AON Tower

The AON Center. I am not sure it’s a proper name for this beautiful tower.

Aon doesn’t even own the building, they share the rent with ketchup maker Kraft Heinz. The real owner is Mark Karasick.

Companies come and go away. The architecture stays immortal.

Some people call it the Amoco building. Where is Amoco now?

Other people remember it as the Big Stan. Because it used to be headquarters of the Standard Oil Company of Indiana, long gone as well.

But that’s a proper name: the Big Stan.

Edward Durell Stone designed the Big Stan back in the 1970s. It is a strong and elegant tubular steel-framed structure, same as the former World Trade Center towers.

But he made a tiny mistake in his design. It measured only half of an inch but it led to really big consequences.

Edward Durell Stone fell in love with Carrara marble back when he traveled in Europe after his graduation from Harvard. Michelangelo used this brilliantly white marble to carve his famous David. And Edward ambitiously decided to clad his 80-tower in Carrara marble as well.

But to save money he went with inch and half panels which proved to be too thin for temperamental Chicago weather. Two inches, just a half of an inch more, might have been more prudent panel thickness.

Carrara marble is the best in the world for a reason. Because it is almost pure calcite, the main mineral which forms marble.

But calcite has a structural flaw. Depending on temperature it expands and contracts differently along different crystal axes. It also readily absorbs water which causes cracks during freeze and thaw cycles.

To cut story short Carrara marble panels began to crack and buckle, steel fasteners became loose. Finally 350-pound slabs started to fall off the building. One of the panels smashed the car down on Randolph Street, another panel got wedged on the roof of a neighboring building.

At first they tried stainless steel straps to hold the marble in place. It didn’t help.

At the end all Carrara marble has been removed and replaced with white granite panels from North Carolina.

Was it expensive? Yes, about 80 million dollars. It almost doubled the cost of the building.

Half of an inch miscalculation. Small mistake. Big consequences.

Something to ponder about when you are making your next decision.