今回のテーマは『剛心』です。
子供の頃、指の上に本を乗せて上手くバランスをとって遊びませんでしたか？
これは本の重心を見つけているんですよね。
これと同じで、建物にも重さの中心『重心』があります。
実際には不可能ですが、重心を捉えれば家でさえ指に上手に乗っかります。
建物の重さに中心があるように、建物の強さにも中心があります。
それを『剛心』と呼びます。
建物の強さにはバラツキがあります。
南側に窓が多く北側は少なかった場合、南側は弱く、北側は強くなり、弱い南側が揺れやすくなります。
この場合、剛心が北側に寄っています。
地震のとき建物は、重心を基準に揺れます。
建物の強さが均一であれば、建物全体がバランスよく揺れます。
しかし実際には強さは均一ではありません。
揺れやすい南側、揺れにくい北側。
重心を起点に、建物がねじれるように揺れます。
よく建物の被害でいわれる「ねじれ」とは、このことです。
専門的にいうと、重心と剛心の距離が離れるほど、『ねじれ』が大きくなります。
逆をいえば、重心と剛心をできるだけ近づけることで、この現象は防げます。
耐力壁のバランスを考えることの重要性は、ここからきています。
弱い部分を強くし、重心と剛心をできるだけ近づけるのが目的なのです。
また2階が強く1階が弱いとき、その差が極端にあると、やはりより多くの力が1階に掛かってしまいます。
強さのバランスを整えることは、地震から建物を守る上でとても重要なことなのです。
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