“The people were incredibly welcoming, but they're still rebuilding”

Just a few days after the lockdown-affected Christmas of 2020 – on 29 December to be precise – a 6.4-magnitude earthquake struck central Croatia. In the town of Petrinja, which was hit especially hard, the renovation, consolidation and rebuilding work continues to this day.

Bönzli and Heitmann, both second-year Master’s students in civil engineering, decided to spend a week in Croatia ahead of their Master’s projects, which will start in September and be completed this winter. The students will track two separate rehabilitation projects, under the supervision of Igor Tomic postdoctoral researcher at EPFL’s Earthquake Engineering and Structural Dynamics Laboratory (EESD), headed by Prof. Katrin Beyer. "We have been actively involved, Katrin Bayer and myself, since the earthquakes of 2020, explains Tomic. First as advisers, then as consultants for the static-dynamic analysis of the Cathedral of Zagreb, for example. It is a very broad cooperation that we have established, which has led the students to go to Croatia."

Bönzli, who wanted to work on an applied project, was immediately drawn to an opportunity to study the post-earthquake condition of a parish house in Petrinja. “My father is an architect, so I grew up with someone who spent a lot of time at building sites,” she says. “I really enjoyed the experience, although I’m less interested in the aesthetic side of architecture.”

For her project, Heitmann will focus on assessing failure mechanisms and proposing retrofit solutions for the Church of Assumption of the Virgin Mary in the nearby village of Pokupsko. Her main aim will be to determine how the old building can be restored. “From a sustainability perspective, I think it’s important to work with existing structures,” says Heitmann. “I’m also interested in architecture, so this is a great opportunity to do something that spans both disciplines.”

For the first leg of their trip, Bönzli and Heitmann headed to Zagreb on Easter Sunday. “We spent the day exploring the city before leaving for Petrinja, which is where the building I am working on is situated,” says Bönzli. “The man at the car hire place asked what could possibly interest us about visiting such a remote location!” Central Croatia is a largely rural region off the main tourist track. “As an American, I was excited to explore a place where the architecture is completely different,” says Heitmann. “I was struck by how welcoming the people were.” Even though more than two years had passed since the earthquake, the students were shocked at the extent of the damage: every building was cracked or under reconstruction. With resources in short supply, the locals have learned to adapt.

Piled from floor to ceiling

The parish house in Petrinja is a perfect example of this resourcefulness: the earthquake left the building uninhabitable, so the church community has turned it into a warehouse. For her project, Bönzli will need to work around masses of furniture in every room, often piled from floor to ceiling. “There’s a huge amount of work to do,” she explains. “Initially, I struggled to see how I’d go about measuring the walls, producing the drawings, photographing the cracks and other damage, and modeling the building.” Ultimately, Bönzli will need to calculate the forces the parish house was subjected to during the earthquake, understand how the damage occurred, and work out how to strengthen the building so it can withstand future tremors.

War and earthquake damage

In Pokupsko, a village 40 km outside Petrinja, stands the damaged parish church that Heitmann will study. Also known as the Church of Saint Ladislaus after its patron saint, it was built between 1736 and 1739. The building has a tortured history: it was restored after a 6.0-magnitude earthquake in 1909 only to suffer extensive damage from Serbian artillery fire in 1991 during the Croatian War of Independence. The December 2020 earthquake was the latest in a long line of incidents to have shaken the church to its foundations. “Local people have a strong emotional attachment to the church,” says Heitmann. “As a result, it’s important to be able to safely rebuild the structure and restore its purpose.”

In the aftermath of the earthquake, local engineering firm Studio Arhing set about analyzing the building and its damaged walls and ceilings, reviewing the condition of the wooden roof structure, and conducting thermal imaging tests. Heitmann’s task will be to produce her own assessment. “It’s a very distinctive structure with lots of walls and arches,” she explains. “I’ll need to run various analyses to understand how it was damaged, how it might collapse and how the structure will behave. The support I’ve received from the Croatian engineers working on the renovation project has been invaluable. So far, it has been an insightful look at the kinds of challenges I’m likely to encounter in my career.”