The world’s oldest ongoing construction project

Jun 19, 2020 · 10 min read

By Brian McManus and Erica Corder

A old photo with the beginnings of the Sagrada Família’s construction in the background.
The Sagrada Família during construction in 1905. (Image: Baldomer Gili i Roig / Wikimedia Commons; Illustration: MODULUS)

Barcelona is a city characterized by regimental city blocks and wide, linear streets — an orderly departure from the narrow, winding streets of most European cities.

The inside of the Sagrada Família, complete with impressive stone structures and stained glass.
Inside the Sagrada Família. (Image: SBA73 / Flickr)

Nestled in these rigid streets are some of the most unique and distinctive buildings ever created: the Casa Milà, Casa Batlló and, perhaps most famously, the yet to be finished Sagrada Família. Inspired by nature, these complex structural forms and designs defied all conventional architectural rules at the time of their conception.

These stand-out structures are the work of one the world’s greatest architects, Antoni Gaudí. Born in 1852 to a family of craftsmen, Gaudí found much of his inspiration and meaning in architecture by following the patterns of nature, which he considered a gift from God and the ultimate blueprint to the world. As Gaudí himself once said, “originality consists of going back to the origins.”

Gaudí’s structures, made without the aid of computers, utilize complex, intertwining parabolic forms that are so difficult to build that his magnum opus, the Sagrada Família, is still under construction — 135 years since it began.

The inspired mind behind the Sagrada Família

Sagrada Família began construction in 1882, under the guidance of architect Francisco De Paula del Villar, who resigned a year later.

A black and white portrait of Antoni Gaudí.
Antoni Gaudí. (Image: Pau Audouard Deglaire / Wikimedia Commons)

A 30-year-old Gaudí swooped in to claim the project. The budding architect sensed it would be his opportunity to make a masterpiece, leaning on his signature style: a combination of Gothic and Art Nouveau architecture.

But Gaudí was not just a master architect with a distinctive approach. His edge was also a byproduct of his mastery of engineering.

Gaudí understood how the parabolic shapes he would become known for impacted the structural integrity of buildings. He leaned into the physics of his structures and worked with the laws of nature, rather than against them.

Sculptures of people surround blue tinted windows on the outside of the Sagrada Família.
Sculptures on the façade of the Sagrada Família. (Image: Wikimedia Commons)

Gaudí bridged art and science in his work, and understood that nature is defined by the laws of mathematics. To the devout Roman Catholic, this adherence to the natural world in his work was the ultimate celebration of God.

The view of the Sagrada Família looking up.
The ceiling of the Sagrada Família. (Image: SBA73 / Flickr)

Gaudí erected columns mimicking trees and skeletons. He designed arches mirroring rib cages, spiral stairways inspired by seashells, and spires emulating crystals. Inside structures that stand stories tall, Gaudí ceilings rivaled the natural beauty of forest canopies.

Translating these natural phenomena to stone is notable in its own right, but it’s made all the more impressive considering one critical factor: Gaudí designed all of this without computational analysis.

Inside the Sagrada Família. (Left image: Jnolan14 / Wikimedia Commons; Right image: Alvesgaspar / Wikimedia Commons)

The physics behind an optimal stone structure

The challenge that Gaudí faced was one of material strengths. His primary building material was Montjuïc stone, a type of sandstone.

Like all stone, sandstone is strongest when it is in compression, wherein it can withstand enormous pressure. But when in tension, tiny imperfections in the stone allow cracks to grow, which leads to fractures along those cracks.

The Sagrada Família stands tall amidst a backdrop of blue sky.
The exterior of the Sagrada Família, complete with cranes, circa 2010. (Image: Peter Broster / Wikimedia Commons)

The challenge with constructing structures using stone is primarily one of keeping the stone in compression at all times. This has been a well known building technique for thousands of years, but it was the Romans who truly saw and utilized its potential.

Unlike Ancient Greek structures characterized by vertical posts and horizontal lintel constructions, Roman buildings are defined by arches and domes. They raised massive structures, like their famous aqueducts and the Pantheon dome, which still stand today.

The arches’ strength is derived from how it transfers weight to the ground. Compare a post and lintel structure and an arch spanning an equal distance.

As long as the posts remain vertical, they will remain entirely in compression as the downwards force of gravity squeezes the stones against the unmoving ground.

However, the horizontal beam — the lintel — will experience both compression and tension as it sags from its own weight. The upper surface will bend inwards, causing compression, while the lower surface will experience tension as it is stretched. This severely limits its structural strength.

On the left is a depiction of a post and lintel structure, and how it handles force. On the right is an arch structure.
Comparison of the post and lintel construction and the arch construction, and how each handles force. (Illustration: Real Engineering)

Meanwhile, an arch allows the force to be distributed neatly across the arch. There are no hard corners to allow bending stresses to develop. The reaction force from the ground can travel smoothly up the same path as the force of gravity takes downwards, allowing the stones to remain in compression, and thus allowing the arch to carry more weight.

This imaginary path is called the line of thrust, and modelling it was Gaudí’s speciality. Designing with these concepts in mind is relatively simple for a 2D structure like an aqueduct. Using the concept to form something as complex as the Sagrada Família, however, is immeasurably difficult — but it’s exactly where Gaudí’s genius shines.

Gaudí’s innovative structural method

Gaudí didn’t need a computer to design the Sagrada Família. He just needed string.

When a flexible material like a rope hangs down over a gap, it has no rigidity. It remains only in pure tension. Hang a rope from a single point and it will hang directly downwards, the entire length of string in tension. Hang it between two points and it will sag downwards, until once again the entire string is in tension. Add more weight and it will sag with a deeper parabolic arch to allow it to remain in tension.

Depiction of tension in strings spanning a gap. (Illustration: Real Engineering)

With this pattern in mind, strings can be used to map out lines of thrust. But because the goal of stone structures is to be in compression, rather than tension, the best way to model the structure is flipping the string model upside down. That’s exactly what Gaudí did.

When he began designing the Sagrada Família, Gaudí started with a 3D model. He first drew the structure’s footprint on a piece of wood at a 1:10 scale. Next, he designated where support columns would exist by attaching anchor points for strings. Then, he gradually added the strings to those anchor points, which hung down from the upside down model.

A photo of an upside down model of strings and weights tied to them.
A replica of Gaudí’s model of the Sagrada Família. (Image: Canaan / Wikimedia Commons)

Gaudí tied weights to the strings that were scaled to represent the actual weight those support columns would need to hold, at about one ten-thousandth of the actual weight. This weight was scaled to represent the difference in stiffness between the string and the rock that it represented.

Gaudí tinkered with the string model for a decade before he was content with its final form. This model was the skeleton of the Sagrada Família upon which Gaudí planned to carve his masterpiece.

Gaudí devoted the rest of his life to designing this model with intricate facades depicting stories from the Bible and adding detail that few would see.

As the project wore on, Gaudí withdrew from life, rejecting any additional projects. In 1914, he made his home inside the Sagrada Família workshop. His obsession with his final masterpiece took over his life. He neglected his personal appearance, and became a disheveled recluse. His days were devoted to praying and overseeing the design and construction of the structure.

A black and white photo of a group of men.
Gaudí shows the Sagrada Família to Cardinal Francesco Ragonesi in 1915. (Image: Josep Branguli / Wikimedia Commons)

Then, on June 7, 1926, disaster struck. On his daily walk to confession, Gaudí was struck by a tram. Passersby did not recognise the ragged man as the famous and beloved architect, mistaking him for a homeless man and heartlessly leaving him to die in the street.

It took hours before a passerby finally hailed him a cab to a hospital, but once there he received only basic care. By the time he was recognized, it was too late. He died on June 10, 1926.

The funeral that followed shut down Barcelona streets. The residents of the city showed up en masse, marching his coffin to its final resting place in the crypt of the Sagrada Família, which was less than a quarter complete at the time.

A grainy black and white photo of a crowded funeral procession.
Gaudí’s funeral in 1926. (Image: Wikimedia Commons)

The future of the Sagrada Família

With the mastermind behind the complex project gone, completing it would become a Herculean task, made even more difficult by the outbreak of the Spanish Civil War in 1936. During the war, Catalan anarchists broke into the Sagrada Família and destroyed Gaudí’s original plans and models.

At this point, the building could have been abandoned, or its vision changed. But as Spain shook off the cobwebs of post-war poverty in the 1950s, a new generation of engineers and craftsmen devoted their skills to completing Gaudí’s vision, employing tools and materials that he never had access to.

Attempts to piece together the shattered models back together were difficult, as pieces were either too damaged or missing completely. But the designers that took up Gaudí’s vision worked with what they had — and they continue to do so still.

The construction has spanned generations of both engineers and technology. Production rates have sometimes increased, but the construction still sees its fair share of delays.

A grainy sepia photo of the Sagrada Família under construction.
Construction on the Sagrada Família in the early 1900s. (Image: Txllxt TxllxT / Wikimedia Commons)

Sandstone comes in a wide variety of colors, due to differences in impurities. These can range from blues and greys right through to pink and red. When construction began on the Sagrada Família, the distinctive and highly durable tan sandstone being used was in plentiful supply from the nearby Montjuïc quarry.

But as the decades passed, the stone became more scarce. In 1954, when work began on the outer Passion façade the hands of the sculptures were left idle as availability of the stone was limited to what was left over in stock on hand from the quarries or what could be scavenged from other buildings in Catalonia.

When stone was not available, they resorted to bricks, which can be seen in many of the basilica’s towers, though recycled stone was always preferred when possible. As the project drew on however, these sources also began to run dry, creating a bottleneck in their supply chain that halted work.

The project’s managers were forced to start looking for alternative supplies from around the world. Over the past few decades of construction, the Sagrada Família has incorporated variants of sandstone and granite that match the color and physical properties of the original stone as much as possible.

A photo comparing newer, white and pink colored construction versus older, darkened stone.
New and old construction on the Sagrada Família. (Image: Benjamin Nolan / Flickr)

Over the years, construction technology has also improved, evident in the giant modern cranes that crowd the structure. Meanwhile, 3D modeling softwares have slowly caught up with Gaudí’s genius. Early civil engineering software was too simple to recreate his designs, but architects working on the project discovered that aeronautical engineering design software could model the complex curves Gaudí envisioned. The project actually resulted in a merging of these softwares to improve civil engineering modeling techniques.

Cash flow has increased drastically over the years too. For decades, the only money the Sagrada Família received was through meager donations to the church.

Today the church has no shortage of funding, being one of the most popular tourist sites in the world. Since it opened its doors to the public, Sagrada Familia has been raising a consistent and reliable cash flow from the millions of tourists that visit it every year, paying for the materials and workers needed to complete it.

But those workers are not always easy to come by. The art of sculpting has slowly withered away over the past century, and skilled craftsmen are difficult to come by. Here, technology has come in to lend a helping hand.

A photo of intricate carved structures on the outside of the Sagrada Família.
The Nativity façade on the Sagrada Família. (Image: Wikimedia Commons)

Sculptures still play an important role in the process, completing delicate tasks like surface texturing, but today most of the heavy-duty cutting is done by CNC machines, which can cut the stone exactly to requirements.

This ensures perfect fits between pieces and for the internal steel supports, which are made of tensioned steel bars. These bars are routed through the center of the structure’s stones, compressing them together to form larger panels. This advancement allows pieces to be partially assembled before being lifted to the tower and assembled like a giant stone lego set, drastically decreasing construction time.

This represents a far cry from the stone-by-stone construction of Gaudí’s time, but the advancements will lead to the conclusion of this chapter of Gaudí’s legacy. The Sagrada Família’s expected completion in 2026 will finally make Gaudí’s obsession a reality, 100 years after his tragic death.


To measure is to know.