أسئلة وأجوبة الخبراء: لماذا انهارت كل هذه المباني في الزلزال المزدوج بفنزويلا؟

More than 500 people have been killed in Venezuela following powerful back-to-back earthquakes, with many more injured. Rescue teams have also been trying to locate people trapped in collapsed buildings.

Here, Raffaele De Risi, associate professor in civil engineering at the University of Bristol, answered our questions about the role building design may have played in the disaster.

Venezuela is in an active seismic zone. Why do you think there have been so many devastating building collapses?

Indeed, Venezuela is a seismically active country. Hazard levels can easily be checked on several websites, such as the Global Seismic Hazard Map from the Global Earthquake Model Foundation.

The number of building collapses is unfortunately linked to several factors rather than a single one: these range from the age and construction type of the buildings to their level of maintenance, to local soil amplification (when seismic waves pass from hard bedrock into softer surface soils) , and proximity to the source.

In addition, both events were shallow (the mainshock particularly so) , which further contributes to such extensive devastation. More broadly, it is not possible to identify a single cause; it is generally a combination of factors.

How would you recommend that countries construct their buildings in a country like Venezuela with the earthquake risks that it has?

Modern seismic building codes are very effective at preventing this kind of catastrophic collapse, so for new construction, they are central to the answer. The crucial point is enforcement: a code only protects people if it is properly applied, and construction quality is controlled.

It’s also worth remembering that much of the world’s building stock predates current codes, which have evolved as scientific understanding has advanced, often incorporating lessons directly from events like this one.

For that existing building stock, seismic retrofitting and strengthening are essential, since we cannot simply rebuild everything. And for strategic buildings such as hospitals and power plants, modern solutions like base isolation can keep them not just standing but operational, and have performed extremely well in recent earthquakes.

How can buildings be retrofitted to enhance their protection against earthquakes?

Nowadays, there are many retrofitting techniques, and the right one depends on the building type, for example, reinforced concrete, steel or masonry.

Broadly, a retrofit either increases the building’s strength and stiffness or reduces the forces it has to withstand, for instance, through base isolation (an engineering technique which decouples a building from its foundations) or energy dissipation devices. What matters most is this: before any retrofitting, a bespoke assessment is essential.

The goal is to reduce the unknowns about the structure through detailed surveys and material testing, and to build models capable of diagnosing it, so that the intervention can target the building’s specific weaknesses rather than applying a generic fix.

There have been numerous ‘pancake’ building collapses as a result of the earthquakes here, how do these happen and how can they be prevented?

A “pancake” collapse occurs when the vertical elements that support a building’s weight (primarily its columns) fail. The floors then lose their support and fall one on top of another. This is one of the deadliest forms of structural failure. In older buildings, pancake collapses are often caused by brittle failure.

Columns that were not properly designed and detailed to deform and absorb energy simply break. The problem may be made worse by an open or weak ground floor, which concentrates the damage at a single level.

The engineering approach used to prevent this is known as capacity design. The principle is to determine in advance where the structure should sustain damage and to ensure that this damage occurs in a controlled, ductile manner, typically in the beams.

The columns, joints, and foundations are deliberately designed to be stronger so that they remain intact while the beams safely absorb and dissipate the earthquake’s energy. This principle is often summarised as “strong column, weak beam.” A building designed in this way can sway and dissipate energy rather than losing an entire storey and collapsing.

Combined with proper structural detailing and strong connections that prevent a local failure from spreading through the building, capacity design is a central feature of modern building codes. It is also a major reason why well designed modern buildings are far less likely to suffer pancake collapse.

It looks like there are many buildings that have been damaged but have not collapsed. Does this typically result in many other buildings having to be demolished in the aftermath of earthquakes, and how is this decided?

Yes, and this is actually a sign that the buildings performed as intended. For ordinary structures, the goal of seismic design is not to survive undamaged, but to protect life: the building is allowed to be damaged, absorbing the earthquake’s energy, provided it does not collapse, and people can evacuate safely.

A building that is badly damaged but lets everyone out has done its job, even if it must later be demolished. Afterwards, each of these buildings has to be assessed, usually in two stages.

First, rapid inspections tag buildings for immediate use (broadly, safe, restricted, or unsafe to enter) to keep people out of danger while aftershocks continue; an “unsafe” tag does not mean the building is condemned, only that it cannot be occupied until checked properly.

Then a detailed engineering assessment determines how much of the original capacity remains and whether repair is feasible. Whether a building is repaired or demolished depends on several factors: whether repair is technically possible, how much strength is left, whether the building is permanently leaning (which often makes repair uneconomic) , and ultimately, the cost of repair against the cost of rebuilding.

This is why major earthquakes are frequently followed by extensive demolition (as across central Christchurch, New Zealand, after 2011) , even where buildings did not collapse. Far from being a failure, it reflects the design philosophy at work: the buildings spent themselves to save the people inside.

Raffaele De Risi does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.