasty thermal twitching to the sensitive glass it's bonded to. This kind of sealant is strong, Nicholas says, and has equally strong research to back it up.

Putting It All Together

So you want to build a wall of glass. A big one. The only thing standing between a design on paper and a building on the ground, Nicholas explains, is the size of the oven you can use to bake-bolster the glass plates after spraying on the Low-E coating. The largest in North America crank out 7x14 foot walls of hardened glass—but the largest in world, found in China and Germany, are capable of staggering 49x10 foot sheets. Once you've got your nice giant sealed slabs, they can be placed together between metal spacers—like pictures in a frame—and assembled into a wall. A curtain, actually, as its known. The hanging curtain is pretty much exactly as it sounds—a plane of unified glass plates, sealed, coated, and stuck together. And how does stand up, especially when you stack it 60 stories high? With zero effort at all. Glass curtain walls bear no weight whatsoever, but are instead hung—quite literally—onto the skeleton of a building. Again, think Christmas tree. Gravity keeps everything in place (though only after exhaustive curtain testing, including a barrage of two by fours and subjection to a wind tunnel). Once the glass is thoroughly abused, hang it up, and let it do its work.

Disaster Averted (Sort of)

How to Build a Glass Tower that Doesn't Fall Down

So what about the poor Hancock Tower? A solution you should be very glad you had nothing to do with. Every single pane of glass in the entire building had to be replaced. Price tag: $7 million, and a five year delay. But what about all of those over 5,000 defective windows that weren't damaged or exploded out onto the street? Robert Campbell of the Boston Globe tracked them down, and reported they found a genteel retirement, sold for $100 each as picture frames, coffee tables, and greenhouse parts. And that infamous, shameful plywood? Repurposed to board up abandoned Bostonian buildings.

The lesson was a bitter one, and at 790 feet tall, a hard one to ignore. A poorly-designed stool only fails one sitter. A flawed skyscraper design, on the other hand, looms over an entire city. But on such a scale, no mistake goes unnoticed or to waste. The teetering shard-shaking mess that was the Hancock Tower is a case study in how not to do something—and a testament to just how well it's done now. The building itself still stands, now proudly, despite its dubious past. But walk through any major urban area, and you're likely to see one of its highly reflective nephews. And 40 years later, thanks to smart materials and good design, you don't need to be afraid to look up.

Photos by Adam Pieniaze, Allie Caufield, and Michael Shellenbarger/ University of Oregon Library