Why did so many buildings collapse in the double earthquake in Venezuela?

More than 500 people have died in Venezuela following successive powerful earthquakes, and many more have been injured. Rescue teams are also trying to locate people trapped under collapsed buildings.

Here, Raphael De Risi, Associate Professor of Civil Engineering at the University of Bristol, answers our questions about the role the building’s design may have played in the disaster.

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

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

Unfortunately, the number of collapsed buildings is related to several factors rather than a single factor: they range from the age of the buildings, the type of construction, their level of maintenance, local soil amplification (when seismic waves pass from the solid foundation to the softer topsoil), and proximity to the source.

In addition, both events were superficial (the main shock in particular), which also contributed to this widespread devastation. More broadly, it is not possible to pinpoint a single cause; It’s generally a combination of factors.

How do you advise countries to build their buildings in a country like Venezuela with the earthquake risks it is exposed to?

Modern building codes for earthquake resistance are very effective in preventing this type of catastrophic collapse, so for new construction, they are key to the answer. The crucial point here is implementation: the law only protects people if it is properly implemented and the quality of construction is controlled.

It is also worth noting that much of the world’s building stock predates current rules, which have evolved as scientific understanding has advanced, often incorporating direct lessons from events such as these.

For existing building stock, retrofitting and strengthening is essential, because we cannot simply rebuild everything. For strategic buildings such as hospitals and power plants, modern solutions such as core insulation can keep them not only in place but in operation, and have performed very well in recent earthquakes.

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

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

In general, retrofitting either increases the strength and rigidity of a building or reduces the forces it must withstand, for example, through base insulation (an engineering technique that separates a building from its foundations) or energy dissipation devices. What is most important is that before any retrofitting is carried out, it is necessary to carry out a custom assessment.

The aim is to reduce unknowns about the structure through detailed surveys and testing of materials, and to build models capable of diagnosing them, so that intervention can target specific weak points in the building rather than applying a general fix.

There have been many ‘pancake’ building collapses as a result of earthquakes here, how does this happen and how can it be prevented?

A “pancake” collapse occurs when the vertical elements supporting the weight of a building (primarily its columns) fail. Then the floors lose their support and fall one on top of the other. This is one of the most serious forms of structural failure. In older buildings, pie collapse is often caused by brittle failure.

Columns that are not properly designed and detailed to deform and absorb energy simply break. The problem may be exacerbated if the ground floor is open or weak, concentrating the damage on one level.

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

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

Combined with appropriate structural details and strong connections that prevent local failure from propagating throughout the building, capacity design is an essential feature of modern building codes. It is also a major reason why modern, well-designed buildings are less likely to collapse.

There appear to be many buildings that were damaged but did not collapse. Does this typically lead to the demolition of many other buildings following earthquakes, and how is this determined?

Yes, this is actually a sign that the buildings are performing 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 energy of the earthquake, provided it does not collapse, and people can evacuate safely.

A building that was badly damaged but allowed everyone outside had done its job, even if it had to be demolished later. Next, each of these premises must be evaluated, usually in two stages.

Buildings that are damaged but allow people to get out alive have done their job. Ronald Pena R

First, rapid inspection tags are placed on buildings for immediate use (wide, safe, restricted, or unsafe to enter) to keep people out of danger while aftershocks continue; An “unsafe” sign does not mean that the building is condemned, it just means that it cannot be occupied until it has been properly inspected.

A detailed engineering evaluation then determines how much original capacity remains and whether repair is possible. Whether a building will be repaired or demolished depends on several factors: whether repair is technically possible, how much strength remains, whether the building is permanently leaning (which often makes repair uneconomical), and, ultimately, the cost of repair versus the cost of rebuilding.

For this reason, major earthquakes are often followed by large-scale demolitions (such as in downtown Christchurch, New Zealand, after 2011), even in cases where the buildings have not collapsed. Far from being a failure, it reflects the design philosophy at work: buildings have spent themselves saving the people inside.

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