地基承载力特征值与基础底面尺寸确定修改.ppt

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1、基础设计的典型步骤：确定基础埋深求得持力层承载力选择基础底面尺寸验算持力层承载力是否满足p≤f验算软弱下卧层验算地基变形调整基础尺寸结束否是否否是Terzaghi(1943)极限承载力公式在整体剪切破坏情况下（密实砂土和坚硬粘土地基）对条形基础：对直径为b的圆形基础：对边长为b方形基础：对于宽为b，长为l的矩形基础，按b/l值在条形基础（b/l→0）及方形基础（b/l=1）的承载力之间插值而得。Terzaghi(1943)极限承载力公式在局部剪切破坏的情况下（软粘土或松散砂地基上），建议用调整抗剪强度指标，即用：——相应于局部剪切破坏的承

2、载力因素,由查下图中的虚线或由查图中的实线。代替，极限承载力采用下式：魏锡克(Vesic)极限承载力公式中心荷载作用下条形基础的地基极限承载力基本公式与Terzaghi公式相同，但其承载力系数不同。式中：承载力因素，分别由下式确定或查表承载力因素φNcNqNrNq/NctanφφNcNqNrNq/Nctanφ0123456789101112131415161718192021222324255.145.385.635.906.196.496.817.167.537.928.358.809.289.8110.3710.9811.6312.3

3、413.1013.9314.8315.8216.8818.0519.3220.721.001.091.201.311.431.571.721.882.062.252.472.712.973.263.593.944.344.775.265.806.407.077.828.669.6010.660.000.070.150.240.340.450.570.710.861.031.221.441.601.972.292.653.063.535.074.685.396.207.138.209.4410.880.200.200.210.220.230

4、.240.250.260.270.280.300.310.320.330.350.360.370.390.400.420.430.450.460.480.500.510.000.020.030.050.070.090.110.120.140.160.180.190.210.230.250.270.290.310.320.340.360.380.400.420.450.472627282930313233343536373839404142434445464748495022.2523.9425.8027.8630.1432.6735.49

5、38.6442.1646.1250.5955.6361.3567.8775.3183.8693.71105.11118.37133.88152.10173.64199.26229.93266.8911.8513.2014.7216.4418.4020.6323.1826.0929.4433.3037.7542.9248.9355.9664.2073.9085.3899.02115.31134.88158.51187.21222.31265.51319.0712.5414.7416.7219.3422.4025.9930.2235.1941

6、.0648.0356.3166.1978.0392.25109.41130.22155.55186.54224.64271.76330.35403.67496.01613.16762.890.530.550.570.590.610.630.650.680.700.720.750.770.800.820.850.880.910.940.971.011.041.081.121.151.200.490.510.530.550.580.600.620.650.670.700.730.750.780.810.840.870.900.930.971.

7、001.041.071.111.151.19魏锡克(Vesic)极限承载力公式须指出的是：许多地基承载力公式都可写成Terzaghi公式的形式，且这些公式中的Nc、Nq相同，但Nr差别较大。由Vesic公式算得结果与实际分析结果误差不大，且偏于安全。Vesic根据影响承载力的各种因素对公式进行修正。例如：基础底面的形状、偏心和倾斜荷载、基础两侧覆盖层的抗剪强度、基底和地面倾斜h和土的压缩性等其它因素的影响等，到目前是为止，考虑得最多，最全面。基础形状的影响:对方形或圆形基础,采用半经验的基础形状因素加以修正,修正后的极限承载力公式为:Sc

8、、Sq、Sr---基础形状因素，按以下公式确定：矩形基础：式中：b为基础宽度，l为基础长度偏心和倾斜荷载的影响：分析表明，偏心和倾斜荷载作用下，极限承载力将有所降低。对于偏心荷载，如为条形基础