8 Critical Seconds: How the 30-storey building collapsed into a layer cake

Former President and Professor Emeritus of the Asian Institute of Technology (AIT) (Civil Engineering), Dr. Worsak Kanok-Nukulchai, has offered his theory and assumption on how the new 30-storey building of the State Audit Office (SAO) in Chatuchak collapsed via the St. Venant Torsion theory

Former President and Professor Emeritus of the Asian Institute of Technology (AIT) (Civil Engineering), Dr. Worsak Kanok-Nukulchai, also a fellow of the Royal Society of Thailand, 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.

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

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.