Flexible structure - flexible structure

One of the architectural structures that withstand earthquakes. In contrast, there is rigid structure. This refers to a structure that gives the building sufficient flexibility to deform, and lengthens the natural period of the building's shaking (the time it takes for the building to shake and return to its original position), thereby reducing the overall seismic force acting on the building. In contrast, rigid structure is a structure that uses earthquake-resistant walls and braces to resist earthquake forces and make the building strong, in order to minimize the deformation of the building during an earthquake. This method of rigid structure is generally used for low-rise buildings, but it is not appropriate for skyscrapers. Low-rise, strong buildings have little deformation and are subjected to relatively large destructive forces, but the aforementioned earthquake-resistant walls and braces can adequately resist them. On the other hand, flexible buildings with long periods are like "the wind in a willow," and only small destructive forces act on them overall, but they do deform to a certain extent. With the nature of earthquake response now understood, it has become possible to rationally build skyscrapers with large deformation capacity in Japan, assuming a location on solid ground. Before such scientific evidence was discovered, it was believed that the reason why the ancient five-story pagodas in Japan had never collapsed in an earthquake was because they possessed similar structural properties to the skyscrapers mentioned above, which is a very interesting idea.

[Takuji Kobori, Hiroo Kanayama]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

地震に対する建築の構造の一つ。対比して剛構造がある。建物に十分に、しなやかな変形能力を与えたうえで、建物の揺れの固有周期（建物が1回揺れて戻ってくるまでの時間）を長くして、作用する地震力を全体として小さくしようとする構造をいう。これに反して地震力に抵抗する耐震壁やブレース（筋かい）を設け建物を強剛につくって、地震時の建物の変形をできるだけ少なくしようとするのが剛構造であり、低層ビルには一般にこの剛構造の手法がとられるが、超高層ビルではそれは適切ではない。低層の強剛な建物は変形が少なく、それなりに大きな破壊力が作用するが、前述の耐震壁やブレースで十分に抵抗させられる。一方、周期の長い柔軟な建物には、地震を「柳に風と受け流し」全体として小さな破壊力しか作用しないが、それなりの変形は生ずる。こうした地震応答の性質がわかってきたことが背景にあって、堅い地盤立地を前提に、日本にも変形能力の大きい超高層ビルを合理的に建設することが可能となった。このような科学的裏づけがみいだされる以前に、古くから日本にある五重塔が地震で倒れたことがないのは、前記の高層ビルの構造に似通った性質が備わっていたためと解釈されていて、まことに興味深い。

［小堀鐸二・金山弘雄］

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