Sliding / OT factor of safety - updated LC's 2

[cec17]

I know this has been harped on numerous times, however, many of the threads are old and appear to be based on older load combinations (i.e. 0.6D + 1.0W ; from what I can tell this LC is now 0.6D + 0.6W).

My question is what factor of safety are you using for LRFD and ASD load combinations, and where do you pull this language from in IBC or ASCE7?

I interpreted, based on older threads, that when using "older" service load combinations (i.e. 0.6D + 1.0E) it would be a FS of 1.0 because you've reduced the DL by 0.6. However, the "newer" load combinations also reduce the EQ loads to service level (0.6D + 0.7E) so you've reduced both numerator and denominator. Likewise, when using strength load combinations (i.e. 0.9D + 1.0E) you would be required to have a FS of 1.5 (see example below).

I am thinking that using service load combinations with 0.7E or 0.6W would still require a FS = 1.5, and I should be using 1.5 for BOTH service and strength load combinations. Is this overly conservative?

I know they should, but where in ASCE or IBC does it say that foundations require a factor of safety based on the load combinations used? All I see in 2018 IBC is 1807.2.3 which says retaining walls require FS = 1.5 (it then goes on to contradict itself and say the LC in 1605 shall not apply to this requirement).

Sincerely,
Confused

 

[Celt83]

The load combinations are checked against unity (1.0)

From my memory only section 1807.2.3 in IBC 2021 retains the stability safety factors for retaining walls and here you perform all possible combinations of loading with load factors of 1.0 or 0 on variable load types 1 on permanent loads except seismic is factored by 0.7 to bring it down to service level.

 

[lexpatrie]

You mentioned 1.0W, that would be based on "ultimate" wind loads circa 2010 ASCE.

So the 0.6D + 0.6W that's current, is more or less the same 0.6D + 1.0W back when service level wind was a thing and the strength combos had 1.4 for wind (or something in that neighborhood).

Back around 2000 there were service level earthquakes and that switched,

and there's with ASCE 7-22, I think, "factored snow". So the phase out of service level "design events" is now almost done.

If that helps any.

As to the 0.6 D and the 1.5 factor of safety on sliding, they are about the same thing if you don't get too deep into it. I feel like SK Ghosh had an article on it quite a while ago.