1、桩基设计
目录
一. 设计任务 ··································································· 错误!未定义书签。
二. 设计依据与具体计算 ··············································································· 1
1. 桩的轴向承载能力 ··················································································· 2 1) 极限承载能力 ······················································································· 2 2) 粘性土中桩的表面摩阻力和端部承载力················································ 2 3) 非粘性土中的桩表面摩阻力和端部承载力 ············································ 2 2. 轴向荷载桩的土反力 ··············································································· 3 1) 轴向荷载传递曲线(t-z曲线) ································································· 4 2) 桩端荷载—位移(Q-z)曲线 ···································································· 9 3. 侧向荷载桩的土反力 ············································································· 10 1) 软粘土的侧向承载力 ·········································································· 10 2) 硬粘土的侧向承载力 ·········································································· 14 3) 砂土的侧向承载力 ·············································································· 14
三. ANSYS模型计算 ·················································································· 18
1、桩基设计
一.
设计任务
导管架平台桩腿插入土层中,设计土层参数如表1所示,依据相关理论依据和规范中的相关公式计算桩基轴向荷载-位移关系(t-z曲线)(得到极限承载力),桩端载荷-位移曲线(Q-z曲线)和水平荷载-位移关系(p-y曲线)进行桩基设计,并用ANSYS软件计算,校核桩体强度。假设泥线处桩顶作用载荷:
竖直荷载:Q= 300+10×2+4 ×1000×9.8=3175200N 水平荷载:F= 50+2×28 ×1000×9.8=568400N 弯 矩:M=10×4×1000=40000N.m
假设桩体许用弯曲应力 σ =358MPa,桩腿外径D=0.889米,壁厚t=0.030米。
二.
设计依据与具体计算
由规范可知
1
1. 桩的轴向承载能力
1) 极限承载能力
包括扩脚桩在内,桩的极限承载能力QD应由下式确定;
Q??=????+????=??????+??????
式中:
????—表面摩擦阻力,kN; ????—端部总承载力,kN; f—单位侧摩阻力,kPa; As—桩侧表面积,m2;
q—单位桩端承载力,kPa; Ap—桩端总面积,m2.
2) 粘性土中桩的表面摩阻力和端部承载力
对于粘性土中的管桩,沿桩长度上任一点的轴向摩阻力f(应力单位),可按下式计算:
f=ac
式中:
a— 无量纲系数;
c— 所讨论点的土的不排水剪切强度(以应力单位计)。
系数a可由下式计算:
a=0.5???0.5??≤1.0 a=0.5???0.25??>1.0
????=′ ??0
限制条件是a≤1 式中:
??—对所讨论点; ′??0—所讨论点的有效上覆土压力,kPa;
对于端部支承在粘性土中的桩,单位端部承载力q可由下式计算,其单位为kPa:
q=9c
3) 非粘性土中的桩表面摩阻力和端部承载力
非粘性土中管桩的轴向侧摩阻力可按下式计算:
2
′
f=K??0tanδ
式中:
K —无因次侧向土压力系数(水平与垂直有效正应力之比); ′??0—计算点的有效上覆土压力,kPa ; δ —土和桩壁之间的摩擦角。
对于非堵塞的开口打人桩,通常假设拉伸和压缩荷载的K值均为0.8是合适的。对于充分排的桩(形成土塞或端部封闭的桩),可假设其K值为1.0。如果没有其它资料,可用表2来选择δ。对于长桩,f不可能按上面公式随上覆土压力无限地线性增加。在这种情况下,将f限制在表2给出的值内是恰当的。
表2非粘性硅质土的设计参数 土—桩摩擦极限表面摩极限单位端密度 土的类别 Nq 角/度 擦力值/kPa 承力值/Mpa 极松 砂 松 砂质粉土 15 47.8 8 1.9 中密 粉土 松 砂 中密 砂质粉土 20 67 12 2.9 密实 粉土 中密 砂 25 81.3 20 4.8 密实 砂质粉土 密实 砂 30 95.7 40 9.6 极密 砂质粉土 密实 砂 35 114.8 50 12 极密 砾砂 对于端部支承在非粘性土中部的桩,其单位端部承载力q可由下式计算,其单位为应力单位:
′
q=??0N??
式中: ′??0— 桩尖处的有效上覆土压力,kPa;
N??— 无量纲承载力系数,推荐的N??的值可从表2查出。
轴向摩擦力f作用于桩的内部和外部。然而,总的阻力除外部轴向摩擦力加端部环形面的承载力之外,还有内部轴向摩擦力或土塞的端部承载力这两者中的较小值。对于形成土塞的桩,可假设其支承力作用于桩的整个横截面上。对于没有形成土塞的桩,其支撑力只作用于桩的环形面积上。可根据静力计算结果判定是否形成土塞。例如,在非堵塞条件下打人的桩,在静载荷下可能形成土塞。 2. 轴向荷载桩的土反力
桩基础的设计应能够承受设计轴向静力和循环荷载。土的轴向抗力是由轴向的桩—土粘结或荷载沿桩侧面的传递和桩端的承载力组合而成的。在任一深度的动员的桩—土的剪力传递和桩的局部位移的图形关系可以用t-z曲线来表示,同
3
样,可动员的端部承载力和端部的轴向位移可以用Q-z曲线来表示。
1) 轴向荷载传递曲线(t-z曲线)
在没有更明确的准则以前,对非钙质土建议采用下列曲线,这些曲线见图1
图1 典型的桩的轴向荷载传递曲线(t-z曲线)
Z(in) t/tmax z/D t/tmax 0.0016 0.3 0 0 0.0031 0.5 0.100 1.00 0.0057 0.75 1.00 ∞ 0.008 0.9 0.01 1 0.02 0.70~0.90 0.70~0.90 表3 粘土t-z关系表 表4 砂土t-z关系表
式中:
z—桩的局部位移,mm; D—桩的直径,mm;
t—可动员的桩土粘结力,kPa;
tmax—桩土的最大粘结力或由上面计算的桩的单位面积的表面摩擦力,kPa。 算例
a. 第二层和第四层粘土
取7米处土层为例
该层土的有效有效上覆土压力
′??0=6.1×9.6+ 7?6.1 ×8.6+=66.3??????
设计抗剪强度
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