25% Reduction - Extend to Sliding?

[calvinandhobbes10]

Curious--- for those of you who take advantage of the 25% reduction in seismic overturning effects prescribed in ASCE 7-10 12.13.4, do you also apply this force reduction to the sliding check? In my experience sliding is often the controlling design check these days. I understand it reads "overturning effects", however I think there's a case to be made for sliding being lumped in with it---if a shearwall is a vertical cantilever with a fixed base, you have two reactions, M and V, due to the seismic force applied. I could make a case that both M and V are reactions of the "overturning effects".

Also, have any of you ever heard of a one-third increase in the sliding friction coefficient under transient loads? E.g., 0.35 turns to 0.467 under seismic check? I have a geotech report that mentions this, first I've encountered.

 

[KootK]

I've not been applying it to shear. That said, when last I looked into it, I saw mixed signals. It's been a while though so you may want to fact check what follows.

One NEHRP source says that the reduction is to account for the fact that the vertical distribution of shear is skewed upwards to envelope wall forces. If this is the story that we're telling, then shear shouldn't be reduced because it's the same total shear just lowered in space to yield a more realistic overturning moment.

Another NEHRP sources say that the reduction is to account for foundation rocking. The foundation is allowed to rock, and when it does, it ceases to draw additional seismic load (ish). If this is the story that we're telling, then shear should be reduces as the same rocking that limits moment also limits shear (both a function of a limited attraction of seismic force).

I favor the first story.