Harold Tobin is director of the Pacific Northwest Seismic Network and a 91探花professor of Earth and space sciences. Tobin studies tectonic plate boundaries with a focus on how faults work and the conditions inside them that lead to earthquakes. He also serves as Washington state鈥檚 seismologist.

“This region along the East Anatolian Fault has a well-known history of seismic activity, and it had been identified by Turkish as a place of high seismic hazard,” Tobin said. “However, its known history does not include earthquakes of magnitude 7 or above since seismometers existed to measure them, though historic records indicate earthquakes of up to magnitude 7.4 have occurred. The scale and size of this magnitude 7.8 quake and the one that followed are both larger than what was most likely anticipated. The fact that there was a second large and damaging quake, the magnitude 7.5 that occurred about nine hours later, is not unprecedented globally, but is very uncommon, especially at this size.

“It is not typical for a rupture on one fault to trigger a slip on another fault, but it鈥檚 also not that uncommon. For example, the , also clearly had slip along two different faults.

“The surprising size of the two earthquakes and the length of the fault zone makes them very remarkable events. We have seen very, very few on-land, strike-slip fault earthquakes as large as this in the past century, anywhere. For comparison, the San Andreas Fault in California has not had a comparable quake since the . The only other U.S. event of similar scale in the era of instrumental records was the in Alaska. That was also a strike-slip fault, involving the lateral motion of two crustal blocks, as opposed to the converging motion of a subduction zone fault. Fortunately that earthquake affected a sparsely populated region.”

In southern Turkey and Syria, “the risk remains elevated, unfortunately, because aftershocks are expected for some time 鈥� weeks to months to even years. Besides the 7.8 and the 7.5, there have been three aftershocks of magnitude 6.0 or larger already, and more can be expected. People in the region need to remain vigilant that more aftershocks may occur. It is also possible, though less probable, that additional, very large earthquakes could occur, even ones as large as, or larger than, the 7.5 and 7.8. Adjacent segments of the faults could still have built-up strain to be released.”

, 91探花professor of civil and environmental engineering, studies older buildings with substandard details and connections to develop advanced computer methods that can identify weak points. She then creates rehabilitation methods to improve the structural performance of these buildings.

“It is devastating to watch the aftermath of this earthquake followed by aftershocks,” Lehman said. “Clearly we have to think about the magnitude of aftershocks and simple mechanisms to reinforce brittle structures.”

“Although every building is unique in its geometry, function and seismic demands, it is well understood that reinforced concrete buildings without seismic detailing are particularly vulnerable in earthquakes. In modern reinforced concrete design, we improve the seismic performance by using steel with very high strain capacities at fracture and closely spaced hoop-shaped reinforcement to encase the main reinforcing bars. Even if the two buildings have the same strength, only the building with the high-strain capacity steel and the encased rebar will be able to sustain the earthquake demands without collapsing. Otherwise the response is ‘brittle.'”

“Many countries are studying important technologies to prevent building collapse in moderate to large earthquakes. The knowledge and development of the technologies is the first step, but implementation and construction methods are also very important. We have seen over decades that improvement in codes leads to improvement in seismic response.”

“The most important thing right now is the humanitarian aspect of this tragedy: ensuring people who have been displaced have warm shelter and basic human necessities and evacuating structures that have a high probability of collapsing in a large aftershock. I am thankful for every person who is helping with that effort.”

, lecturer of international studies at the UW, is a retired foreign service officer. His expertise includes humanitarian emergencies, disasters from natural and human causes and public-private partnerships in disaster response.

“Turkish authorities will probably mount an effective response,” Ward said. “They have a lot of experience and international support. The situation in northwest Syria will be far more dire, where the seemingly endless civil war will make emergency response much, much harder.”

While there is currently no explanation for why the Champlain Towers South building collapsed, , a 91探花 professor of civil and environmental engineering, has been studying photographs, videos, drawings, reports and permits to investigate this tragedy to understand what happened.

Lehman studies older buildings with substandard details and connections to develop advanced computer methods to identify weak points. She then creates rehabilitation methods to improve their structural performance. She has analyzed buildings that have been damaged or have collapsed in earthquakes all over the world including in California, Chile, Taiwan and New Zealand.

91探花News asked her to help us understand why buildings fail.

What happens when a building collapses like this?

It is important to remember that a building collapse needs to be triggered by some type of load or movement that it does not normally experience, which then causes widespread structural deficiencies. For example, in seismic regions, buildings that are standing right now might be damaged or collapse if an earthquake strikes and their structural system does not have the strength and deformability to resist the earthquake demands.

Clearly the Champlain Towers South building did not experience an earthquake, but this analogy demonstrates that a building that is standing does not just collapse. Something needs to initiate that collapse.

So what do we know so far?

The building was constructed of cast-in-place reinforced concrete systems, where concrete floors are cast on site. It was constructed using a slab-column system 鈥� slabs of concrete held up by columns of concrete 鈥� as its primary gravity system, or the support for vertical loads. The building’s primary lateral system, which supports horizontal loads such as wind, was two isolated walls. Walls are stiff and strong in the direction of their primary axis but much more flexible and weaker in the perpendicular direction.

Each section of the building differed when it comes to the lateral system. In the eastern portion of the building (the section that appears to collapse after the full central section had collapsed), there was a single wall to the west of the stairwell that framed the stairwell in the North-South direction, meaning it was much more flexible in the East-West direction.

The central portion of the building (the section that appears to collapse first) did not have any structural components to resist lateral forces.

There were two significant differences to the western portion of the building, which remained standing. First, there was a core wall, which framed the stairwell, elevator core and mechanical room. The core wall had wall sections in both the East-West and the North-South directions, so it provided strength and stiffness in both directions.

In addition, the most western portion of the building had surface-level parking on the pool-deck (level 1) level. The 鈥渃eiling鈥� of that portion of the building (level 2) had beams between the columns. Beams and columns framed together are capable of resisting horizontal forces and movements. This system was on the second floor only.

Finally, reports from a structural engineering firm in 2018 and 2020 indicated that the slab, beams and columns supporting the pool deck sustained significant degradation of the concrete and corrosion of the reinforcing steel. This slab formed the ceiling of the garage and was exposed to the ocean environment.

Many people are jumping on this corrosion as the explanation for why this collapse happened. What do you think?

As with any tragedy, there are many quick explanations for what caused it. This has happened with this tragic event, but in many cases those quick explanations are not sufficient.

There is nothing that suggests corrosion alone is what allowed partial collapse of every floor of the building. The type of corrosion seen in this building is not uncommon in older buildings near the ocean. Does corrosion weaken a structural member, such as slab, of a building? Yes. Does corrosion cause expansion of the steel reinforcing bars, and can this expansion crack and damage to the concrete to further expose the bars which had been protected from corrosion by this concrete cover? Yes. But there have been examples where buildings with corrosion were damaged and the failure was limited to the slab and multiple (or all) floors did not collapse. Corrosion limited to the 鈥渃eiling鈥� slab of the garage should not lead to building collapse.

In short, corrosion is not the sole cause 鈥� it existed before the collapse. There are other important issues, and likely more than one of them contributed to this unimaginable failure.

What are other likely culprits?

This is a list of the typical culprits to consider when a building fails instead of just sustaining damage to or failure of a component or part of a floor. This is not an exhaustive list. And, as I and others have pointed out, likely more than one of these played an important role in the collapse.

• Inadequate design 鈥� for example, a building is not designed for the expected load and deformations, or the reinforcement is not placed in the right location or is too short
• Construction errors, or the construction does not meet the structural drawings
• Poor connections 鈥� an example of this is the slab-column connection damage . This can lead to the partial collapse of a floor, and in some cases, it has also caused partial or full collapse of a building. One example is buildings that collapsed in the (Figure 3 in )
• Inadequate materials 鈥� examples include:
  • the actual strengths of the materials do not meet the specified strengths
  • materials have deteriorated or been damaged
  • bars are weakened from corrosion due to loss in area
  • reinforcing bars have inadequate bond capacity which is needed to transfer forces from the reinforcing steel to the surrounding concrete
• Inadequate foundation strength
• Loss of or weakened soil strength
• No building redundancy 鈥� to understand the word redundancy, here is a simple analogy: if a single leg of a chair fails, the three remaining legs are not sufficient to hold the chair up. This is an example of not having redundancy

Again, this is not an exhaustive list but suggests that we need to look at more than corrosion alone.

What’s next?

Right now, we do not know what initiated the damage to or collapse of this building. It will take some time to determine why this building failed. We need to be patient and allow the structural engineers from the National Institute of Standards and Technology, structural-engineering practitioners and structural-engineering faculty to fully study this building using forensic investigative tools to fully understand what happened.

Once this is complete, we can work collaboratively with building code committees, counties and cities to improve inspection practices. In addition, it is expected that we will also use this tragedy to develop new rehabilitation methods for corrosion and new resilient slab-column connections.

For more information, contact Lehman (she/her) at delehman@uw.edu.