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Nirvana

why not include the load of water?

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ALL IN ONE, EX. 208.2

why the self-weight of water not included in Fy?

the original question in ex. 208.1

Thanks.

Ex. 208.2 part 1.PNG

Ex. 208.2 part 2.PNG

Ex. 208.2 part 3.PNG

Ex. 208.2 part 4.PNG

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without reading thoroughly, I would say because the water level is not guaranteed, so you would conservatively use the weight of dry soil in calculating your resisting moments and sliding due to friction etc.

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For the ground water to raise to the top of the wall it needs to start below the footing.  The downward pressure/force for the hydrostatic load above the footing is cancelled by the hydrostatic uplift below the footing.

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23 hours ago, MA_PE said:

For the ground water to raise to the top of the wall it needs to start below the footing.  The downward pressure/force for the hydrostatic load above the footing is cancelled by the hydrostatic uplift below the footing.

From your angle, the hydrostatic force would be an extra uplift buoyancy from the water pressure difference between the top and bottom side of part 2 (thickness), rather than of cancellation. So the extra uplift buoyancy which is equal to the water occupied by part 2 of wall should be included based on your thinking.

I think it should be included the self-weight of the water behind the wall between the levels 129 and 118. The final resistance moment will be more.

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1 hour ago, Nirvana said:

From your angle, the hydrostatic force would be an extra uplift buoyancy from the water pressure difference between the top and bottom side of part 2 (thickness), rather than of cancellation. So the extra uplift buoyancy which is equal to the water occupied by part 2 of wall should be included based on your thinking.

I think it should be included the self-weight of the water behind the wall between the levels 129 and 118. The final resistance moment will be more.

Exactly and like tj_PE said it's conservative to use the weight of dry soil in calculating your resisting moments and sliding due to friction etc.

Also note that for part two they are computing the effective vertical stress from the top at elevation 132 down to the base of the footing El. 115.  When going from El 129. to El 115: first they they use 360 psf (3 ft of dry soil from 132 to 129) and then (120-62.4) the bouyant weight of soil for 14 ft (El129 - El115).  Therefore they don't consider the footing at all and the vertical pressures from hydrostatic loads will cancel themselves at elevation 115.

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