樁筏基礎受水平地震力作用之簡易分析

Simplified Analysis for Piled Raft Foundation Subjected to Horizontal Motion of Earthquake

李旻儒(Min-Ju Lee)

指導教授 ： 張德文

淡江大學/工學院/土木工程學系碩士班/碩士(2016年)

https://doi.org/10.6846/TKU.2016.00888

摘要

本研究探討樁筏基礎受震反應，嘗試開發簡易分析程式EQPR (EarthQuake analysis for Piled Raft foundation)進行分析。該分析係建構於筏基的水平振動力平衡之上，藉有限差分法以及代表樁土元素的彈簧力進行求解，具簡易快速特性。研究並與三維有限元素軟體MIDAS-GTS NX 進行動力分析比較。地震紀錄係採用加速度歷時縮放法，配合建築物耐震設計規範及解說規定，將地震紀錄縮放至不同地震等級PGA值，以供耐震性能評估分析使用。本研究並藉EQPR分析探討樁筏基礎參數變化，同時藉由美國太平洋地震工程研究中心所建議的機率分析法(PBEE analysis)探討基樁耐震性能特性，討論不同耐震性能目標下樁身最大位移和彎矩狀況。
以新北市林口區大型煤倉樁筏基礎構造為例，研究發現: EQPR分析與有限元素分析在基礎位移量的預測上較為一致，但上構力的模擬會使得不同分析結果產生差異；基樁彎矩和剪力將受筏樁介面之邊界條件影響而產生差異。本案例中，有限元素預測的結構位移和內力均較EQPR稍大。大型樁筏基礎在水平地震力作用下，作用於基樁的地震力皆一致，故可藉單樁方式進行耐震分析。由於EQPR分析係以筏基下方等值墩基提供傳遞力，故墩基彈簧模式選取至為重要。當樁徑減少將會降低筏基位移量，當樁長縮短將會造成筏基位移量增加，樁數變化結果並不明顯。當改變煤灰土層剪力波速時，隨著剪力波速上升，筏基絕對位移量會下降；而當改變煤灰層土壤厚度時，隨著厚度增加，筏基位移量將會下降。上部結構對基礎位移之影響顯著，若將上部結構物納入分析，筏基之絕對位移量將會降低。上構和筏基位移比值R將介於0.5~1.2間。 研究同時發現場址軟土層厚度將對上述觀察造成影響。另以基樁耐震性能而言，若以水平地震觀察，筏基內力之重要性不及樁基，其設計仍以基樁耐震性能為主軸。本研究係以上構力均佈作用於筏樁的方式討論基樁之耐震性能，研究發現無論在中小地震、設計地震和最大考量地震下，基樁的耐震性能皆無虞，惟作用樁頂的起始力將會影響基樁彎矩容量，分析者必須慎之。

關鍵字

樁筏基礎、波動方程、耐震性能分析、地震力、動力分析

並列摘要

This study intends to discuss the simplified modeling for seismic responses of piled raft foundation subjected to horizontal earthquakes. The simplified modeling is suggested solving the differential equation based on force equilibriums of the raft with central difference formulas. The pile-soil-pile and soil elements underneath the raft were modeled by springs. This study also uses three-dimensional finite element analysis on MIDAS GTS NX program to verify the simplified solutions. Seismic performance of the piles was assessed using one-dimensional wave-equation analysis with PBEE procedures. For the moderate, design and the maximum consideration earthquakes, the seismic behaviors of the piled raft foundation could be evaluated using target PGAs based on calibration method. The maximum displacement and bending moments of the piles could be checked to reveal the assessment.
Numerical examples were conducted for piled raft foundations underneath the coal bunkers located at Linkou District in New Taipei City. The study reveals that: Foundation displacements obtained from the EQPR and 3D FEM analyses were found agreeable, however the super-structural load will affect the solutions. The internal stresses of the piles would be affected by the boundary conditions assumed at the interface between the pile head and the raft. In the case studied, the finite element analysis would result in larger internal stress and foundations displacement than the simplified analysis. The grouped pile foundation can be analyzed by a single pile because the horizontal seismic motions in any piles were found consistent. The simplified analysis was able to obtain the seismic force transmitting through the equivalent piers to the foundation. The foundation displacement will be decreased when reducing the pile diameter, whereas it will be increased by reducing pile length. Varying the number of piles seems not affect the results significantly. The foundation displacement will be slightly decreased with the increase of the shear wave velocity of soft soils. As the thickness of the soft layer increases, foundation displacement also tends to decrease. The displacement ratio R between the super-structure and the raft was found in a range of 0.5~1.2. With uniform loads applied at the piled raft foundation, the seismic performance of the piles were found safe at respective earthquake levels. The analysts should be cautious knowing that the loads on the pile head would affect moment capacities of the piles, in which the assessment could yield different result.

並列關鍵字

Piled raft foundation, Wave equation, dynamic analysis, horizontal earthquake, seismic performance

參考文獻

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