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Christopher Mark was born in 1956, the eldest of three sons of a civil engineer named Robert Mark.

His mother was a classical pianist, but his mother, for reasons that later became clear, wasn’t present in Chris’s initial, and somewhat halting, telling of his own story.

His father, however, was impossible to hide.

His father had moved the family to Princeton the year Chris was born.

Robert Mark had grown up in the Bronx and studied engineering at City College of New York.

A few years out of college, he’d made a name for himself with his deft use of photoelastic models to test the effects of physical stress on virtually any object.

He was testing fighter jets and nuclear subs for the Defense Department when Princeton hired him to test parts of small but expensive nuclear reactors it was about to build.

His work saved Princeton so much money that the university ignored his lack of graduate education and invited him to be a professor in the engineering department.

He accepted.

There, his life was biffed onto a radically different course.

“A kid asked a question,” recalled Chris.

“He’d just come from some art history class, where they had these running arguments about Gothic cathedrals
— if certain elements in the buildings are there for aesthetic reasons or structural reasons.

The kid asks my dad:
Can you answer the questions using these models you have?”

The answer was yes.

It would be a bit like reopening a cold case using new DNA technology.

A 12th-century builder had no concept of gravity and only Roman numerals to work with:
He couldn’t multiply or divide.

And yet an engineering movement that started in roughly 1135 A.D. proceeded to generate structures more improbable and accomplished than anything built anywhere in the world over the next 700 years.

As if to further bewilder historians, their architects had left next to no written records.

Any tourist who has stumbled into Chartres soon asks the obvious question:
What’s holding this roof up?

By the time the question was put to Robert Mark, scholars had pretty much given up looking for an answer.

“An insuperable barrier separates their approach to building from ours,” wrote one of the leading historians of Gothic art, before dismissing any hope of figuring it out.

But then Mark deployed his stress-testing gizmos to investigate Gothic cathedrals.

“Robert’s big thing was showing that this technique that came from aerospace could be used for concrete,” says Rob Bork,
a former student and current professor of medieval architecture at the University of Iowa.

“The work was not only original but essentially unique.”

Mark began by taking a vertical slice of, say, Chartres and replicating it in a special kind of plastic.

He’d then hang fishing weights from various points on the plastic replica,
like ornaments on a Christmas tree,
to simulate the actual external forces acting upon various parts of the cathedral.

There was the direct load of the overhead stone, of course,
but also the winds.

(To estimate the winds in the 12th century, he found anemometer readings in rural France going back a century. Not perfect, but good enough.)

He placed his fully loaded plastic model in an oven, where it was subjected not just to heat but also light.

Warmed, the plastic model revealed its stresses, sort of like the way an MRI reveals damage to soft human tissue.

The models had their own haunting beauty.

They turned art history into science.

They generated testable hypotheses.

They predicted exactly which stones inside Chartres or any other cathedral might be overstressed by their loads.

But the power of Mark’s methods became clearest when he traveled to France to visit cathedrals.

The buildings behaved exactly the way his models suggested they should.

“There should be cracking in the mortar here,” he would say to some French stonemason at Chartres,
and the stonemason would invariably reply,

“We repointed that only last year!”

For centuries, the damage inside Chartres had been repaired by workers who never understood why certain stones always needed replacing.

Now this guy from Princeton could not only tell you why
— he could explain the buildings in ways that not even their builders could have done.

Mark founded a program at Princeton that combined architecture and engineering.

His plastic models yielded insights beyond the cathedrals’ weak spots.

They proved that certain Gothic features that art historians assumed essential were mostly decorative
and other Gothic features that seemed decorative were structural,
preventing the roof from collapsing.

An example:
The pinnacles on top of the outer piers had been thought to be mainly for show,
but they actually pre-stressed the mortar beneath them and thus prevented it from cracking and weakening the entire structure.