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Re: FOS for overturning
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- Subject: Re: FOS for overturning
- From: <ErnieNSE(--nospam--at)aol.com>
- Date: Thu, 25 Jun 1998 20:12:25 EDT
Let's first define overturning about what point, and what are the conditions.... Ideally, we check for overturning about the lower right hand corner (Case 1 only first), call it point "O", assuming that the soil is rigid and non yielding. If we apply enough lateral load to tilt it sideways, the footing will rotate about point "O". We assume that this rotaton point is pinned and restrained from moving laterally both sides and vertically downward only. >From basic statics, for equilibrium, summation of overtuning moment about a point equals zero.. The lateral external force will cause a clockwise overturning moment about point "O". The ftg dead wt will cause a counterclockwise resisting moment about point "O"(forget about the 2/3 dl factor for now). The external uplift force is more com0plicated to analyze. If the resultant sum of the vertical forces(dl + uplift) is downward, we include the uplift force in our calculation for clockwise overturning about point "O". This is because point "O" is restrained from moving downwards(see assumption above). Do not consider for now the lateral soil bearing pressure resisting the oveturning. If the uplift force is greater than the dead wt. of the ftg, then the system is unstable since we assume that point "O" is not restrained vertically upwards.(which is close to reality). The footing will continue to move move upward until the downward force becomes greater than or equal to the upward force. As the ftg lift up above the soil, there is no lateral restraint anymore so the ftg slides sideways. So, the contribution of the uplift force(it does not matter where it is applied) on overturning about point "O"(considering point "O"'s vertical and lateral restraint conditions) depends on it's magitude compared to the downward force. However, if point "O" is restrained from moving vertically upward also(by some kind of connection at the edge of the footing to the rigid foundation) then, the uplift force will always be considered in overturning, no matter what(until the upward ftg restraint is exceeded). This is just the theoretical analysis. In practice, we need to use our judgement in defining the actual point of overturning(for a soft soil, the footing may not actually rotate about point "O"), the actual vertical and lateral restraint at the overturning point and the contributions of other materials on the stability of the system(lateral soil bearing capacity, soil friction at the sides of the ftg, dl of slab above soil, etc). Just my theoretical opinion. Ernie Natividad
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