Seismic performance and influencing factors of pile foundation of bridges in permafrost regions
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摘要: 为研究多年冻土区桥梁桩基础抗震性能及影响因素,以中国多年冻土区广泛存在的高承台桩基础为研究对象,通过拟静力试验结合有限元方法探讨了多年冻土区桥梁桩基础地震破坏特征及冻土层物理力学特性变化对其抗震性能的影响规律。结果表明,随着土体温度的降低,桩–冻土体系的水平承载力、初始刚度均呈增大趋势,桩身位移在土体冻结前后变化显著。土体初始含水率的改变对桩–冻土体系的水平承载力及桩身位移的影响较小,但以土体最优含水率为界限,界限含水率两侧桩–冻土体系的刚度变化存在较大差异。土体压实度的改变对桩–冻土体系的水平承载力及桩身位移的改变影响较小,桩–冻土体系的初始刚度随着压实度的增大而增大。因此,在冻土区桩基础桥梁抗震设防中应当充分考虑桩周冻土物理力学特性变化对其抗震性能的影响。Abstract: In order to study the seismic performance and influencing factors of the pile foundation of bridges in permafrost regions, the pile foundation of bridges with elevated caps widely used in the permafrost regions of China is taken as the research object. The seismic failure characteristics of the pile foundation of bridges in the permafrost regions, and the influences of the physical and mechanical properties of frozen soil on its seismic performance are discussed by using the quasi-static tests and combining with the finite element method. It is found that the lateral bearing capacity and the initial stiffness of the pile-frozen soil system increase with the decreasing temperature of the frozen soil. The displacement of piles changes significantly before and after soil freezing. The change of the initial moisture content of soil has small influences on the lateral bearing capacity of the pile–frozen soil system and the displacement of piles. Take the optimal moisture content of soil as a boundary, the stiffness changes of the pile-frozen soil system are quite different under both sides of the boundary moisture content. The change of soil compaction degree has small influences on the change of the lateral bearing capacity of the pile–frozen system and the displacement of piles. The initial stiffness of the pile-frozen soil system increases with the increase of compaction degree of soil. Therefore, the physical and mechanical properties of the surrounding frozen soil of piles should be fully considered in the seismic performance evaluation of bridges with pile foundation in frozen soil regions.
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0. 引言
在建筑工程领域中,对于水平荷载作用下桩基的受力分析,常见有极限地基反力法、弹性地基反力法、复合地基反力法、弹性理论法和p-y曲线法等[1-2]。弹性地基反力法又包括地基系数常数法、k法、c法、m法以及吴恒立[3]的双参数法。张有龄给出了地基系数为常数时的桩身响应解析解,N.B.ypdh与众多学者给出了桩身内力与变形的幂级数解,更有采用[4]纽玛克法、有限差分法与有限元法来求解桩身内力与变形。
上述常用方法中对于多层地基情况的处理略显粗糙,如目前建筑桩基[6]与公路桥涵桩基领域[7]最常采用各层地基按其地基系数以权重进行折算,得到一个地基系数的等效值。近年来,Pise[5]对双层地基水平受荷桩进行了数值求解,赵明华等[8-10]对成层地基中桩的受力与试验做了大量工作,并尝试用无网格法分析计算,戴自航等[11]采用有限元与有限差分进行数值计算,竺明星等[12]利用矩阵传递法依次求解多层地基中的桩身各点内力,詹红志等[13]也采用类似矩阵传递方法对抗滑桩嵌固段多层岩层进行了计算。
本文不同于先前学者从桩身形函数利用幂级数角度出发,引入张氏法的解析解函数形式,利用节点内力变形连续条件,建立全桩全节点统一矩阵线性方程,引入边界条件后一次性求解所有节点的变位与内力,并将该计算方法应用于多层地基桩基的水平响应计算。
1. 线性方程解力学模型
1.1 力学模型建立
不同于竺明星等[12, 14]建立三参数地基系数模型并利用Laplace变换求解桩身响应的方法,本文在理论推导过程中不特别假定地基系数的分布模式,但考虑到设计人员使用上的便利性,以单层地基m值与多层地基m值分别演示计算过程。
根据Winkler理论,假定地基是服从胡克定律的弹性体,且每层地基厚度为hj,如图1所示。
将桩身沿深度方向分成
n 段,桩单元依次编号为1,2,···,n ,桩结点编号为0,1,2,···,n ,结点对应各自坐标值。记桩身水平位移为y(z) ,桩身转角为φ(z) ,桩身弯矩为M(z) ,桩身剪力为F(z) ,M0,F0 表示桩顶作用的弯矩与水平力,Mi,zj,Fi,zj 表示第i桩单元的zj 节点处的弯矩与剪力,对于第i 桩单元,第i 段内地基系数ki 以该段内的积分中值定理为原则,即ki=∫zizi−1k(z)dzzi−zi−1。 (1) 约定弯矩以桩左侧受拉为正,剪力以使桩顺时针转动方向为正,水平位移以坐标正向为正,而截面转角以逆时针转动为正。
1.2 微分方程及解答
对于第
i 段,满足如下微分方程:EId4yidz4+kibyi=0。 (2) 令
Ai=4√kib4EI ,则可得到第i 段z∈[zi,zi−1] 的挠曲线方程yi(z) 解析解与挠曲线各阶导数:yi(z)=Ci1e−Aizsin(Aiz)+Ci2e−Aizcos(Aiz)+Ci3eAizsin(Aiz)+Ci4eAizcos(Aiz)。 (3) 将以上各函数表达式整理成矩阵形式,如式(4):
yi=[fi1(z)fi2(z)fi3(z)fi4(z)]⋅[Ci1Ci2Ci3Ci4]T ,y(1)i=[gi1(z)gi2(z)gi3(z)gi4(z)]⋅[Ci1Ci2Ci3Ci4]T ,y(2)i=[pi1(z)pi2(z)pi3(z)pi4(z)]⋅ [Ci1Ci2Ci3Ci4]T ,y(3)i=[qi1(z)qi2(z)qi3(z)qi4(z)]⋅ [Ci1Ci2Ci3Ci4]T 。} (4) 令各系数矩阵表达式如下:
[P*i,zi−1]=[EIpi1,zi−1EIpi2,zi−1EIpi3,zi−1EIpi4,zi−1],[Q*i,zi−1]=[EIqi1,zi−1EIqi2,zi−1EIqi3,zi−1EIqi4,zi−1],[f*i,zi−1]=[fi1,zi−1fi2,zi−1fi3,zi−1fi4,zi−1],[g*i,zi−1]=[gi1,zi−1gi2,zi−1gi3,zi−1gi4,zi−1]。} (5) 则桩身
n 段各个节点处的弯矩、剪力、挠度与转角记成:[B]=[V*]⋅[C*] ,其中[B]=[M1,0...Mn,n−1M1,1...Mn,nF1,0...Fn,n−1F1,1...Fn,ny1,0...yn,n−1y1,1...yn,nφ1,0...φn,n−1φ1,1...φn,n], (6) (7) [C*]=[[C1,j][0]...[0][0][C2,j]...[0][0][0]...[0][0][0]...[Cn,j]]j=1,2,3,4。 (8) 1.3 全桩线性方程组的构建与解答
保证桩身每一结点处内力与位移是连续的,以此思路建立桩身全结点的线性方程组如下:
Mi,zi=Mi+1,zi ,Fi,zi=Fi+1,zi ,yi,zi=yi+1,zi ,φi,zi=φi+1,zi 。} (9) 最终记成如下矩阵形式:
[ξ*]⋅[C]=[H] ,其中[C]=[[C1,j][C2,j][C3,j]...[Cn−1,j] [Cn,j]]j=1,2,3,4,[H]=[M0F00...0Hn−1,znHn,zn], (10) [ξ*]=[[P*1,z0][0][0][Q*1,z0][0][0][P*1,z1]−[P*2,z1][0][Q*1,z1]−[Q*2,z1][0][f*1,z1]−[f*2,z1][0][g*1,z1]−[g*2,z1][0][0]......[0][0][ξ*4n−1,n][0][0][ξ*4n,n]]。 (11) 上式矩阵运算表示了全桩全结点内力与位移值需要满足该线性方程组,引入桩顶与桩端的边界条件后,等式右侧矩阵也为常数阵,这样可通过Gauss消元等多种方法求解线性方程组,解得桩身每一段的四组参数
Ci1,Ci2,Ci3,Ci4 ,再将系数C矩阵回代式(8)即可。2. 基于m法的设计计算与结果验证
2.1 单层地基算例验证
某建筑物[2]采用桩基基础,直径d=1.5 m,埋入并支持在非岩石类土中,入土深度h=15 m,桩头在地面处自由,作用有水平荷载H0=60 kN和M0=700 kN·m,C25级混凝土的弹性模量Ec=2.8×104 MPa= 2.8×107 kN/m2,地基的反力系数的比例系数m=9400 kN/m4,土的内摩擦角
φ =22°,黏聚力c=15 kN/m2 ,重度γ=20 kN/m3 。b=KφK0d=0.9(1.5+1)=2.25m ,EI=0.85×2.8×107×π×1.5464=59.3×105kN⋅m2 。分别将n取5,10,15,20进行了桩身内力与位移计算,本文法计算结果与传统m法计算的桩身弯矩绘制成曲线图,如图2所示。按规范法计算桩身最大弯矩为766.9 kN·m,将桩等分20段后桩身弯矩最大值为762.8 kN·m,相比规范法误差0.53%。从上图2看出,桩身弯矩随深度增加总体呈现先上升后下降的过程,弯矩极值出现在距离桩顶(1~3)d范围之间(d为桩身直径),弯矩零点位于距离桩顶6d位置左右。
图3,4中看出3种端部约束下的弯矩、位移曲线重合度较高,仅在桩端附近处弯矩曲线出现了分叉发展的趋势。位移零点出现在距离桩顶4d位置处,相比桩身弯矩的6d变化范围缩小了33.3%。本算例所得的桩身弯矩极值、弯矩与位移零点所在的桩身位置符合目前国内外学者的研究结果,如赵明华等[15-16]曾建议桩影响范围取3~5d,冯忠居等[17]建议取(2~8)d等。
2.2 多层地基算例验证
某圆形[12, 18]截面灌注桩[10]桩径d=1.0 m,地面处桩顶剪力Q =150 kN,弯矩M =0,桩的弹性模量E =2.1675×10 kN/m2。桩侧有两层地基土体:第一层为流塑状回填土,层厚为2.0 m,相应的地基反力系数m为3000 kN/m4;第二层为硬塑状黏性土,桩身在该层土体中的长度为10.0 m,相应的地基反力系数为20000 kN/m4。
表1为不同计算方法的计算结果,从中可知本文线性方程解与精确解之间的桩顶位移误差为2.3%,最大弯矩误差为0.285%。
笔者在本算例基础上,改变地层情况再次进行桩身弯矩与位移计算,分别将地层视为全为上层土的单一地层与全为下层土的单一地层(简称“上层土地基”与“下层土地基”),将计算结果分别绘制成曲线图5,6用以对比分析。由图5可以看出,桩身弯矩极值出现在距离桩顶(1~5)d之间,本例中双层地基情况与“下层土地基”情况都在8d位置处达到了弯矩零点;“下层土地基”与“两层土地基”均在深度5d处为位移零点。
3. 结论
假设地基为弹性材料,分段建立梁挠曲线微分方程,通过结点内力与位移的连续条件一次性建立桩身全结点的线性方程组,求解得到各点内力与位移。以两个算例验证了线性方程解法在单层地基与多层地基中桩身响应计算的正确性,并对桩底不同边界条件、桩身周围不同地层进行了计算与讨论,得出如下结论:
(1)在桩顶水平荷载的作用下,桩身弯矩最大值出现在距离桩顶(1~5)d范围内,桩顶附近土层抗力越差,最大弯矩所出现的位置将越深。
(2)在距离桩顶(6~8)d位置附近将出现弯矩函数零点,且下降段所处区间受桩中部土层的抗力大小控制。
(3)桩身位移最大值出现在桩顶,距桩顶5d位置处出现位移零点。桩端不同的边界条件对桩身的位移影响较小,而桩周土层的抗力大小对桩身的位移起到控制作用。
(4)同一种情况下的桩身水平响应,其弯矩零点所出现的位置将比位移零点所出现的位置滞后(2~3)d。
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表 1 不同变量土体力学参数
Table 1 Mechanical parameters of soils with different variables
试验工况 含水率/% 干密度
/(g·cm-3)温度/℃ 围压/kPa 弹性模量
/kPa黏聚力/kPa 内摩擦角/(°) 1 10% 1.75 -5 50 2 10% 1.75 -5 100 355.95 24.76 3 10% 1.75 -5 200 85723.19 4 14% 1.75 -5 50 5 14% 1.75 -5 100 568.05 25.99 6 14% 1.75 -5 200 54347.82 7 16% 1.75 -5 50 8 16% 1.75 -5 100 500.67 25.36 9 16% 1.75 -5 200 49083.27 10 14% 1.75 2 50 11 14% 1.75 2 100 15.57 19.52 12 14% 1.75 2 200 5433.04 13 14% 1.75 -9 50 14 14% 1.75 -9 100 798.41 30.49 15 14% 1.75 -9 200 158985.18 16 14% 1.65 -5 50 17 14% 1.65 -5 100 407.62 24.16 18 14% 1.65 -5 200 51916.93 19 14% 1.85 -5 50 20 14% 1.85 -5 100 638.96 27.85 21 14% 1.85 -5 200 123211.26 表 2 有限元模拟工况
Table 2 Finite element simulation conditions
模拟工况 含水率/% 温度/℃ 干密度/(g·cm-3) 模型1 14 2 1.75 模型2 14 -5 1.75 模型3 14 -9 1.75 模型4 10 -5 1.75 模型5 14 -5 1.75 模型6 16 -5 1.75 模型7 14 -5 1.65 模型8 14 -5 1.75 模型9 14 -5 1.85 -
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