In June 1926, an elderly man in scruffy clothes was hit by a tram while crossing the street on his way to church. A few days later, he died in a hospital for the homeless.

The man was Antoni Gaudí, who would become known as “God's architect.”

Gaudí left behind him a titanic task. He died in the middle of the construction of the Sagrada Familia, an emblematic and towering church located in Barcelona, ​​northeast of Spain.

From a distance, the church rises above the city skyline like a giant bristling organic being. Up close, it looks even more like a living being. The hypnotically detailed facades, depicting biblical scenes, seem to spring from the stone.

Intended to continue long after his death, the construction of the Sagrada Familia posed extraordinary challenges to those who continued the work.

Gaudí's sketches and models were destroyed during the Spanish Civil War in 1936, leaving his successors with little material to work with.

The towering towers were once thought to be too tall for their foundations and vulnerable to the wind.

However, 100 years after his death, the church's central pinnacle, the Tower of Jesus Christ, stands complete at 172.5 metres. This makes the Sagrada Familia the tallest church in the world.

And to celebrate the centenary, Pope Leo XIV will preside over a mass in the church this Friday and will bless the recently completed tower of Jesus Christ, the highest in the place.

Ancient inspiration

As a young, immensely ambitious and devout Catholic architect, Gaudí had two goals, says Gijs van Hensbergen, art historian and Gaudí biographer.

"Create, first of all, a Bible in stone, which is the Holy Family. But also correct all the errors of previous architectural styles."

To do this, you would have to turn to one of the wonders of the ancient world, the Taq Kasra Arch in the ancient city of Ctesiphon in modern-day Iraq, an imposing early example of an extremely stable structure called a catenary arch.

Gaudí was convinced that the catenary arch would allow him to correct what he considered an annoying element of the cathedrals of his time.

To support their vaulted ceilings, the great neo-Gothic churches relied on flying buttresses—stone fingers that extended from the upper walls to the lower levels. Gaudí considered them “crutches” to support a building that could not support its own weight.

Instead, he turned to the catenary arch for the building's 18 towers. If they were based in this way, the towers could support their own weight.

“He was clearly someone fascinated by mathematics, but always because it was the work of the Creator,” says Van Hensbergen. Gaudí believed that gravity and the catenary arch were divine inventions.

“That, for him, would have made it a wonderful leitmotif, if you will, a nod to God as the great architect.”

As his knowledge of structures and forces developed, he felt more confident in removing vaults and arches from the basilica that were not structurally necessary.

“He redesigned the temple three times,” says Liam Duff, a structural engineer who leads a team working on the Sagrada Familia at engineering firm Arup.

He used branched, stripped columns on the interior of the nave, the central space of the church. Inspired by nature, the columns were built to resemble trees, with their branches extending upward to support the weight of the roof and towers above.

From the church's nave, illuminated by the orange and red hues filtered through the west stained glass windows, the tree-shaped columns give the illusion of weightlessness.

heavy towers

Despite the simple and economical design of the towers, the construction of one of the tallest posed a huge challenge: it was too heavy.

The Tower of the Virgin Mary measures 138 meters high, only surpassed in height by the central Tower of Jesus Christ, at 172.5 meters.

When the Tower of the Virgin Mary was being built, it was realized that the columns below would be overloaded by its weight if it were built using traditional masonry methods or in reinforced concrete with stone facing.

One solution was an internal steel structure to support the tower, clad with thinner concrete panels to lighten the tower's load. With this plan, in 2014 the Sagrada Familia hired the services of a team of structural engineers from the company Arup, from the United Kingdom.

“The Sagrada Familia is a legendary place, isn't it?” says Steve McKechnie, a structural engineer at Arup who has been involved in the project since its inception. “It's something you would dream of participating in and the opportunity just presented itself.”

Arup engineers had a radical idea to address the weight of the tower.

They suggested completely dispensing with the steel structure and reinforced concrete. Instead, they proposed using a thinner layer of stone tensioned with internal steel tendons. It was a significant change from the previous plan.

“It was a big change,” says McKechnie. “There was a time when we thought our services were not going to be necessary, and the project was put on hold.”

Fortunately that has changed, says McKechnie.

The theory behind the idea of ​​prestressed stone panels is that stone becomes extremely strong if it is compressed, but if you do the opposite of compression and try to separate it, it easily cracks under tension and loses strength.

When the wind blows against a tower and pushes it back, it puts stress on the windward side of the tower. This stress puts the stone tower at risk of cracking and damage.

The shape of the catenary arch helps keep the stone in compression using the tower's own mass. But the use of internally prestressed panels with steel tendons adds even more compression, making it stronger.

With this design, when the wind blows, very little stress is produced on the side where the wind blows.

The tendons located above and between the windows also helped prestress areas that might otherwise be prone to cracking. This was important, as Gaudi's designs were filled with windows to illuminate the basilica below.

The Tower of the Virgin Mary and five other central towers, including the Tower of Jesus Christ, were completed with the use of prestressed stone panels.

“Gaudí would have been very excited by the possibilities” of these new technologies, believes Van Hensbergen.

The architect experimented with different methods on the Tower of St. Barnabas on the Nativity Facade, which was completed while he was alive.

It began at the bottom with sandstone extracted from the nearby Montjuïc mountain, whose color varies subtly from gray to beige, green, ocher, gold, purple and red. On top, he used Portland cement.

“It was new material in Barcelona at that time,” says Duff.

His experiments paid off. “It is impressive to know the quality of the construction from Gaudí's time,” says Fernando Villa, director of technology and innovation at the Sagrada Familia.

Gaudí's church, still unfinished

While dignitaries such as Pope Leo XIV gather to celebrate the centenary of Gaudí's death, construction of the church is still far from complete. Among other areas, the Facade of Glory, the main one of the church, is still to be finished.

Even after its completion, modern technologies will help maintain the building. Changes in wind, temperature and building movement over time can cause cracks to proliferate both inside and outside the church, Villa says.

Previously, the Sagrada Familia relied on climbers to inspect the building's cracks and flaws, a task that required two years to check each part of the basilica. Today, drones and artificial intelligence are used to detect cracks that require attention.

“Once we have trained the AI, it will be possible to scan the entire basilica in a month,” explains Villa.

Far from being an immovable stone monument, the Sagrada Familia responds to the elements almost like a living being. This constant movement and evolution reflects the organic design of Gaudí's church, says Villa.