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Factor of Safety for Crest Toppling 1.5 required?

Factor of Safety for Crest Toppling 1.5 required?

(OP)
Hello,

I am reviewing a retaining wall project which uses anchor wall software. I understand the minimum factor of safety for crest toppling is 1.5 (NCMA). In the report 1.5 falls under the “Default Minimum”, but the engineers are using different minimums when analyzing crest toppling, ranging from 0.95 to 1.5 to design their walls. Why?

Would anyone be able to explain to me please why they would have used safety factors below 1.5 for some of their walls?
Leniency with crest toppling?

Thank you!
Tomas

RE: Factor of Safety for Crest Toppling 1.5 required?

Anchor wall? I am assuming you mean a segmental retaining wall with "geogrid" soil reinforcement?

I always use the default minimums, but I would say that it relates to risk of failure and what "stuff" the wall is retaining (soil + structure, or just soil, roadways, etc.). In general, I think crest toppling as little to do with the overall stability of the wall. If you could rationalize the wall as akin to risk category I (ASCE 7), then maybe, you could justify backing off on some of those factors for basic analysis. but, 0.95 - not sure why that makes sense. Though, the safety factors are generally lowered when you perform a seismic analysis. If you are using the software, turning on seismic analysis will show you that automagically (at least Keystone's does).

Maybe someone else has a different take.

"It is imperative Cunth doesn't get his hands on those codes."

RE: Factor of Safety for Crest Toppling 1.5 required?

Crest toppling is a simple overturning analysis of the gravity wall section above the uppermost reinforcement level. A minimum factor of safety of 1.5 would be appropriate for an earth pressure loading like the rest of the wall design.

There can be issues with the modeling that can overstate the loading at the top of the wall which can be noted but there is really no justification for using lower FS's for normal loadings. There is a case be made for lower FS's with seismic analysis or impact loading but not normal loadings.

One would have to look at the exact situation to see why the numbers are the way they are, not just say 1.1 is ok which would be questioned by most engineers.

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