Foundation Stability Factors of Safety 3

[HDStructural]

This may have been discussed in the past but I couldn't find this specific aspect of the foundation discussion. Sorry for bringing this topic back into discussion. This should be a simple yes or no answer.

In the past, I've checked all ASD load combinations for my footing stability, and when wind or seismic controlled, I used a FOS of 1.0 due to the 0.6D in the load combination (would be a 1.5 FOS for 1.0D combinations)

Couldn't I ignore all 0.6D load combinations and check the 1.0D + 0.6W or 1.0D + 0.7E load combo with a safety factor of 1.5. The other load combinations with 1.0D (D + L, D + S, etc..) would already be checked with a safety factor of 1.5 for stability. This makes the analysis easier as a 1.5 FOS would apply to all of the combinations I am checking, so I wouldn't need to mess with different FOS's and try to trick enercalc or other spreadsheets.

Wanna make sure I'm not crazy to stop checking certain load combinations in my footing analysis.

 

[human909]

The 0.6 and 0.9 factors are de facto exactly the same. Because wind is reduced so much the end result is the same, just the reduction factor looks quite odd because ASD load don't play nice with actual predicted max wind loads.

I don't understand this.
1. 0.6 and 0.9 are not the same. They are factors that are suppose to account for the variability the load. Even if the end result is the same in some circumstance it doesn't make it the same.

2. "Because wind is reduced so much the end result is the same." I'm not sure what code you are referring here where wind gets reduced. But if that is the case then that is another oddity.

 

[driftLimiter]

Lets convince the geotechs to give ultimate loads and be done with ASD forever.

Would also like to mention that our code allows us to reduce seismic effects of overturning by 25% at the foundation.

 

[human909]

Lets convince the geotechs to give ultimate loads and be done with ASD forever.

I'm onboard with that! While here with AS code we have long left ASD ,our geotechs still mostly give allowable bearing stresses rather than ultimate. Occasionally you get a good one that gives you the ultimate value AND/OR the reduction factors used to obtain allowable values.

 

[hardbutmild]

I think they use ASD all around the world.

Would also like to mention that our code allows us to reduce seismic effects of overturning by 25% at the foundation.

This is interesting, I have never heard of that. It maybe comes from an idea of a dynamic response of a rigid block - you need a larger force to overturn the block if the load has a large frequency compared to a static load.
It's just my attempt to explain this, it may be very wrong.