方形樁帽之極限強度預測-以四樁承載之樁帽為例

Estimating the Ultimate Load of Square Pile Caps: Take the Pile Cap Supported by 4 Piles as an Example

當建築基地土層承載力不足，可採用基樁承載傳遞建築物重量以保安全。然而其間聯結之鋼筋混凝土樁帽在底層柱之極高軸壓力作用下，設計時需配置適當之厚度避免剪力破壞，並且為確保樁帽不致發生撓曲破壞尚需配置適量的撓曲鋼筋。本研究針對4支樁承載之126座方形樁帽，依據軟化壓-拉桿模型，建議一新解析方法用以計算樁帽之極限強度。藉由分析值與試驗結果之比較，探討分析方法之精確度。經由比對發現，ACI規範(2019)斷面法採用樁帽之全寬度計算樁帽之撓曲強度，易高估由撓曲破壞主控樁帽之極限強度。本研究建議之分析方法，可以精確地判別破壞模式及預測樁帽之極限強度。

 

When the bearing capacity of the soil layer of the building base is insufficient, foundation piles can be used to transfer the weight of the building to ensure safety. However, the reinforced concrete pile caps of the interconnected piles have to be designed with appropriate thickness to avoid shear damage under the extremely high axial pressure of the underlying columns, and the appropriate amount of bending reinforcement is also required to ensure that the pile caps will not be damaged by bending. In this study, a new analytical method is proposed to calculate the ultimate strength of 126 square pile caps supported by 4 piles based on the softened strut-and-tie (SST) model. The accuracy of the analytical method is investigated by comparing the analytical values with the test results. It was found that the ACI Code (2019) cross-section method uses the full width of the pile cap to calculate the bending strength of the pile cap, which tends to overestimate the ultimate strength of the pile cap controlled by the bending damage. The proposed analytical method can accurately identify the damage pattern and predict the ultimate strength of the pile cap.