Buckling

A phenomenon in which, when pressure is applied to building components, etc., the deformation increases suddenly if the pressure exceeds a certain limit. Building columns support the weight of the upper part (compressive force). When a relatively slender column receives a compressive force exceeding a certain limit, the deflection (deformation) increases suddenly, and a phenomenon known as bulging (the surface is pushed out and bulges) occurs. This phenomenon is called buckling (bifurcated buckling, Figure A ). Buckling phenomena are characterized by the change in the type of deformation at the buckling point, and the sudden decrease in the column's ability to support the weight of the upper part at the moment of buckling. There are many cases in which braces are used as earthquake-resistant elements in the structural surfaces of columns and beams, and a similar phenomenon occurs with these. The susceptibility of columns and braces to buckling depends on the shape and length of the cross section (slenderness ratio). The slenderer they are, the more likely they are to buckle. In addition to column buckling, there is also plate buckling. When force is applied to the top of a bent plate (folded plate), the top of the plate will move to the other side at a certain limit of force (snap-through buckling, Figure B ). In this case, if there is no damage when it bends to the other side, it can withstand further increases in force. The final example is the buckling that occurs when a can is crushed. When a can made of thin aluminum plate is subjected to a compressive force that exceeds a certain limit, it will neck down and suddenly collapse (yield buckling, Figure C ). This buckling phenomenon also occurs in large-scale structures such as oil tanks, causing localized deformation known as an "elephant's foot." The likelihood of this phenomenon occurring depends on the thickness and shape (width-to-thickness ratio) of the plate.

In structures, these buckling phenomena are often accompanied by a sudden decrease in strength and an increase in deformation, so they require careful consideration in the design.

[Takuji Kobori, Hiroo Kanayama]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

建築などの部材に圧力が加えられたときに、限度を超えると急激に変形が増大する現象。建物の柱は、上部の重さ（圧縮力）を支えている。比較的細長い柱が、ある限度以上の圧縮力を受けると、急にたわみ（変形）が増加し、いわゆる、はらみ出し（面が押し出されてふくれること）が生じる。この現象を座屈という（分岐座屈、図A）。座屈現象の特徴は、座屈点を境に変形の型が変化することと、柱では座屈の瞬間に上部の重さを支える能力が急激に低下することである。柱や梁(はり)の構面に耐震要素としてブレース（筋かい）を用いる例も多いが、これにも同様な現象が生じる。柱やブレースの座屈のしやすさは、断面の形と長さ（細長比(ほそながひ)）に依存する。細長いものほど座屈は生じやすい。柱の座屈のほかに板の座屈がある。折り曲げた板（折板(せっぱん)）の頂部に力を加えると、ある限度の力で折板の頂点が逆側に移る（飛移り座屈、図B）。このときは、逆側に反り返るときに破損がなければ、それ以後も力の増加に耐えられる。最後の例は、缶をつぶすときに生じる座屈である。薄いアルミ板でつくられた缶は、ある限度を超えた圧縮力で、くびれが生じ急激につぶれる（屈服座屈、図C）。貯油タンクなどの大規模構造物にもこの座屈現象が生じ、「象の足」とよばれる局所的変形が生じる。この現象の生じやすさは、板の厚さと形状（幅厚比(はばあつひ)）に依存する。

　構造物において、これらの座屈現象は急激な耐力低下や変形の増大を伴うことが多いので、設計上周到な考慮を必要とする。

［小堀鐸二・金山弘雄］

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