Structural Damping

This past week has seen a number of earthquakes occurring in this region of the world and as of this morning another one, of 7.8 Richter scale magnitude in Ecuador. The tragedies have thrown up a number of information on the impact of earthquakes on building, in particular this one which was on my Facebook feed last night.

Initially, the Kumamoto-jo. a hilltop Japanese castle located in Chūō-ku, Kumamoto in Kumamoto Prefecture originally built in 1467, was reported to have been damaged but as it turned out it was still standing despite damage to the stone walls and roof. Even then this was by design, meaning the roof tiles were constructed to fall off during earthquakes.

If you read the article carefully you would infer that ancient architects thought through design carefully. The design of the castle had to withstand strong hurricanes and minor tremors which meant the structure had to be strong and heavy. However, during a major earthquake strong tremors can be detrimental to a strong inflexible structure, causing it to collapse.

This was where the kawara (heavy tile roofing) came into play by providing enough weight to make the castle stable against hurricane and small tremors. Yet, in the face of a strong tremor the tiles would fall off, transforming a heavy, strong structure into a light, flexible structure. This is a good example of a paradox at work, that is, a structure that is stiff yet flexible when required to be so as to be able to provide structural damping against earthquakes.

Structural Damping – a measure of energy dissipation in a vibrating structure that results in bringing it to a quiescent state.

This principle was proposed in 1937 by Professor Ryo Tanabashi who specialized in building structures and had taught as a professor at the University of Kyoto and Kinki University in Osaka. Professor Tanabashi had proposed to define and estimate the seismic resistance of structures as the energy absorption until failure of the structure at the plastic limit state.

This was considered revolutionary back in the day but is today the basis for the design of structures to withstand earthquakes after it was verified by Professor G.W. Housner in 1956 who examined the failure of holding-down bolts at an oil refinery in the aftermath of the Alaska earthquake. Professor George Housner who died in 2008 at the age of 97 is regarded as the father of earthquake engineering, having developed what is regarded as the most complete mathematical system to analyze the effects of ground shaking on structures.

You can read what Professor Tanabashi wrote about flexible structure on page 4 of Earthquake Resistance of Traditional Japanese Wooden Structures published in 1960 by the Disaster Prevention Research Institute of the Kyoto University here. Below is the paragraph I am referring to on page 4 :-

The theory of rigid structures of Dr. Sano states that, in order to be safe from earthquake, structures should have sufficient stiffness to withstand the lateral force of earthquake. This theory itself seems to be unanimously agreeable. However, Dr. Majima maintained that as a result of providing a structure with ample stiffness the structure would be rigid and then the action of ground motions would increase so that a more stiffness would be required for the increase in the earthquake action, since the seismic action is larger as the structure becomes more rigid. He therefore concluded that if we can make the structures being more flexible the seismic action on them would be much weaker and the structures would still be safe.

How does an understanding of structural dampening advance our study of Tai Chi Chuan? Simple, in push hands we are dealing with forces exerted on our body by the opponent. To prevent ourselves from being unbalanced we would need to have the means to keep our balance in the face of the forces acting on us. One solution is to be structurally strong and heavy. The other solution would be to be flexible and keep moving to prevent the forces exerted by the opponent from disrupting our balance.

The third less obvious solution would be to keep a pliant structure, one that is strong enough to absorb smaller forces but will morph to adapt to stronger forces, something similar to what is written above about structural damping. If this is a viable method then why is it not used more?

The reason is because it is not so straightforward to train your body to be structurally stiff yet pliant. As simple as it sounds it is also incredibly difficult to do so due to the requirements for good awareness and control of your body when you are learning how to build this structure.

A good example was when I attempted to teach my student – I would fix one thing and move on to the next. When I adjusted the second part, she would lose the first part even when I stressed not to lose the adjustment. You can imagine how much more difficult it is when there are not just two, but many points to adjust.

This is why an in-depth adjustment of the body structure is normally not recommended for beginners because they have not developed the necessary amount of control of mind and body to be able to do it with lesser amount of difficulty. How do we train our body to structural dampen an external force is by understanding the requirements that I wrote about on page 50 and page 55, TaijiKinesis Vol 2 – Learning the Taijiquan Form. It is not impossible to Master Tai Chi Today once you know where to begin your learning.

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