今回のテーマは『断面欠損』です。
在来軸組工法は、梁と柱を組み合わせて造ります。
一方の木材に『ほぞ穴』と呼ばれる穴をあけ、もう一方の木材の先端を細くして、ほぞ穴に差し込みます。
木材の一部を欠いているほぞ穴部分が、断面欠損の状態です。
昔の建物は柱も梁も大きく、ほぞ穴をあけても残っている面積が大きいので接合部に耐力がありました。
現在の一般的な在来軸組工法も伝統工法からの流れをくんでいますが、使用されている10.5㎝角や12㎝角の柱に18㎝程度の梁を取り付けても、その接合部は簡単に変形してしまい、耐力が期待できません。
特に梁は、梁材の真ん中で柱や他の梁を受けることがあります。
その場合、梁の真ん中にほぞ穴を開けなければなりません。
部材の途中の断面欠損は構造上弱くなりがちです。
ですので、きちんと計算をし掛かる力に負けない強度の梁材を見極め、適切な金物で補強します。
ちなみに柱は、通し柱を除いて断面欠損は先端になります。
上部の梁を受ける部分ですね。
この場合、構造計算上の支障はほぼありません。
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