桩基础设计习题讲解

HOHAIUNIVERSITY土木与交通学院岩土所基础工程习题课基础工程习题课HOHAIUNIVERSITY土木与交通学院岩土所基础工程习题课基础工程FoundationEngineering基础底面尺寸的确定【例1】某住宅承重墙厚240mm，采用墙下条形基础；地基土表层为杂填土，厚度0.80m，重度17.5kN/m3，其下为粘土层，重度18.5kN/m，承载力特征值fak为170kPa，孔隙比0.86。地下水位在地表下1.0m处。若已知上部墙体传来的竖向荷载标准值为195kN/m。试确定基础底面尺寸。HOHAIUNIVERSITY土木与交通学院岩土所基础工程习题课基础工程FoundationEngineering解:1.确定基础埋深:为了便于施工，基础宜建在地下水位以上，故选择粘土层作为持力层，初步选择基础埋深d=1m。2.对持力层土进行深度修正:由86.0e85.0，查表得：0b，0.1d。HOHAIUNIVERSITY土木与交通学院岩土所基础工程习题课基础工程FoundationEngineeringHOHAIUNIVERSITY土木与交通学院岩土所基础工程习题课基础工程FoundationEngineering埋深范围内土的加权平均重度：3/7.170.12.05.188.05.17mkNm持力层土的承载力特征值kPafa9.178)5.00.1(0.17.171703.取1m长的条基作计算单元基础宽度mmdfFbGak0.323.10.1209.178195HOHAIUNIVERSITY土木与交通学院岩土所基础工程习题课基础工程FoundationEngineering取该承重墙下条形基础宽度b＝1.25m。4.验算:KPabdbFAGFGkkk17625.125.1120195＜KPafa9.178满足要求。HOHAIUNIVERSITY土木与交通学院岩土所基础工程习题课基础工程FoundationEngineering例：某住宅的承重砖墙厚b0＝240mm，传至基顶的荷载标准值Fk＝220kN/m，埋深d＝1.3m。地基土为很厚的细砂，其承载力特征值fak＝140kPa，天然重度＝19kN/m3，基础材料用MU10砖和M5砂浆砌筑。试设计刚性条形基础。2.0b3.0d2(0.5)1403.019(1.30.5)185.6/aakdmffdkNm1.0lm解：查表知。地基承载力进行深度修正：，取单位长度进行计算，则有:mmmdfFbak138038.13.1206.18522020HOHAIUNIVERSITY土木与交通学院岩土所基础工程习题课基础工程FoundationEngineering考虑采用“二一间隔收”砌筑方法，设砖基础台阶数为n阶，按砖的模数，基底宽度应为：b=b0+2×60n=240+120n≥1380mm，则有：5.91202401380n则基底宽度应为b=b0+2×60n=240+120×10=1440mm3m(可不作宽度修正)。验算：（阶）。n应取整数，即n=10（阶）kPafkPabGFpakkk6.1858.17844.13.144.120220满足要求。HOHAIUNIVERSITY土木与交通学院岩土所基础工程习题课基础工程FoundationEngineering按“二一间隔收”砌筑方法，基础台阶数为n=10阶，则基础高度应为：H0=120×5+60×5=900mm。查表知:砖基础的宽高比允许值为，验算：1:1.55.11tan5.11900224014402000HbHbbi满足要求dt=d-H0=1.3–0.9=0.4m0.15m；bt=b0+2b1=0.24+2×0.06=0.36m。HOHAIUNIVERSITY土木与交通学院岩土所基础工程习题课例题：桩基础设计如图所示，柱截面尺寸：bc=450mm，hc=600mm，荷载标准组合时作用于柱底的荷载为：Fk=3040KN，Mk=160KNm，Hk=140KN，拟采用混凝土预制桩基础，方桩边长bp=400mm，桩长15m。已确定单桩竖向承载力特征值Ra=540KN，承台混凝土C20，钢筋HRB335级，试设计该桩基础。y600450例图4-2HKFKMKXHOHAIUNIVERSITY土木与交通学院岩土所基础工程习题课500xy600450例图4-2HKFKMK解：C20混凝土：ft=1.1N/mm2;HRB335级钢筋：fy=300N/mm2。（1）桩的类型和尺寸已确定，桩身结构设计从略。（2）初选桩的根数：n＞Fk/Ra=3040/540=5.6根，暂取6根（5.6×1.2=7根）。HOHAIUNIVERSITY土木与交通学院岩土所基础工程习题课120040032004002000600140070830500xy600450例图4-2HKFKMK4006004001200（3）初选承台尺寸桩距（按规范表）：mbsp2.14.00.30.3承台长边：ma2.3)2.14.0(2承台短边：mmb5.00.2)6.04.0(2暂取承台埋深：1.4m，承台高h=0.9m(注：h＞0.3m），桩顶深入承台50mm，钢筋保护层取70mm。承台有效高度（近似取）：mmmh830830.007.09.00承台有效高度精确计算：mmdcahh773214705090020HOHAIUNIVERSITY土木与交通学院岩土所基础工程习题课单桩承载力验算：（4）桩顶荷载计算：取承台及上土的平均重度γG=20KN/m3单桩承载力验算：桩顶平均竖向力：kNRkNnGFQakkK5405.53664.10.22.3203040120040032004002000600140070830500xy600450例图4-2HKFKMK4006004001200HOHAIUNIVERSITY土木与交通学院岩土所基础工程习题课09.4766482.11.5966.595.5362.142.1)9.0140160(5.536)(22maxmaxminKNKNRKNxxhHMQQaikkk120040032004002000600140070830500xy600450例图4-2HKFKMK4006004001200单桩竖向承载力验算符合要求。HOHAIUNIVERSITY土木与交通学院岩土所基础工程习题课单桩水平力标准值：120040032004002000600140070830500xy600450例图4-2HKFKMK4006004001200KN./nHHkk32361401KNKN3.2360EIR0ax3h估算单桩水平承载力特征值：因此单桩水平承载力满足要求。HOHAIUNIVERSITY土木与交通学院岩土所基础工程习题课120040032004002000600140070830500xy600450例图4-2HFM4006004001200承台相关承载力计算采用荷载效应基本组合时柱底内力：KN.F.FK41043040351351KNm.M.MK216160351351KN.H.HK189140351351HOHAIUNIVERSITY土木与交通学院岩土所基础工程习题课扣除承台及上土的自重后的桩顶竖向净反力设计值：kNnFN68464104KN.KN....).(xx)HhM(NNimaxmaxmin66034764480684214219018921668422120040032004002000600140070830500xy600450例图4-2HFM4006004001200HOHAIUNIVERSITY土木与交通学院岩土所基础工程习题课C1=600a0x=700300120040032004002000a0y=175225C2=600600140070830500ⅠⅠⅡⅡxy600450例图4-2HKFKMK（5）承台受冲切承载力验算a.柱边冲切承载力验算计算公式：000002hfahabFthpxcyycxlilNFF2.084.000xx000haxx2.084.000yy000hayyHOHAIUNIVERSITY土木与交通学院岩土所基础工程习题课C1=600a0x=700300120040032004002000a0y=175225C2=600600140070830500ⅠⅠⅡⅡxy600450例图4-2HKFKMKa.柱边冲切承载力验算KNNFFil410404104βhp--受冲切承载力截面高度影响系数，当h≤800㎜时，βhp取1.0；当h≥2000㎜时，βhp取0.9；其间按线性内插法取用。992080090080020009011..hpHOHAIUNIVERSITY土木与交通学院岩土所基础工程习题课C1=600a0x=700300120040032004002000a0y=175225C2=600600140070830500ⅠⅠⅡⅡxy600450例图4-2HKFKMK80502084308402084000......xx0.1843.083.07.0000haxx867.12.025.084.02.084.000yy25.025.0210.0830.0175.00000yyyha取KNFKNhfahablthpxcyycx41045350200000柱边冲切承载力验算满足要求。HOHAIUNIVERSITY土木与交通学院岩土所基础工程习题课C1=600a0x=700300120040032004002000a0y=175225C2=600600140070830500ⅠⅠⅡⅡxy600450例图4-2HKFKMKb.角桩向上冲切承载力验算0111121max)2()2(hfacacNthpxyyx2.056.011xx011haxx2.056.011yy011hayyλ1x、λ1y—为角桩冲跨比取值范围0.2-1.0；HOHAIUNIVERSITY土木与交通学院岩土所基础工程习题课C1=600a0x=700300120040032004002000a0y=175225C2=600600140070830500ⅠⅠⅡⅡxy600450例图4-2HKFKMKb.角桩向上冲切承载力验算yyyyxxxx,aa,,aa,m.cc01010101216053702084305602056011......xx36612021005602056011......yyKNNKNhfacacthpxyyx4.7647.1509)2()2(max0111121角桩向上冲切承载力验算满足要求。HOHAIUNIVERSITY土木与交通学院岩土所基础工程习题课C1=600a0x=700300120040032004002000a0y=175225C2=600600140070830500ⅠⅠⅡⅡxy600450例图4-2HKFKMK（6）承台受冲剪承载力验算00hbfVths0.175.1斜截面受剪承载力计算公式：βhs=（800/h0）1/4=（800/830）1/4=0.991当h0800㎜时，取h0=800㎜；当h02000㎜时，取h0=2000㎜.HOHAIUNIVERSITY土木与交通学院岩土所基础工程习题课剪跨比与以上冲跨比相同。对Ⅰ-