Risa Foundation Factors of Safety

[Stazz]

When I have Risa design my footings in Risa foundation, shouldn't the program optimize the size such that all of the factors of safety are above 1.5? When I design them the report says that some of the load combo's produce safety factors under 1.5 which is highlighted in red. Are these factors actually ok? Am I missing something?

 

[JoshPlumSE]

The stability checks were added as an afterthought to the program. They are not currently included in the optimization routines.... If we were writing the program from scratch today then that's not how I would do it. However, that's what we've got for now.

Maybe when we release a version 4.0 then it will have optimization for OTM and sliding as well.

Some additional thoughts:
1) When using the 0.6DL + WL Load combinations, the safety factor need only be 1.0. That's becasue the 1.5 factor of safety has already been account for with the reduced dead load.

2) Personally, I'd like to see us both optimize for OTM stabiliy AND allow the user to adjust this factor on a per LC basis. I know that my old company used to apply higher factors of saftety when DL or LL cause some overturning. Also, we used to use different factors of safety for wind and seismic.

Josh

 

[Stazz]

Thanks Josh

I was just looking for information about whether to consider a factor of safety along with the 0.6DL+W cases. It seems to be a debated topic with mixed opinions. I'm glad these cases DO account for the 1.5 FOS because it kills the size of my footing when I use them both.

Can I ask where you found that the 0.6DL case already includes the FOS? I can't find any hard published evidence that talks about this. It should be in the IBC.

 

[WillisV]

Stazz...it should be but its not - see my post here for explanation:

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