After the earthquakes: Experts discuss building codes in Türkiye and the U.S.

Experts reflect on building collapses in the wake of the 2023 Türkiye earthquakes and the importance of earthquake-resilient design in both the U.S. and Türkiye.

On Feb. 6, 2023, two major earthquakes — with magnitudes of 7.8 and 7.5 — occurred nine hours apart in the southern region of Türkiye. As of March of the same year, an estimated 3 million people in Türkiye have been displaced and around 320,000 buildings were heavily damaged or collapsed. In northwestern Syria, which was also affected by the earthquake, more than 9,100 buildings collapsed. This earthquake sequence was the most devastating to hit Türkiye in more than 20 years and the fifth-deadliest earthquake of the 21st century.

The East Anatolian Fault, on which the two earthquakes occurred, is a “sister fault” to California’s San Andreas Fault, Ross Stein, CEO of Temblor, said in a recent webinar on the Türkiye earthquakes. The two faults have similar lengths, shapes and histories. “If it can happen in Türkiye, it can happen in California,” Stein said.

We sat down for a chat with two experts, Mustafa Erdik, Professor of Civil Engineering at Boğaziçi University in Istanbul, and Evan Reis, Executive Director and co-founder of the U.S. Resiliency Council, to consider the building collapses observed as a result of the Türkiye earthquakes and explore lessons that architects, engineers and policymakers in Türkiye and the U.S. may learn from this catastrophe.

What happened in Türkiye?

The two earthquakes were associated with the East Anatolian Fault system, which runs northeast-southwest through southern Türkiye, close to the Syrian border. This left-lateral strike slip fault accommodates the motion of Earth’s tectonic plates. The westward motion along the fault, at a rate of between 6 and 10 millimeters per year, makes the region prone to quakes. (For the seismotectonics of the region see Erdik et al., 2023.)
 

The East Anatolian Fault passes through the southeastern provinces of Hatay and Kahramanmaraş, which suffered great damage in the quakes. Both regions experienced maximum shaking of 9 on the Modified Mercalli Scale, which describes the intensity of shaking experienced by buildings and people.

Hatay sits atop a basin filled with sedimentary rocks, which are considered “soft.” As seismic waves pass from hard bedrock to softer sedimentary rocks, the waves slow down and increase in amplitude — just like how ocean waves slow down and increase in height as they approach the shore. Though Hatay was farther from the two epicenters than Kahramanmaraş, the basin amplified the ground motions and increased the damage, Erdik says.

The pair of quakes came consecutively, impacting the distribution of damage too. “Kahramanmaraş was essentially affected by two earthquakes,” Erdik says.

What types of buildings collapsed?

In Kahramanmaraş, 55% of buildings were damaged or collapsed; in Hatay, 58% of buildings were damaged (Tao et al., 2023). In both provinces, many of the buildings that collapsed were old “low code” buildings — those not built to modern seismic standards, Erdik says. A recent study reports that in Kahramanmaraş, about 97% of the collapsed buildings were constructed prior the significant 1997 seismic code updates (Binici et al., 2023).

“The main difference between a low code building and a high code building is the ductility,” Erdik says. Ductility is the ability of a material to bend or change shape without breaking when a large stress is applied, like stretching out taffy candy before it snaps. The opposite of ductile materials are brittle ones that break under too much stress, like snapping off a piece of KitKat bar.

A low code building behaves in a brittle fashion, so when a quake strikes and the strength of the structure is exceeded, the building is more likely to break instead of bend, resulting in either severe damage or collapse.

“Brittle structures would be okay in a moderate earthquake,” says Erdik, because the stress is not too large. “But whenever the ground motion exceeds a certain level, brittle buildings collapse. This is why the damage is almost binary. It either collapses or stays intact — no medium damage.”

Modern seismic codes in both California and Türkiye aim to ensure new buildings behave ductily by using high-quality concrete reinforced with steel bars. But the hazard remains for old, brittle buildings. In California alone, there could be as many as 17,000 old nonductile concrete buildings.

In addition to ductility, “soft story” buildings can contribute to the likelihood of collapse. Soft story buildings have large, open-plan spaces on the ground floor, often used for shops or parking garages. In an earthquake, the soft story ground floor (that’s also supporting the upper floors) collapses, and there’s a progressive “pancake” collapse of the whole structure.

“This is the worst type of collapse for saving people,” Erdik says, because when the floors start to collapse on top of each other, people have no time to escape. Pancake-type collapse likely contributed to the majority of the 50,000 casualties in Türkiye, many of whom were crushed or trapped in the rubble of collapsed buildings.

In contrast, modern buildings that were built to Türkiye’s current seismic codes — buildings that are ductile and have sufficiently strengthened soft stories — were less likely to collapse, although some still did.

Modern school buildings, many of which were constructed with ductile reinforced concrete (Ozturk et al. 2023), fared well in the Türkiye quakes. Since 2017, there has been support from the World Bank, Global Facility for Disaster Reduction and Recovery and the EU to build more seismically resilient schools that follow Türkiye’s 2018 seismic code. Only about 5% of the 20,000 education buildings in the earthquake-affected areas collapsed or were either severely or moderately damaged.

Modern hospitals in the region also performed well, especially those with base isolation, Erdik says. With base isolation, flexible pads made of rubber and lead are installed beneath the building. The pads separate the building from the ground, so shaking is reduced up to 5 to 6 times that of a building without base isolation.

But when it comes to healthcare, building collapse is not usually the biggest problem: Rather, loss of power and water, or nonstructural damage — when a building is damaged but still standing — significantly impacts hospitals. Fifteen percent of health facilities in the worst-hit region were unusable due to nonstructural damage such as cracks in nonsupporting walls or damage to ceiling tiles, Erdik says. It’s relatively easy to fix the minor damage and get the health facilities back up and running again, he says. “If buildings are built properly, there is no problem.”

However, not all modern buildings were constructed properly, as some, built long after the 1998 code, still collapsed. Widely covered at the time of the earthquakes, the Rönesans Rezidans, constructed in 2011, completely collapsed. Construction was likely in accordance with regulations — the walls were all built with sufficiently ductile materials. But there were architectural design flaws that made the building susceptible to collapse.

“Buildings still suffer damage due to the design problems, not construction problems,” Erdik says. In Türkiye, there are fewer checks to ensure that architects and structural engineers are properly qualified. In the U.S., meanwhile, just like doctors must have a license to practice medicine in each state in the U.S., professional institutions, like the National Society of Professional Engineers, license U.S.-based engineers.

But in Türkiye, “any [engineering] graduate with a four-year university degree can sign any project he wants,” says Erdik. “We are trying to introduce an ‘Institution of Professional Engineering’ to control the design process in Türkiye.” A professional institution in Türkiye would provide assurance of competence of structural engineers and architects, he explains. Such a professional institution would also encourage continuing professional development of its members.

A brief history of building codes in Türkiye

In the past, destructive earthquakes have resulted in updates to Türkiye’s seismic code. The first seismic design codes were published in 1940, following a magnitude 7.9 earthquake in 1939. More than 33,000 people died and more than 140,000 buildings were destroyed. The 1940 seismic code was similar to the Italian seismic code at that time. It provided the same guidance across Türkiye, regardless of proximity to a fault or how many earthquakes had occurred in a given location.

In 1942, that changed. Three levels of seismic hazard were identified and applied to provinces across Türkiye: hazardous, less hazardous and no hazard. The 1940 codes therefore did not apply to buildings in the nonhazardous zone. In 1947, the codes took into account the impact of soil type beneath buildings. A 1975 update explicitly mentioned ductility for the first time. The 2007 code updated guidance for retrofitting buildings.

“The code became decent in 1998,” Erdik says. “It was very similar to the 1997 Uniform Building Code in the United States.” The 1998 code mentions earthquake-resistant building design principles and brings in the concepts of rigidity, stability and strength. The most recent update to Türkiye’s seismic code was in 2019, which addressed an additional four types of buildings: high-rise, seismically isolated, cold-formed steel and wooden.

How is what happened in Türkiye comparable to what could happen in California?

In the Türkiye earthquakes, many of the buildings that collapsed had deficiencies, such as soft stories or nonductile concrete, according to Reis of the U.S. Resiliency Council. And while less common, “there are plenty of older concrete buildings built in the 1970s or 1980s in the U.S.,” Reis says. “If a large earthquake occurred, you could see the same style of structural failures that you saw in Türkiye.”

But in California, there is tighter control on the design process of each building, which, as Erdik points out, is needed for Türkiye. In the U.S., architects and their teams of design professionals must be licensed by a state board. In California, architects must even take a supplementary exam to understand the challenges of practicing in California in large part because of its earthquake hazards. The design team must submit plans for new construction projects to city building departments who ensure that the design meets seismic and other codes. “It’s extraordinarily complex and rigorous,” Reis says.

The conscientious checks don’t end with the design. “During construction, the structural engineer of record and the architect make regular observations,” Reis explains, to ensure that construction is going to plan and the materials are correct.

But overall, in the U.S., Reis says, if an event like the Türkiye double quakes occurred, “we’re unlikely to have the same level of impact.” That will remain true as long as codes are effectively enforced to ensure that buildings are both designed and constructed properly.

What can Californians learn from the Türkiye earthquake?

“There’s a lot of confirmation of what we already know; buildings with poor design, soft stories and [those that] were nonductile sustained the most damage,” Reis says. The lesson, he says, is that “we need to retrofit [buildings] in California.”

Retrofitting buildings includes processes such as reinforcing masonry walls and soft stories, or bolting buildings to their foundations. And in California, there are programs that can help residents shore up their homes’ seismic protections. For example, the U.S. Resiliency Council worked on a legislative initiative to encourage retrofits. “We appropriated around $250 million in grant funding for retrofitting soft story apartment buildings,” Reis says. This initiative will help make affordable housing in California safer.

To minimize seismic risk even further, the U.S. Resiliency Council developed a rating system to “provide an objective, highly credible, consistent way to measure the performance of buildings [in an earthquake],” Reis says. The rating system can then be used in financial initiatives to encourage people to make their homes safer. For example, the California Earthquake Authority offers a 10% to 25% earthquake insurance premium discount to owners who undertake a seismic retrofit of their homes.

What about life safety objectives?

The life safety objectives for buildings aim to prevent buildings from collapsing during an earthquake. But buildings that only ensure the safety of the people inside may still experience substantial damage that may eventually result in demolition. Should buildings be constructed to exceed these life safety objectives in order to ensure their full usability after an earthquake?

To do so, engineers would have to be concerned about a building’s contents and nonstructural elements, in addition to basic structural seismic safety, Erdik says. This is important for critical buildings like hospitals and fire stations.

“But for residential buildings, it’s better to have minor damage and repair the damage,” Erdik says. “That’s the most rational thing to do. It’s just too much effort or expense to make these buildings completely functional — without damage — after an earthquake.”

Reis takes a different perspective when it comes to California.

“In the U.S., lenders and insurers want better performing buildings — buildings that not only will be safe, but will also minimize damage repair costs and recovery time,” Reis says. “We’re working to get beyond this minimum idea of life safety. If I want to go from just a bare-bones code building to a resilient building, sometimes it’s one or two percent more construction cost. And that’s extraordinary.”

“There are many examples where smart engineering meant that you paid no more for the building, and yet, it will perform a lot better,” Reis continues. “We are working to incentivize that.”