離心機振動台試驗樁土互制行為之數值分析

Abstract

本研究利用SAP2000程式針對含有不同上部結構配置之樁基礎離心機振動台試驗進行數值模擬分析，藉以探討結構物慣性力及地盤運動效應對於樁基礎受震反應之影響。離心機模型試驗係由楊宗翰（2016）完成，模型為埋置於乾砂土層中之樁基礎，砂土相對密度為80%及50%，經離心機80倍重力場放大後土層厚度為25.2 m，樁頂設計質量塊以模擬上部結構質量，搭配三種不同樁長之情況以代表不同結構動態系統特性。輸入運動採簡諧波，並考慮不同之振動頻率及振幅。 數值模型方面使用梁-彈簧-阻尼模式建立一套結合地盤反應及結構-樁-土互制分析之模型，以同時考量樁基礎受震時地盤運動及結構慣性力之效應，並以離心機振動台樁基礎受震試驗進行驗證，確定數值分析模型之適用性。土壤受震時之地盤運動以剪力梁模式進行模擬，以SAP2000沿垂直方向設置分布剪力彈簧模擬土層，以雙曲線骨架曲線搭配簡易梅新規則（Masing rule）定義土壤彈簧之非線性遲滯行為；樁-土間之互制行為則以溫克基礎模式（Winkler model）建立數值模型，以雙曲線函數表示之p-y曲線搭配簡易之梅新規則，模擬土壤受樁基礎側向擠壓而呈現之非線性行為。

This study uses SAP2000 to simulate the dynamic responses of different structure-pile-soil models under centrifuge shaking table testing, considering both the inertia effect from the structure mass and kinematic effect of ground on the pile. The centrifuge models consisted of piles embedded in dry sand under 80g centrifuge acceleration. The thickness of sand in the prototype was 25.2 m with relative density of 80% and 50%. A mass block was placed on the top of the pile to simulate the mass of the structure. Different extended lengths of the pile above the ground were adopted to represent different dynamic characteristics of the structural system. In the numerical model, a mass-pile-soil model connected to a shear beam model is established to consider both inertia and kinematic effects at the same time. The shear beam model adopts a series of vertically distributed horizontal springs, which adopts a hyperbolic shear force-displacement curve and the Masing rule for the hysteretic response, to simulate the seismic ground responses. For the pile-soil interaction behavior in the mass-pile-soil model, beam elements are used to model the pile, while the spring elements are used to model the soil reactions. The soil springs are defined by hyperbolic p-y curves and their hysteretic response is modeled by the Masing rule. A series of verification analyses show that the numerical ground, pile and structure responses from the proposed combined model are in good agreement with the centrifuge test results.