Chartres Cathedral, a UNESCO World Heritage Site in northwestern France, is considered the pinnacle of Gothic architecture and is the epitome of medieval engineering, using the physical properties of stone and innovative architectural techniques to achieve its massive scale and spectacular stained glass.
When you visit Chartres, a city in northwestern France, you’ll be greeted by a historic structure that is also a UNESCO World Heritage Site. It’s Chartres Cathedral. Colorful, magnificent, and, for some, awe-inspiring, this stunning structure is the epitome of Gothic art and the epitome of Western medieval materials science and architectural engineering.
Chartres Cathedral is primarily made of stone, although some wood and glass were also used. Unlike wood, which is susceptible to fire and moisture and will decay over time, stone is resistant to fire and does not rot, making it extremely durable. This makes stone the perfect material for building tall cathedrals. However, one material engineering property of stone posed a major structural challenge to Gothic architects. The problem was the difference between compressive and tensile stresses in stone.
Compressive stress is the resistance created within a material by a compressive force applied from the outside. If the compressive stress is high, the material can resist being deformed by external compressive forces. Tensile stress, on the other hand, is the resistance to an external force that tries to stretch a material. A high tensile stress resists external forces that try to stretch the material. Because these two forces act perpendicular to the surface of the material (normal), they are sometimes referred to as normal stresses.
Stone has a high compressive stress, which is why you can build a vertical column tens of meters long and not have it crack at the bottom, but tensile stress is a different story. Stone has less tensile stress than wood, so if you were to make beams to support the roof of a building out of flat, long stones, they would crack easily under their own weight and break if you didn’t do it right. So it seemed impossible to use flat stones to support the heavy ceilings of Chartres Cathedral, which had to have a large interior space. But Gothic architectural engineers cleverly overcame the huge problems that stone posed. Instead of flat beams, they used stone to create arches. By creating an arch, the bricks distributed the weight of the top brick to the bricks below on either side, allowing the load to be borne in compressive stress rather than the tensile stress of the stone. The large compressive stresses in the stone prevented the material from cracking and also prevented the bricks from sliding over each other and causing the building to collapse. These arches were not only used in Chartres Cathedral, but in many other large medieval buildings.
Early medieval architects opted for a “cylindrical vaulted ceiling,” in which the arches were simply stretched horizontally. However, there was one problem with using cylindrical vaulting for large buildings. The walls had to be thicker to support the weight of the building, and the windows couldn’t be as large as they were. The wider the window, the less the wall could support the load. For this reason, cylindrical vaulted ceilings were not well suited to the Gothic style, which demanded great height and maximum light.
Gothic architecture was not just about structural stability, but also about creating a sacred and mystical atmosphere for the people of the time. The light entering the interior of a cathedral symbolized heavenly glory, and large windows with stained glass were essential to maximize that light. Medievalists believed that this light filled the cathedral and brought them closer to the presence of God. In this context, Gothic architects sought not only to solve structural problems, but also to optimize the aesthetic use of light and space. The result was an innovative combination of arches and buttresses that gave Gothic cathedrals a unique aesthetic that preserved the mystical atmosphere of the interior without losing the majesty of the exterior.
The subsequent “cross-vaulted” structure was the first step toward overcoming the shortcomings of cylindrical vaulting. Crossed vaulted ceilings are two cylindrical vaulted ceilings crossed vertically to create an arch structure with four columns instead of two walls. This concentrated the load of the building on the four columns, which were then supported by buttresses. By using cross-vaulted ceilings, architects were able to create wider windows and build taller structures. However, even with these advances, cross-vaulted ceilings still had some limitations on how large windows could be, as architects had to find ways to efficiently reinforce the columns to support the load against the outward force of each column, and the exterior walls still had to support the load of the roof.
The solution Gothic architects found to the problem of creating buildings that were tall, grand, and able to utilize light from the outside for aesthetic purposes was the wolf-vaulted ceiling, which combines a pinnacle arch with a cross-vaulted ceiling structure. A pinnacled arch is an arch whose top is pointed upward rather than rounded. This allows the arch to be much taller and the top load to be transferred more directly to the columns than with a rounded arch. By using a wolf-arch ceiling structure, the load of the building is transferred directly to the columns at the corners of the arch, minimizing the load of the roof on the exterior walls and allowing for large windows in the building. The large stained glass windows you see in Gothic cathedrals are proof of this. But even so, the outward thrust concentrated on each column was still a major concern. This was especially true when large-scale arches were used, spaced far apart to accommodate large windows. One solution was to have large supports and buttresses projecting outward from the columns, but Gothic architects were reluctant to use them because they could be detrimental to the aesthetics of the building and, if done incorrectly, could block light from the windows. Eventually, Gothic architects came up with a solution to remove all the unnecessary parts of the buttresses: aerial buttresses. They didn’t block light from entering the building, they were aesthetically pleasing, and they were an economical building technique because they allowed the building’s load to be safely transferred to the ground using minimal materials.
As you can see, Chartres Cathedral is a laboratory of engineering, with materials science and architectural engineering at its core. If you’re traveling to France in the future, it’s worth taking a few minutes to visit Chartres Cathedral to appreciate the beauty of its architecture and the engineering behind it.
