Dr. Bakko's Structural LAB.

For the people who wants to live in Japan safely

Tale of Stiff, Strong and Safety:House edition

Japanese word “GASSHIRI” has a meaning “solid”, “robust” or “stable”. It can be said “stiff and strong”. “GASSHIRI house” will offer safety life for natural disasters.


□■□ Question ■□■

To realize “GASSHIRI”, which is more important, “stiff” or “strong”? Or is “stiff and strong” necessary?


□■□ Answer ■□■

Only “stiff” or “strong” are not enough. And “too, stiff” may causes an adverse effect. Balance of stiff and strong is important, so extra care is necessary when setting built-in garage, large opening and so on.



Difference between “stiff” and “strong”

Though we often use those two words “stiff” and “strong” in daily life, some people sometimes confuse “stiff” and “strong”. Definitely, there is a tendency that the stiffer the more strong. But there are some materials which are stiff but weak, or soft but strong.


Stiffnessthe force required to deform the material certain amount

Strongnessthe force that the material can tolerate until collapse


Diamond represents “stiff but weak” materials. What happen if you hit the diamond by steel hammer? Perhaps diamond gets crashed and hammer gets only slight damage on its surface. It means diamond is stiffer than steel, but weaker than steel. As a familiar example, glass is stiff and weak material, too. Only “stiff” may result in making house fragile.


Alloy used to frame of glasses represents “soft but strong” materials. Although it is enough soft to be bent easily by your fingers, it can restore completely after you stop bending.


Vibration feature of “stiff building” and “soft building” under earthquake

In comparison with single house and skyscraper, it can be said that single houses sway fast and bumpy and are “stiff buildings”, and skyscrapers sway slow and unsteadily and are “soft buildings”. And in “stiff buildings”, there are “stiff” and “soft”. The newest houses well designed against earthquake are “stiff”, and old houses with heavy roofing tiles are “soft”. Here, difference of “stiff” and “soft” in “stiff buildings” is explained.


Relation of shaking intensity of each floor under earthquake in stiff buildings is follows.

Base 1st floor 2nd floor Roof

The shaking is amplified as an upper floor.


Then, amplification of shaking becomes smaller as the building is stiffer. In the case that the building is adequate stiff, shaking of roof becomes around 1.5 times of base. If building is soft, it would be over 3 times of base. Even if strong enough to withstand earthquake force, safety of residents is not ensured unless stiffness is enough. Building structure might not get damaged, but amplification of shaking causes overturning of furniture and crack of interiors.


And building becomes softer as the walls and columns get more damaged by earthquakes. Due to the accumulation of slight damage caused by several small earthquakes, building may be shaken greatly on next huge earthquake.


Balance of “stiffness” and “strong” is important

In wooden house, walls withstand earthquake force. In reinforced concrete or steel house, it is rare that adopting ramen structure (withstanding earthquake force by columns and girders). Therefore walls and braces are main structural elements regardless of structural style.


There is interrelation between stiffness and strong. So there are few single houses with “stiff and weak” or “soft and strong” if Japanese building code is satisfied. It will offer appropriate stiffness and strong. To improve safety for earthquake, only adding walls is necessary. But it doesn’t mean that adding more walls simply results in safer.


Supposing that you still worry about the safety of the house which designed well, and you decide to retrofit the house.


“On 1st floor, there are a few spaces to set new walls, so please increase walls by 20%. On 2nd floor, there are a lot of spaces to set new walls, so please increase walls by 100%.”

Is this meaningful?


In regulation of wooden building (depends on quantity of walls), safety improves by 20%. In normal structural calculation (depends on stress of each element), safety improves by 20%, too. It doesn’t depend on that increase amount of the 2nd floor walls. After all, the safety is decided depending on weaker layer, 1st floor or 2nd floor. Setting many walls on one floor is not effective.


You may say“It’s just a from point of view on regulation or calculation. You mentioned that amplification of shaking becomes smaller as building is stiffer, right? If so, we can say that setting many amounts of walls on 2nd floor and making building stiffer is effective.”

But it’s not true.


If design is appropriate, building will withstand earthquake throughout itself. Both 1st floor and 2nd floor deform to get away with earthquake. In the case of too stiff 2nd floor, total deformation of building may become smaller. But almost all deformation would occur on 1st floor. After all, it may cause adverse effect rather than improving safety by 20%.


The building easy to lose balance

Walls are the most important elements for earthquake, so the buildings having difficulty to set walls evenly are easy to loose balance (e.g. unusual footprint, unusual appearance or very large opening).


Of course those are not bad, and it is possible to make such buildings safer. But there is no usual solution. Measures vary depending on the building. It is important that architect who specializes in structure participate it.