A water tower weighing 3000 kN is supported by four columns inclined at 15? from

Question

A water tower weighing 3000 kN is supported by four columns inclined at 15? from vertical. The columns are founded on level ground on square spread footings of 4 m width. The depth of embedment of the footings will be 1 m, which is the same as the footing thickness. The soil profile consists of a lightly overconsolidated clay with a uniform undrained shear strength of 30 kPa, and drained strength parameters c? = 5 kPa and ?? = 22?. Compute the FS against foundation bearing capacity failure for drained and undrained conditions. Use FS = qu,net / qall,net . Indicate whether the critical loading condition is undrained (immediately after construction) or drained (a long time after construction).

Explanation / Answer

Drained : shear strength = c + normal stress * tan 22 . SO normal stress = 61.8 KPa. Now footings at 15 degree to vertical. so force on footing at 15 degee to vertical = 3000kN * cos 15 = 2897.77 kN. On each footing = 2897.77/4 = 724.44 kN. stress = force /area = 724.44/16 = 45.27 KPa. SO FS = 61.8 KPa / 45.27 KPa = 1.36. ................................. Undrained: c =0 so normal stress = 30/tan22 = 74.25 KPa. SO FS = 74.28/45.27 = 1.64

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