固定樁頭樁韌性行為之研究

Abstract

樁基礎在耐震設計上通常會被設計為彈性結構物，然而對於固定樁頭樁，其樁頭受到樁帽或基礎版束制之影響，在地震下不免會發生損害，此時基礎若採韌性設計，允許基礎在高強度地震下有非彈性反應，將可使基礎設計更為合理可行。當耐震設計時考量樁之韌性消能行為，其位移韌性容量會是個設計所需之重要參數，其代表樁之非彈性變形能力。為探討樁基礎之位移韌性容量，本研究透過參數研究，以固定樁頭單樁為研究標的，探討其韌性容量之影響因子，並利用理論分析針對凝聚性與非凝聚性土層中樁基礎之位移韌性容量發展簡易評估公式，並以一壁式橋墩群樁基礎耐震設計為示範例，示範此簡易評估公式於樁基礎韌性設計之應用，並進一步探討基樁韌性設計對基礎設計結果之影響。 由參數研究結果發現基樁位移韌性容量與斷面過強度比之間成顯著正相關。除曲率韌性容量外，斷面之過強度比在固定樁頭樁之位移韌性容量上亦扮演相當重要的角色。此參數研究結果亦可從理論推導所發展的簡易公式獲得到驗證。由簡易評估公式可知固定樁頭樁之位移韌性在線性土壤之情況下，直接係樁斷面之過強度比與曲率韌性容量之函數，可作為韌性容量之下限值，而在土壤為非線性的情況下，因土壤之非線性行為之參與，可使得樁之位移韌性容量更為加大。本研究進一步提出的類壁式橋墩群樁基礎耐震韌性設計流程，並利用所發展之簡易評估公式於設計流程中以估算結構系統之韌性容量，以展現基礎韌性設計之適用性與影響。此設計流程簡單，適合於工程初步設計使用。 另一方面，由於基樁斷面強度常有弱化現象，本論文專章討論斷面撓曲強度產生弱化現象之機制，並進一步探討其對韌性容量之影響。經參數研究發現斷面產生弱化現象係因混凝土在達尖峰強度後強度下降所致，其中以軸壓力大小所造成之影響最為顯著。具弱化現象之斷面，其斷面過強度比因在高軸壓力下相當小，使樁所能發揮之位移韌性容量也相對有限。

For fixed-head piles subjected to seismic loading, they usually sustain large flexure curvature demands at the pile heads when the pile cap undergoes a horizontal displacement. For this type of pile, it is cost effective to design the piles as ductile members to absorb the earthquake energy, instead of elastic ones, which are normally adopted in conventional seismic design. Consequently, the displacement ductility capacity of a pile is an important concern when the ductile design is applied to piles under seismic loading. This study performs parametric analysis to investigate the influencing factors of the displacement ductility capacity of a fixed-head pile. In addition to the curvature ductility, the results of the parametric analysis show that the over-strength ratio of the pile section is another important factor that influences the displacement ductility capacity of the pile. Furthermore, this study develops simple formulae to estimate the displacement ductility capacity of a fixed-head pile in cohesive and cohesionless soils. The formulae have three parameters: the sectional over-strength ratio, curvature ductility capacity, and a modification factor considered for soil nonlinearity. For demonstrating the applicability of the proposed formulae, this study further applies them to seismic design of the pile foundation of wall piers. On the other hand, the softening in flexural strength of a RC pile is commonly observed in either experiments or numerical analysis. This study performs a parametric study on its mechanism. The results show that the softening in flexural strength of pile sections is due to the loss of concrete strength and the softening degree of flexural strength is significantly influenced by the axial force level on the pile section. For this type of pile section, its sectional over-strength ratio is small and the displacement ductility capacity of the pile is accordingly small.