報告書番号

382037

タイトル（和文）

軟質地盤上の大型基礎の動的挙動に関する実証研究（６）地震動に対する基礎の制震効果とその特性

タイトル（英文）

STUDY ON DYNAMIC BEHAVIOR OF LARGE-SCALE FOUNDATION ON SOFT GROUND (6) -LOSS MECHANISM OF EARTHQUAKE MOTION ON FOUNDATION-

概要 （図表や脚注は「報告書全文」に掲載しております）

本研究は，地震時における基礎の制震効果の特性を詳細に解明することを目的として，二つの典型的な基礎についての地震観測記録を解析し，それらの挙動を調べたもので，主な成果は次のとおりである。（１）超軟弱地盤の表層２ｍを直径３０ｍにわたり地盤改良した基礎と，砂礫地盤上に巾４５ｍ，長さ９０ｍ，厚さ約２１ｍの大型コンクリート基礎について，地震時の基礎上と地表面のデータを比較した結果，前者の水平成分の振巾が後者に比べて，波の周波数が高いほど小さくなることが判明した。（２）地盤改良による基礎の直径を約半分に減じても，基礎の制震効果の特性に変化は生じないことを確認した。（３）入力損失理論と重複反射理論による検討の結果，基礎の制震効果は後者によりほぼ説明することができることが判明し，この効果を今後設計に取入れることが期待される。

概要 （英文）

IN ORDER TO EXAMINE THE LOSS MECHANISM OF EARTHQUAKE MOTION ON FOUNDATION EXACTLY,SEVERAL EARTHQUAKE OBSERVATIONS ARE PERFORMED FOR TWO KINDS OF FOUNDATION.ONE IS THE CEMENT-MIXED SOIL IMPROVED FOUNDATION SETTLED ON THE ULTRA SOFT CLAYEY GROUND(B-BASE),WHICH IS 30M IN DIAMETER AND 2M IN THICKNESS,AND ANOTHER IS A LARGE-SCALE CONCRETE BASE OF 45M(W)X90M(L)X21M(H) CONSTRUCTED IN SANDY GRAVEL LAYER.THE FOLLOWING RESULTS ARE CLALIFIED WITH RESPECT TO THE LOSS CHARACTERISTICS OF THESE FOUNDATIONS DURING EARTHQUAKES. (1)THE HORIZONTAL AMPLITUDE OF EARTHQUAKE MOTION ON BOTH FOUNDATIONS DECREASES IN COMPARISON THOSE ON THE GROUND SURFACE AND THIS PROPERTY IS MORE EFFECTIVE,THE FREQUENCY BECOMES HIGHER, THOUGH THE FOUNDATIONS MAKE ONLY A LITTLE CONTRIBUTION TO REDUCE THE VERTICAL AMPLITUDE OF EARTHQUAKE MOTION. (2)FROM THEANALYSES OF SPECTRAL RATIO BETWEEN THE B-BASE AND GROUND,BEFORE AND AFTER CHANGING ITS DIAMETER,IT IS COMFIRMED THAT LOSS EFFECT OF FOUNDATION IS NOT AFFECTED ESSENTIALLY BY THIS GEOMETRIC TRANSFORMATION IN A DIAMETER. (3)ON THE OTHER HAND,IT IS CLARIFIED THAT THE THEORETICAL TRANSFER FUNCTION BETWEEN THE BASE AND THE GROUND OBTAINED FROM MULTIPLE REFLECTION THEORY OR F.E.M ANALYSIS FAIRLY COINCIDES WITH THE OBSERVED ONE.CONSEQUENTLY,THE LOSS MECHANISM OF THE FOUNDATION IS MAINLY CAUSED BY THE REFLECTION EFFECT OF INPUT ENERGY AT THE BOUNDARY BETWEEN THE RIGIDBASE AND RIGHT UNDER SOFT GROUND BECAUSE OF THE GREAT DIFFERENCE IN BOTH ACOUSTIC IMPEDANCES, AND IT CAN BE SAID THAT THE LOSS EFFECT OF THESE FOUNDATIONS CAN BE ESSENTIALLY ESTIMATED BY MEANS OF SIMPLE MULTIPLE REFLECTION THEORY WITHOUT CONSIDERING THE EFFECT DUE TO THE INPUT LOSS THEORY.

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