Govt-appointed panel focuses its probe on design of collapsed SAO building

The collapsed under-construction building of the State Audit Office (SAO) is found to have an asymmetrical design that could undermine its strength against the earthquake on March 28 to the point that it totally collapsed, according to the panel’s chair, Deputy PM and Interior Minister Anutin Charnvirakul

Deputy PM and Interior Anutin Charnvirakul has revealed that the probe panel he chairs has found that the building’s design was asymmetrical. If a building is asymmetrical, it can be twisted when swayed by strong winds or earthquakes. Engineers must know how to increase the strength of an asymmetrical building to keep it in balance and help it withstand the forces. The panel will investigate further whether the building was designed with safety factors included as required by the law, Mr. Anutin said.

“The panel needs to undertake thorough calculations to get rid of room for doubts and boost confidence in the findings. Engineering principles are scientific, and they don’t universally allow room for discretion,” said Mr. Anutin, who is also a structural engineer.

The minister said the panel’s assumption is in line with the theory and assumption offered by former President and Professor Emeritus of the Asian Institute of Technology (AIT) (Civil Engineering), Dr. Worsak Kanok-Nukulchai, who has produced a video clip to educate the public about the collapse of the SAO building.

“I gratefully thank the professor and also the fellow of the Royal Society of Thailand for sharing what he has found at the building. It’s what we have initially assumed; “that the lift core was not asymmetrical. As it’s swayed by the earthquake, it’s also twisted.”

The design of the building has been brought to public attention as the investigation is progressing. Last Thursday, the Lower House’s standing committee on budget monitoring had invited concerned parties to respond to its inquiries. A representative from the Council of Engineers Thailand (COE), which is another third-party appointed by the government to help investigate the collapse of the SAO building, told the meeting that following an initial examination by his team, they found that the design of the building was “not in line” with the Building Codes under the ministerial regulation B.E. 2550 (2007).

The 2007 ministerial regulation was developed under the Building Code Act B.E. 2522 following the 2004 tsunami to address major earthquakes and their impacts after the tsunami incident, which had caused extensive loss and damage to the country. Unprecedented seismic building codes and standards against earthquakes were then introduced in 2007 and 2018. The regulation was also revised in 2021 to improve earthquake-resistant designs and engineering. 

The representative stressed that the regulation and the existing building codes and standards are valid to deal with earthquakes. But the team has found “irregularities” in the design and construction from the first floor to the middle of the height of the building.

At this point, they will not yet conclude that the design did not abide by the law or violated it, nor will they conclude that this is a prime cause of the collapse of the building because further investigation is needed, he said.

“Normally, we will see first if a building is irregular. There are at least 16 points to check for irregularities in building designs and constructions, and based on our calculations, we have found some of those from the first floor to the middle of the height of the building. So, at this point, we said it’s “not in line” with the regulation. This does not mean they contributed to the collapse of the building either. We need to look into more detail,” said the COE’s representative, in response to an inquiry posted by the committee’s Chair, MP Surachet Pravinvongvuth of the People’s Party, also a civil engineer.

The design of the critical core walls and core lifts of the building, widely acknowledged as the skeleton of a building, was also adjusted, the meeting was told. (Read: Design of collapsed under-construction SAO building “not in line” with regulation)

l Courtesy of Prof. Emeritus Dr. Worsak Kanok-Nukulchai

St. Venant Torsion

Former President and Professor Emeritus of the Asian Institute of Technology (AIT) (Civil Engineering), Dr. Worsak Kanok-Nukulchai, first released a video clip with a length of around 28 minutes on April 14 to explain what would have happened at the SAO building to educate the public.

Based on his long experience and knowledge as a professor in civil engineering, he applied the theory of St. Venant Torsion to explain his assumption and conclusion about the collapse of the building.

According to Prof. Emeritus Dr. Worsak, tall buildings which are designed and built with standard constructioin process and materials and in accordance with building codes will be able to withstand forces from strong winds and distant earthquakes. There is a little chance that such a building will “totally” collapse into a form that engineers term as “pancake collapse”. (Or as he named it following a Thai dessert, Khanom Chun, or steamed layer cake) They can be swayed or damaged during an earthquake, but they will not totally structurally collapse, said the professor, who addressed this kind of building collapse as “tragic”.

There are two types of pancake collapse, according to the professor. The first one is Ground Fall Failure. The other one is Floor Progressive Collapse. 

The Ground Fall Failure occurs when the structural supports of a tall building, particularly interconnected shear walls forming a shear core that normally acts like a skeleton of a building, as well as supportive columns, are destroyed, prompting a building to lose its foundation. With potential energy accumulated within it, a building will fall freely by its own weight in a vertical direction following gravity, prompting it to totally collapse with one floor falling onto the other below like a stack of pancakes. This type of collapse hardly leaves any structures of a collapsed building standing, including its shear core, the strongest part, due to the great loads of the building that fall fast and crash hard.

The Floor Progressive Collapse, on the other hand, occurs when structural supports on top of a building are separated by forces and top floors fall sequentially onto the other floors below, like a stack of pancakes as well. This type of collapse may spare some parts of the building, including the strongest shear core, to still stand.

According to the professor, the SAO building is found to have collapsed within eight seconds. This means a tremendous force was applied to it. The professor suspected that the building was damaged and destroyed following the first type of pancake collapse and applied the theory of St. Venant Torsion to explain the phenomenon.

Prof. Emeritus Dr. Worsak said normally, a tall building, which is built in accordance with building codes and standard construction process and materials, will be able to sway and withstand earthquakes. The design can be symmetrical or asymmetrical, but if it’s asymmetrical, architects and engineers who work on the design must increase the strength of that building. 

When a building is asymmetrical, aside from being swayed by earthquakes, it will be twisted by cyclic torsion following the theory. This will be most critical at the base where restrained warping — or cyclic restrained St. Venant Torsion — occurs and prompts structural supports of a building to start to break to the point that they can collapse, thus the Ground Fall Failure.

According to the professor, the design of the SAO building is found to be asymmetrical as its shear core or lift core was set away from the building’s centre, on the back of the building. 

According to a comparison of three video clips with seconds and frames captured sequentially, the professor said people can see that the shear walls of the lift core on the top floors of the building start to move first when the building was hit by the earthquake on March 28, at the 2nd second. But that move means that by then, on the ground floor, the shear walls were already destroyed and failed, and that’s the reason why the shear walls on top of the building started to move. The supportive columns then broke and failed in the 5th second, as there were no shear walls there, and all forces were transferred to them, the professor noted.

The shear walls of the lift core on top of the building, as seen in another video clip, also fell freely almost simultaneously without touching one another, suggesting that the whole building fell as it lost the foundation and was pulled down by gravity, following a type of ground fall failure. 

All occurred in just eight seconds, the professor said.

“In conclusion, when the earthquake hit the SAO building, which was asymmetrical as its shear core was set on the back of the building, it was twisted while being swayed by the earthquake. 

“This resulted in the shear walls of the lift core and supportive columns being twisted to the point that they broke and failed. The building, without its foundation, was like hanging in the air for seconds before falling vertically due to gravity and crashing, following the rules of physics, F=mg.

“It left almost none of the structures to stand due to its great loads that fell fast and crashed hard, except for a stack of concrete pancakes. And this is the reason why we can hardly see any parts of the shear core of the building left at the scene, while the concrete slabs are so brittle.

“This is a destructive power of potential energy that was accumulated for a long period before being suddenly released — in just eight seconds,” said Prof. Emeritus Dr. Worsak, adding the collapse of the SAO building has set another world record as the tallest building, which has collapsed as complete pancake collapse due to natural forces.

The professor addressed this type of collapse as “tragic”, as it hardly leaves any room for those trapped in a collapsed building to survive.

As of April 18, the Bangkok Administration has updated the number of casualties there, from 44 to 42, as the other two are pending forensic investigation to confirm their identities. Aside from complete bodies recovered at the scenes, parts of bodies are reported to be scattered, prompting some obstacles for authorities to confirm a person’s identity. Only nine workers survived the incident, while 50 more are still missing at the site.