Experiments in Gothic Structure
by Robert Mark
Don'tcha love stone? Sharon too, a fine, smart performer, but I meant like slate and dolomite. We've built a quarter-million years' worth of dwellings from it, and you try making a Great Pyramid out of shittim wood. At first we just heaped up rocks (A), discovering many key principles (gravity, cockroaches, the supervisor). 
It was much later, maybe only six or seven thousand years ago, that we came on the concept of the block, and refinements like doors and plumbing had to wait longer yet.
People who design in stone have spent a lot of time looking for ways to use less of it. Not because they thought the supply would run out, but because it's an engineering problem and folks love the engineering problems. Long story short, by about 1000 AD we could pile up stone blocks to make houses with windows, two and even more stories, and statues on the roof.
And so we come to the Gothic movement. In an effort to get more light into churches, supervisors found that you could put up a big building with thin walls and lots of windows if you put some of the support on the outside. Here's how it worked: A solid wall is easy because the stone down at the bottom will support the weight of the stone up at the top. With care you can even pierce the wall for windows, provided you build in extra strength (thick places) around the hole.
But putting a roof over four high walls will bring the whole thing down, because a peaked roof pushes out on the top of the wall (B). And a high wall catches winds that want to overturn it, too. So you have to add lateral strength to the up-and-down strength of the wall. Heaping up stones is a nearly perfect way to achieve lateral strength. 
If the wall topples, you make a new one with a broader base. Until about 1000, people built churches by artfully heaping up stones into thick walls, not too high, pierced only for small windows. Roman arches (semicircular) carried the strength across the windows and doors and from north to south. But now the Supervisors' Guild began passing around a new secret: If you can separate functions, the walls can handle their own weight while something else makes the building laterally strong (C).
From a house with buttresses to a Gothic church is just a little step. All you do is increase the scale two hundred times, get funding from the devout of the parish for four generations, and for heaven's sake don't make it look like some kind of shack. Use pointed arches and flying buttresses, as they did at Chartres (D). And, maybe the most important thing, get a first-rate supervisor for the job.
Robert Mark followed up the work of those medieval supervisors with technologies they could not use: computer modeling and photoelastic analysis. He and his co-workers built Plexiglas models (E) of notable structures (Chartres, Bourges, St.-Ouen and a dozen others), loaded them, shone polarized light through them, and took color photos. Now many transparent solids alter the polarization of light in a way that depends on the state of stress. A region that's heavily smashed together will appear a different color from one that's under no stress or is being stretched. By looking at the color bands, you can say what parts of the structure are in compression (good for stone, same as in a stable heap of rocks) and which parts are in tension (bad for stone, worse for the mortar that holds the blocks in place). Maybe the results of this study should not be surprising:
—Structures that are still standing, 800-900 years after construction, mostly have their stone blocks in compression, even when the wind blows against them.
—(No finding on structures that fell down and weren't rebuilt)
—If you examine the actual churches and note where they have been repaired over the centuries, it's at the places where the model photos show a state of tension.
—If you can sort out which pieces were built when, you find that additions, such as pinnacles, improved the state of stress in the older parts. Sometimes you see nonperforming bits removed by later supervisors.
You can admire and love Gothic buildings without knowing any of the engineering and science. The spaces sound great; you get exalting effects from light, motion and repetition; the level of skill and effort puts you in awe. But can anyone really look at a wall that isn't there, like the Rose Window wall at the west end of Chartres (here is a superb photograph), and not mutter, "Where did they get the nerve to try hanging all that glass in empty space?" Mark's ambiguous title suggests an answer: The supervisors of the 11th to 13th centuries did careful experiments, shared what they learned, and relied on what they took for rules of thumb but were really solid science and engineering. And they were nervy, too.
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Gothic Structure